Sir: Mlugu TOPIC 1: WAVES
Meaning and definition of wave: -
A wave Refers to
a disturbance that travels through a medium from one location to another
location. Example01 of wave is
Electronic wave the type of wave which can move through a vacuum. Another example02, is slinky wave, a slinky wave as an example of a wave.
When the slinky is stretched from end to end and is held at rest, it assumes a
natural position known as the equilibrium or rest position.
The coils of the slinky naturally assume this position,
spaced equally far apart. To introduce a wave into the slinky, the first
particle is displaced or moved from its equilibrium or rest position.
The
particle might be moved upwards or downwards, forwards or backwards; but once
moved, it is returned to its original equilibrium or rest position. Another example03, sound wave is an example of
mechanical wave. Example,01.
Definitions of
concepts used in wave: -
A pulse is a
single disturbance moving through a medium from one location to another
location.
However, if the first coil of the slinky is continuously and
periodically vibrated in a back-and-forth manner, we would observe a repeating
disturbance moving within the slinky that endures over some prolonged period of
time. The repeating and periodic disturbance that moves through a medium from
one location to another is referred to as a wave.
A medium: Refers
to the space through which a the wave travels such as the most common example
of electromagnetic waves pass through an Air, or water, such as light. Example,
Air as medium below:-
Air, most type of waves do require some sort of interaction with
their medium in order to transport. One of the most common example, on the
earth are sound waves. A sound wave is longitudinal waves which is a type of
wave that moves in direction parallel to the particle of the medium through
which it travels, Unlike electromagnetic waves, longitudinal waves cannot
travel in a vacuum.
NB:
The wave medium is not the wave and it doesn't make the
wave; it merely carries or transports the wave from its source to other
locations.
In the case of our slinky wave, the medium through that the
wave travels is the slinky coils. In the case of a water wave in the ocean, the
medium through which the wave travels is the ocean water. In the case of a
sound wave moving from the church choir to the pews, the medium through which
the sound wave travels is the air in the room.
Wave length: Refers
to a spatial period of periodic wave the distance over which the waves shape
repeats. Its Also the inverse of spatial frequency. Wave length is determined
by considering the distance between consecutively corresponding points of the
same phase, such as crest, though, or zero crossing and is characteristics of
both travelling waves and standing waves as well as other spatial wave patterns., λ, lambda is a symbol which used to
present wave length in order tom avoid confusion its Greek letter.
Frequency: Is the
number of waves passing a point in certain time. We normally use a time of
second so this gives frequency the unit hertz (Hz) , since one hertz is equal
to one wave per second. NB. It’s not
a distance travelled by wave its nor it’s a speed although its linked to both
of these quantities. For water waves and sound waves the unit hertz is usually
good but, radio and Tv waves have such a high frequency that the kilohertz
(kHz) or even the megahertz (MHz) are better units. where, 1kHz=1000HZ
1MHz=1,000,000Hz
. Period, (T) refers to the time that it takes to do
something. When an event occurs repeatedly, then we say that the event is
periodic and refer to the time for the event to repeat
itself as the period. The
period of a wave is the time for a particle on a medium to
make one complete vibrational cycle. Period, being a time, is measured in units
of time such as seconds, hours, days or years. The period of orbit for the
Earth around the Sun is approximately 365 days; it takes 365 days for the Earth
to complete a cycle.
The speed of an object: refers to how fast an object is moving and
is usually expressed as the distance traveled per time of travel. In the case
of a wave, the speed is the distance traveled by a given point on the wave
(such as a crest) in a given interval of time. The SI unit of speed is m/s.
Amplitude: This
is the distance from the centere line or still position to the top of crest or
to the bottom of trough. Be care full with this quantity the Centre line is not
always given in diagram. Amplitude is measured in metres (m) . The greater the
amplitude of wave then the more energy its carrying.
Velocity of wave:
Refers to distance traversed by a periodic or cyclic, motion per unit time (in
any direction), Wave velocity is common usage. In common usage refers to speed,
although, properly, velocity implies both speed and direction. The velocity of
wave is equal to the product of wave length and frequency (number of vibrations
per second) and its intensity.
A transverse wave: This is a wave in which the particles of the
medium are displaced in a direction perpendicular to the direction of energy
transport.
The crest of a wave: Refers to the point
on the medium that exhibits the maximum amount of positive or upward
displacement from the rest position.
The trough of a wave is the point on the medium that exhibits the
maximum amount of negative or downward displacement from the rest position.
A longitudinal wave Refer to a wave in
which the particles of the medium are displaced in a direction parallel to the
direction of energy transport. A longitudinal wave can be created in a slinky
if the slinky is stretched out horizontally and the end coil is vibrated
back-and-forth in a horizontal direction.
A compression is a point on a medium
through which a longitudinal wave is traveling that has the maximum density. A
region where the coils are spread apart, thus maximizing the distance between
coils, is known as a rarefaction.
A rarefaction: Refer to a
point on a medium through which a longitudinal wave is traveling that has the
minimum density. Points A, C and E on the diagram above represent compressions
and points B, D, and F represent rarefactions.
Wave equation
The wave equation shows the relationship between speed,
wavelength and frequency of a wave.
The diagrams below show several "snapshots" of the
production of a wave within a rope. The motion of the disturbance along the
medium after every one-fourth of a period is depicted. Observe that in the time
it takes from the first to the last snapshot, the hand has made one complete
back-and-forth motion.
A period has elapsed. Observe that during this same amount
of time, the leading edge of the disturbance has moved a distance equal to one
complete wavelength. So in a time of one period, the wave has moved a distance
of one wavelength. Combining this information with the equation for speed
(speed = distance/time), it can be said that the speed of a wave is also the
wavelength/period.
Since the period is the reciprocal of the frequency, the
expression 1/f can be substituted into the above equation for period.
Rearranging the equation yields a new equation of the form:
Speed = Wavelength • Frequency. The above equation is known
as the wave equation. It states the mathematical relationship between the speed
(v) of a wave and its wavelength (λ) and frequency (f ). Using the symbols v,
λ, and f , the equation can be rewritten as,v = f • λ
Types of Waves
The following are types of waves according to the media of propagation:
-
·
Mechanical Waves. Example figure below.
Mechanical Waves
Mechanical waves are also called elastic waves as their
propagation depends on the elastic properties of the medium through which the
waves pass
Mechanical waves are divided into three categories:
Transverse waves, longitudinal waves, and surface waves. In transverse waves,
the medium moves perpendicular to the wave direction, and in longitudinal
waves, the medium moves parallel to the wave direction.
In surface waves, both transverse and longitudinal waves mix
in a single medium. In very simple words, an electronic wave is that which
travels in a vacuum, and a mechanical wave is that which needs some medium for
traveling.
Examples of
mechanical waves are Sound waves, Water waves, Ocean waves, Earth quake waves,
Seismic waves
Electromagnetic Waves
Electromagnetic waves are waves that have no medium to
travel whereas mechanical waves need a medium for its transmission. Examples of
electromagnetic waves include light and radio signals.
The following are the
differences between mechanical and electromagnetic waves.
·
Electromagnetic waves travel in a vacuum whereas
mechanical waves do not.
·
The mechanical waves need a medium like water,
air, or anything for it to travel.
·
While an electromagnetic wave is called just a
disturbance, a mechanical wave is considered a periodic disturbance.
Behavior of Waves
Reflection,
Refraction, Diffraction and Interference
of Waves
Explain Reflection, refraction, diffraction and interference
of waves
All waves behave in certain characteristic ways. They can
undergo:
Refraction
Involves the change in the direction of waves as they pass
from one medium to another. Refraction or bending of the path waves, is a
companied by a change in speed and wave length of the waves .Example figure.
Figure 07
Diffraction
Involve change in the
direction of waves as they pass from one medium to another and diffraction
involves change in direction of waves as they pass through an opening or around
a barrier in their path. Water waves have the ability to travel around corners,
around obstacles and through openings. Example, water waves have ability to
travel around corners, around obstacles and through opengs.
Figure08
Interference
These basic properties define the behavior of a wave –
anything that reflects, refracts, diffracts and interferes is labelled a wave.
Figure -09
Reflection
Reflection is the change in direction of a wave front at an
interface between two different media so that the wave front returns into the
medium from which it originated. Common examples include the reflection of
light, sound and water waves. Example figure.
The law of reflection
If the reflecting surface is very smooth, the reflection of
light that occurs is called specular or regular reflection. The laws of
reflection are as follows:
·
The incident ray, the reflected ray and the
normal to the reflection surface at the point of the incidence lie in the same
plane.
·
The angle which the incident ray makes with the
normal is equal to the angle which the reflected ray makes to the same normal.
·
The reflected ray and the incident ray are on
the opposite sides of the normal.
Characteristics of
Reflection of waves:
·
The wavelength, λ of the reflected wave is the
same as that of the incident waves.
·
It obeys the Law of Reflection.
The frequency, f of
the reflected waves is the same as that of the incident waves.
·
Therefore, the speed, v of the reflected waves
is the same as that of the incident waves.
Types of reflection:
·
Diffused: Rough surfaces scatter light in all directions.
·
Specular: Smooth surfaces direct reflected light
at opposite angle.
Spread: Some
surfaces have a combination texture and smooth surface (varnish overcoat on
paper, white label on white bottle).
Refraction
Refraction is the change in direction of propagation of a
wave due to a change in its transmission medium.
The phenomenon is explained by the conservation of energy
and conservation of momentum. Due to change of medium, the phase velocity of
the wave is changed but its frequency remains constant. This is most commonly
observed when a wave passes from one medium to another at any angle other than
0° from the normal.
Refraction of light is the most commonly observed
phenomenon, but any type of wave can refract when it interacts with a medium,
for example when sound waves pass from one medium into another or when water
waves move into water of a different depth.
Snell’s law
Refraction is described by Snell's law, which states that:
“For a given pair of media and a wave with a single frequency, the ratio of the
sines of the angle of incidence
θ1 and angle of refraction θ2 is equivalent to the ratio of
phase velocities ( v1 / v2) in the two media, or equivalently, to the opposite
ratio of the indices of refraction (n 2 / n 1)”
The refractive index or index of refraction n of an optical
medium is a dimensionless number that describes how light, or any other
radiation, propagates through that medium.
Refraction of a light
ray.
The refractive index determines how much light is bent, or
refracted, when entering a material. This is the historically first use of
refractive indices and is described by Snell's law of refraction, n 1 sin θ 1 =
n 2 sin
θ 2, where θ 1 and θ 2 are the angles of incidence and
refraction, respectively, of a ray crossing the interface between two media
with refractive indices n 1 and n 2.
The refractive indices also determine the amount of light
that is reflected when reaching the interface, as well as the critical angle
for total internal reflection and Brewster's angle.
Interference
Interference is a phenomenon in which two waves superpose to
form a resultant wave of greater or lower amplitude.
Interference usually refers to the interaction of waves that
are correlated or coherent with each other, either because they come from the
same source or because they have the same or nearly the same frequency.
Interference effects can be observed with all types of
waves, for example, light, radio, acoustic, surface water waves or matter
waves.
Constructive
Interference
Constructive interference is a type of interference that
occurs at any location along the medium where the two interfering waves have a
displacement in the same direction.
In this case, both waves have an upward displacement;
consequently, the medium has an upward displacement that is greater than the
displacement of the two interfering pulses. Constructive interference is
observed at any location where the two interfering waves are displaced upward.
But it is also observed when both interfering waves are displaced downward.
-In this case, a sine pulse with a maximum displacement of
-1 unit (negative means a downward displacement) interferes with a sine pulse
with a maximum displacement of -1 unit. These two pulses are drawn in red and
blue. The resulting shape of the medium is a sine pulse with a maximum
displacement of -2 units.
Destructive
Interference
Destructive interference is a type of interference that
occurs at any location along the medium where the two interfering waves have a
displacement in the opposite direction.
For instance, when a sine pulse with a maximum displacement
of +1 unit meets a sine pulse with a maximum displacement of -1 unit,
destructive interference occurs.
Diffraction
Diffraction refers to a change in direction of waves as they
pass through an opening or around a barrier in their path.
In classical physics, the diffraction phenomenon is
described as the interference of waves according to the Huygens–Fresnel
principle. These characteristic behaviors are exhibited when a wave encounters
an obstacle or a slit that is comparable in size to its wavelength. Similar
effects occur when a light wave travels through a medium with a varying
refractive index, or when a sound wave travels through a medium with varying
acoustic impedance.
Diffraction occurs with all waves, including sound waves,
water waves, and electromagnetic waves such as visible light, X-rays and radio
waves. Diffraction arises because of the way in which waves propagate; this is
described by the Huygens–Fresnel principle and the principle of superposition
of waves.
The propagation of a wave can be visualized by considering
every particle of the transmitted medium on a wave front as a point source for
a secondary spherical wave. The wave displacement at any subsequent point is
the sum of these secondary waves.
When waves are put together, their sum is determined by the
relative phases as well as the amplitudes of the individual waves so that the
summed amplitude of the waves can have any value between zero and the sum of
the individual amplitudes.
A long slit of infinitesimal width which is illuminated by
light diffracts the light into a series of circular waves and the wave front
which emerges from the slit is a cylindrical wave of uniform intensity. A slit
which is wider than a wavelength produces interference effects in the space
downstream of the slit. These can be explained by assuming that the slit
behaves as though it has a large number of point sources spaced evenly across
the width of the slit
Diffraction of water waves is observed in a harbor as waves
bend around small boats and are found to disturb the water behind them.
Diffraction of sound waves is commonly observed; we notice sound diffracting
around corners, allowing us to hear others who are speaking to us from adjacent
rooms.
Diffraction is observed of light waves but only when the
waves encounter obstacles with extremely small wavelengths (such as particles
suspended in our atmosphere).
The Application of Reflection, Refraction, Diffraction and
Interference of Waves in Daily Life
Mention the application of reflection, refraction, diffraction
and interference of waves in daily life
Application of
reflection of waves
·
Reflection of light waves is used in designing
mirrors. That is concave mirror or convex mirror.
·
Sonar (Sound Navigation and Ranging). Sonar is
used to detect underwater objects (corals / fishes) or to determine the depth
of the water by means of an echo. Sonar equipment emits a high frequency sound
signal which is reflected by the object in the water. The reflected sound wave
is received by the sonar receiver. The time taken for the echo to return is
used to determine the distance of the object below the water surface. Sonar
waves of high frequency is used because itpossessesmore energy, high
penetration power and can travel further through water.
·
The phenomenon of the reflection of sound is
used to determine the distance between the two objects, for example depth of
seabed, depth of cave or width of a valley. The type of sound used must be
ultrasound.
·
Detection
of cracks in metals.
Determination of
frequency of A. C’s.
Applications of
Refraction
·
Refraction has many applications in optics and
technology. A lens uses refraction to form an image of an object for many
different purposes, such as magnification.
·
A prism uses refraction to form a spectrum of
colors from an incident beam of light.
Refraction also plays an important role in the
formation of a mirage and other optical illusions.
Applications of
interference of waves
·
Interference is applied when creating holograms.
A hologram is a photograph of an interference pattern which is able to produce
a three-dimensional image when suitably illuminated.
·
Destructive interference is used in noise
reduction systems such as earphones. The system capture sound from the
environment and use computer technology to produce a second sound wave, which
leads to reduction in the loudness of the noise.
3. Concert halls and auditorium are usually designed in such
a way to reduce the amount of destructive interference. Usually, the walls and
ceiling made in such a way that they absorb rather than reflect sound.
Application of wave
diffraction
·
Diffraction Grating: A diffraction grating is an
optical device that consists of not one but many thousands of apertures.
Spectra produced by diffraction gratings are extremely useful in applications
from studying the structure of atoms and molecules to investigating the
composition of stars.
·
X-ray diffraction: X rays are light waves that
have very short wavelengths. When they irradiate a solid, crystal material they
are diffracted by the atoms in the crystal’s-ray diffraction utilizes an
instrument called a diffractometer to produce diffraction patterns that can be
compared with those of known crystals to determine the structure of new
materials.
The Behavior of Waves
Behavior of Waves are reflection, refraction, diffraction
and interference of Waves. All waves behave in certain characteristic ways.
They can undergo: Reflection, Refraction, Diffraction and Interference. These
basic properties define the behaviour of a wave – anything that reflects,
refracts, diffracts and interferes is labelled as a wave.
Propagation of Waves
The Propagation of
Mechanical Waves
Describe the propagation of mechanical waves
Mechanical waves can be divided into three main categories
according to the ways in which they travel, known as propagation.
·
Transverse
wave is one that vibrates at 90 degrees to the direction the wave is
moving. For example, if you hold a Slinky between two hands and shake it up and
down, you'll get a wave that moves along the Slinky, but the vibrations will
still be up and down. Underwater waves are also transverse.
·
A
longitudinal wave is one in which the vibrations are parallel to the
direction the wave is moving. That's like sending a pulse along the length of a
Slinky, pushing it lengthwise. Instead of peaks and troughs, longitudinal waves
have compressions (areas where the Slinky is bunched together), and
rarefactions(areas where the Slinky is spread apart). Another example of a
longitudinal wave is a sound wave. Although you can't see air molecules, if you
could, you would notice that sound involves air molecules hitting each other,
thereby producing areas with high densities of molecules (compressions) and
areas with low densities of molecules (rarefactions).
·
A surface
wave is a wave that travels along the boundary between two materials. For
example, the kind of water wave you most often see--along the top of water--is
an example of a surface wave. Surface waves move in similar ways to transverse
waves but are a bit more complicated in their behavior.
In the case of an earthquake, you get a
mixture of all three types of waves. The initial earthquake (called the primary
wave) is longitudinal, but the aftershock that comes later (called the
secondary wave) is transverse. Extra surface waves are also created.
The Propagation of
Electromagnetic Waves
Explain the propagation of electromagnetic waves:-
Electromagnetic waves are transverse waves only. Velocity of
all electromagnetic waves is equal to the velocity of light which is
300,000,000 m/s (3x10exp8). Electromagnetic waves do not need medium for their
transmission. Electromagnetic waves are arranged in special arrangement known
as electromagnetic spectrum according to the increase in their frequencies or
decrease in their wavelengths.
The Relationship between Frequency, Speed and Wavelength of a Wave
Determine the relationship between frequency, speed and
wavelength of a wave
Frequency (f) of a wave is inversely proportional to its
wavelength (l). Speed (V) of a wave is constant. That is, f=V/l. That is Speed
of a wave can be expressed as a product of frequency and wavelength. That is, V
= f X l Speed of a wave is expressed in meters per second (m/s). Period (T) of
a wave is a time taken by a wave to complete one oscillation. Frequency is a
reciprocal of the Period.
Mathematically it can be shown as:v = f • λ
where
Ú v =
speed of a wave
Ú λ
=wavelength
Ú f =
frequency
The Refractive Index of a Medium
Determine the refractive index of a medium
Refractive index(n) is a ratio of the velocity (Va) of a
wave in air to its velocity in a medium (Vm). n=Va/Vm. Refractive index can
also be obtained as a ratio of the sine of angle of incidence (Sin i) to the
sine of angle of refraction (Sin r). The speed of the wave depends on the
medium through which it travels. Changing the medium changes the speed.
Absolute refractive index=V in vacuum/V.
Refractive index of a material is a measure of the change in
speed of light as it passes from a vacuum (or air as an approximation) into the
material.
In the equation above, v1is the speed of light in a vacuum.
The bigger the refractive index the slower the light travels in that material -
i.e. the smaller v2is.
Example 1
Light of frequency 4.6 × 1014Hz travels at a speed of 1.24 ×
108ms-1in diamond. Calculate the refractive index of diamond for this colour of
light.
Solution
vdiamond= 1.24 × 108ms-1
c= 3.0 × 108ms-1
Refractive index of diamond for this colour of light = 2.42
·
Sound Waves
·
Source of Sound Wave
Identify source of
sound waves
Sound is oscillation in pressure, stress, particle
displacement, particle velocity, etc., propagated in a medium with internal
forces (e.g., elastic or viscous), or the superposition of such propagated
oscillation.
The sources of sound
·
vibrating solids.
·
rapid expansion or compression (explosions and
implosions).
Smooth (laminar) air
flow around blunt obstacles may result in the formation of vortices (the plural
of vortex) that snap off or shed with a characteristic frequency. This process
is called vortex shedding and is another means by which sound waves are formed.
This is how a whistle or flute produces sound. Also the Aeolian harp effect of
singing power lines and fluttering venetian blinds.
The Concept of
Audibility
Explain the concept of audibility range
Audibility range the range of frequencies that can be heard
by humans or other animals, though it can also refer to the range of levels.
The human range is commonly given as 20 to 20,000Hz, though
there is considerable variation between individuals, especially at high
frequencies, and a gradual loss of sensitivity to higher frequencies with age
is considered normal
Sensitivity also varies with frequency, as shown by
equal-loudness contours. Routine investigation for hearing loss usually
involves an audiogram which shows threshold levels relative to a normal.
Several animal species are able to hear frequencies well beyond the human
hearing range. Some dolphins and bats, for example, can hear frequencies up to
100kHz.
Several animal species are able to hear frequencies well
beyond the human hearing range. Some dolphins and bats, for example, can hear
frequencies up to 100kHz.
The Perception of
Hearing
Describe the perception of hearing
A basic measure of hearing is afforded by an audiogram: a
graph of the minimum discernible sound level at various frequencies throughout
an organism's nominal hearing range.
Behavioural hearing tests or physiological tests can be used
to find hearing thresholds of humans and other animals. For humans, the test
involves tones being presented at a specific frequencies (pitch) and
intensities (loudness). When the subject hears the sound, he or she indicates
it by raising a hand or pressing a button. The lowest intensity they can hear
is recorded.
The human ear
The ear is the organ that detects sound. It not only
receives sound, but also aids in balance and body position. The ear is part of
the auditory system.
Often the entire organ is considered the ear, though it may
also be considered just the visible portion. In most mammals, the visible ear
is a flap of tissue that is also called the pinna (or auricle in humans) and is
the first of many steps in hearing.
Vertebrates have a pair of ears placed somewhat
symmetrically on opposite sides of the head. This arrangement aids in the
ability to localize sound sources.
The human ear is divided into three parts:
The outer ear
The outer ear comprises the pinna (auricle), which is made
of a convoluted plate of flexible cartilage that extends as a nearly closed
tube one-third of the way down the ear canal. This outer third, which is about
eight millimeters (one-third of an inch) long, has small hairs that point
outwards to form a line of defense against small animals creeping in. The roots
of the hairs produce oils and these mix with the secretions from nearby
sweat-like glands to form the basis of wax.
The deep two-thirds of the ear canal (16 millimeters/two-thirds
of an inch long) has a bony wall lined with thin and rather fragile skin which
is devoid of glands. At the far end of the ear canal and stretched across it is
the eardrum (tympanic membrane), which forms the boundary between the outer and
middle ears.
The middle ear
(Tympanum)
The eardrum is a circle of thin skin about eight to nine millimeters
(one-third of an inch) in diameter. Despite its name, it is not flat like the
skin of a drum, but is slightly conical with the curved sides sloping inwards.
The eardrum has three layers.
The inner ear
(Labyrinth)
The inner ear is probably the most remarkably intricate
piece of the body. It makes hearing possible by converting sound into electrical
impulses that then travel along the hearing nerve (the acoustic nerve or
auditory nerve) to the brain. The inner ear also plays a major role in balance.
The balance portions of the inner ear (vestibular labyrinth) can detect
acceleration of the head in any direction whether in a straight line (linear)
or twisting and turning (angular). The electrical signals that arise in
response to head movement pass along the balance nerve (vestibular nerve),
which in due course joins with the hearing nerve to form a single bundle
(statoacoustic, vestibulo-acoustic or eighth nerve, nerve VIII) which then
enters the brain.
The portion of the inner ear that actually hears is the
cochlea. This is a hollow coiled tube set in the very dense bone called the
bony labyrinth (part of the petrous [rocklike] temporal bone). This tube is
filled with fluid, which is much the same as general body fluid (lymph) and
that which surrounds the brain (cerebrospinal fluid – CSF). This inner-ear
fluid is called perilymph. Inside the perilymph is another coiled
triangular-shaped tube called the cochlear duct (Scala media), which contains
the all-important ‘hair cells’ – these convert sound into electricity. These
hair cells are arranged in two groups that follow the coils of the cochlear duct
and spiral upwards from base to apex. There is a single row of inner hair cells
(IHCs), which lie closer to the core of the cochlea (modulus), and three or
four rows of outer hair cells (OHCs), which are further away. In a healthy
young human ear there are about 3,500 IHCs and about 12,000 OHCs. Each hair
cell has a cluster of small rigid hairs (stereo cilia), which project from the
thicker upper surface of the cell into the special fluid that fills the
cochlear duct. This fluid is called endolymph and is remarkable in that it has
a strongly positive electrical charge associated with it – about 80 millivolts
– and is rich in potassium, a metallic element.
The hair cells in their rows are grouped together with their
supporting cells in the organ of Corti. This is a small ridge that sits on a
thin, very flexible membrane called the basilar membrane. The basilar membrane
forms the floor of the triangular cochlear duct. The sloping roof is another
very thin membrane (Reissner’s membrane) and the side wall is a thickened
region rich in blood vessels (the stria vascularis). This structure is
responsible for maintaining the composition of the rather unusual and very
important endolymph.
Adjacent to the base of the hair cells are the nerves that
carry impulses to the brain (the afferent nerves). At least 90 per cent of
these nerves come from the inner hair cells, despite their smaller number. Each
inner hair cell has about 10 nerve endings attached to it and there are,
therefore, about 30,000 nerve fibers in the acoustic nerve.
The hearing nerves travel inwards, along with the balance
and facial nerves, through a canal in the inner part of the skull (variously
called the internal auditory meatus [IAM], internal auditory canal [IAC] or
porus acousticus) to reach the brain stem. This part of the brain deals with
lots of automatic functions such as pulse, blood pressure, general alertness,
balance, and so on.
About half of the hearing nerves from each ear cross over to
the other side of the brain stem and then, on both sides, the nerves pass up
the brain stem through the midbrain, eventually to reach ‘consciousness’ in
what is called the cortex of the brain. For hearing, this conscious region is
located in the temporal lobe portion of the brain, which lies on each side of
the head just above the ear.
Diagram of ear
The Concept of Echo
and Reverberation
Echo
An echo occurs when a sound wave is reflected and hence
arrives to the listener after some time delay after the direct sound.
When we are in a empty space surrounded by mountains and we
shout loudly, we see that faint sound resembling the original sound coming back
after some delay of time. This perception of the reflected wave is nothing but
the echo. This is experienced in remote places, in rooms that are big and empty,
in caves, buildings
Echo is derived from the Greek word which means Sound. The
echo is produced due to hitting of the sound waves with the obstacles which
makes the sound to reflect back
Application of Echo:
Measuring distance
By knowing the speed of sound and measuring the time it
takes to hear the echo, you can calculate the distance of the object. A sonar
device sends out a sound and automatically calculates the distance of an
object. Submarines use sonar to find objects under the water, including other
submarines. The "ping" sound heard in a submarine comes from the
sonar device sending out a sound wave under water.
Fishermen also use sonar to find schools of fish. Since this
is an electronic device, the time it takes for the wave to return can be much
less than the 0.1 second required to hear an echo. For example, if the speed of
sound in water is 1500 m/s and the fisherman's sonar device detects an echo in
0.02s, the distance of the object under water will bed = v x t = 1500 m/s x
0.02s = 30 m (back and forth).
That may mean a school of fish are 15 meters away. Sonar and
radar work on the same principle. Sonar uses sound waves, while radar uses
electromagnetic waves.
Velocity
When a wave bounces off a moving object, the frequency of
the sound changes, according to the relative velocity of the object. (Velocity
is the measurement of speed and direction). If the object is moving toward you,
the frequency or pitch of the sound gets higher. When it is moving away, the
pitch gets lower. The faster the object is moving, the greater the change in
frequency or pitch. This is called the Doppler Effect.
Note: Doppler
radar works on a similar principle to measure the speed of storms in weather
prediction, except that it uses echoes from electromagnetic waves.
Bats can find moths
Bats use echoes to find good tasting moths, while flying
around at night. The bat sends a sharp click or chirping sound and then hears
and processes any echoes off other objects in the area. Bats have large ears
that are very sensitive to sounds in certain wavelengths
Reverberation
Reverberations are the collection of reflected sounds from
the surfaces in an enclosure like an auditorium. It is a desirable property of
auditoriums to the extent that it helps to overcome the inverse square law drop
off of sound intensity in the enclosure.
However, if it is excessive, it makes the sounds run
together with loss of articulation - the sound becomes muddy, garbled.
A reverberation is the same as echo but the distance here is
less. The distance between the source of the sound and the obstacle by which it
is reflected is less in Reverberation. Here, in reverberation the time delay is
less than 0.1 second.
The reflected wave reaches the observer in less than 0.1
second. Now as the delay in time is less than the original sound is still in
memory, the delay between perception of sound and the original sound is
very-very less.
SAMPLE QUESTIONS:
·
How is an Echo Different from the reverberation?
Echo is when long distances are considered and reverberation
is when short distances are considered. Echo is due to the reflection of sound
wave by obstacles or end points like wall etc. But Reverberation is due to the
collection of reflection sounds from the surface which is enclosed completely.
The Speed of Sound in Air
Determine the speed of sound in air
Sound can travel in air (gases), liquids and in solids.
Sound is a mechanical wave hence can not travel in vacuum. The speed of sound
in air is approximately 340 m/s. The speed of sound in air changes with
temperature. As the temperature increase the speed of sound also increases.
Sound travel faster in solids than in liquids and air.
Musical Sound
The Concept of a Musical Sound
·
Explain the concept of a musical sound
Music is organized sound which has some pattern. Music uses
certain frequencies or combinations of frequencies called the musical scale to
produce sounds that are generally appealing to the human ear.
Noise on the other hand, is random and without structure.
Any and all frequencies might occur and their combination is often not
appealing to the ear.
Factors Affecting Loudness, Pitch and Quality of Musical
Sound
Identify factors affecting loudness, pitch and quality of a
musical sound
The musical sounds produced by different musical instruments
have distinct properties that are used to describe them. These include
loudness, pitch and timbre:
Loudness: loudness is the intensity of the
sound which is the perceptual property. It is determined by the amplitude of
sound wave and the number of auditory nerves activated by sound wave. Amplitude
is a
physical property determined by how much air pressure in a
compression or rarefaction deviates from normal air pressure. The larger the
amplitude the louder the sound.
Pitch is an
auditory sensation in which a listener assigns musical tones to relative
positions on a musical scale based on the frequency of sound wave vibration.
Frequency is an objective, scientific concept, whereas pitch is subjective.
Sound waves themselves do not have pitch. It takes a human brain to map the
internal quality of pitch. -Pitches are usually quantified as frequencies in
cycles per second, or hertz.
Timbre is the
tone quality of sound produced by an instrument. It is referred to as sound
quality or sound colour and it is a perceptual property. What makes a
particular musical sound different from another, even when they have the same pitch
and loudness.
·
Identify the different musical instruments
Musical instrument are the device constructed or modified
for the purpose of making music. They are categorized into three categories:
Wind Instruments: This class of musical instruments requires you to
blow into a specific wind instrument by following an order to ensure that the
sound that you desire is produced. The instruments can be expected to work
depending on the principles of frequencies, sound waves, acoustics, resonance
and harmonics. The pitch of the produced sound when you start blowing the
instrument is actually dependent on the length of the air column through which
the waves of the sounds vibrate.
Percussion
Instruments: These instruments require you to strike the surface of the
instrument to generate vibrations to produce your desired note. Percussion
instruments can actually be divided into two types. The first type includes
tuned instruments that are known to produce a definite pitch or a series of
different pitches. Some examples of the tuned percussion instruments include
xylophone, vibraphone, marimba, tubular bells and timpani or kettle drum. The
second type of percussion instruments is the indefinite pitch. Its examples
include triangle, castanets, rattle, cymbals, tambourine, anvil and gong.
String Instruments: These are composed of those instruments that
work based on sound wave vibrations produced by strings. The pitch that can be
produced by these instruments is dependent on the length of air column and the
type and thickness of strings used. Among the most popular string instruments
are guitar, viola, violin, cello, mandolin, harp, double bass and banjo.
The Terms Stationary
Wave, Nodes and Antinodes
Stationary wave
Is a wave in a medium in which each point on the axis of the
wave has an associated constant amplitude
This phenomenon can occur because the medium is moving in
the opposite direction to the wave, or it can arise in a stationary medium as a
result of interference between two waves traveling in opposite directions.
The most common cause of standing waves is the phenomenon of
resonance, in which standing waves occur inside a resonator due to interference
between waves reflected back and forth at the resonator's resonant frequency.
For waves of equal amplitude traveling in opposing
directions, there is on average no net propagation of energy. Traveling waves
have high points called crests and low points called troughs (in the transverse
case) or compressed points called compressions and stretched points called
rarefactions (in the longitudinal case) that travel through the medium.
Standing waves don't go anywhere, but they do have regions
where the disturbance of the wave is quite small, almost zero. These locations
are called nodes. There are also regions where the disturbance is quite
intense, greater than anywhere else in the medium, called antinodes.
Nodes
The locations at which the amplitude is minimum are called
nodes.
Antinodes
The locations where the amplitude is maximum are called
antinodes.
The Frequency of a
Musical Note
The frequency of a musical note is affected by length of
vibrating string and the velocity of the waves. Velocity of the waves depends
on the tension on the string and the linear mass density. The linear mass
density is the mass per unit length. The frequency of a music note depends on
the length, mass per unit length and the tension.
The Difference
between the Fundamental Note and Overtones
Fundamental Note
Fundamental note is the lowest resonant frequency of a
vibrating object. Most vibrating objects have more than one resonant frequency
and those used in musical instruments typically vibrate at harmonics of the
fundamental.
A harmonic is defined as an integer (whole number),n
multiple of the fundamental frequency. Vibrating strings, open cylindrical air
columns, and conical air columns will vibrate at all harmonics of the
fundamental.
Overtone
An overtone is any frequency higher than the fundamental
frequency of a sound. Using the model of Fourier analysis, the fundamental and
the overtones together are called partials. Harmonics, or more precisely,
harmonic partials, are partials whose frequencies are integer multiples of the
fundamental (including the fundamental which is 1 times itself). These
overlapping terms are variously used when discussing the acoustic behavior of
musical instruments.
There are integer multiples of a certain frequency
(fundamental), that are called
harmonics, partial tones (partials) or
overtones. It is important to note that the term 'overtones'
does not include the fundamental frequency. The first overtone is therefore
already the second harmonic or the second partial. The term overtone should
never be mixed with the other terms, as the counting is unequal. The term
harmonic has a precise meaning - that of an integer (whole number) multiple of
the fundamental frequency of a vibrating object.
The Concept of
Resonance as Applied to Sound
Explain the concept of resonance as applied to sound
Resonance is a phenomenon that occurs when a given system is
driven by another vibrating system or external force to oscillate with greater
amplitude at a specific preferential frequency. Frequencies at which the
response amplitude is a relative maximum are known as the system's
resonant frequencies, or resonance frequencies.
At resonant frequencies, small periodic driving forces have
the ability to produce large amplitude oscillations. This is because the system
stores vibrational energy. Resonance occurs when a system is able to store and
easily transfer energy between two or more different storage modes (such as
kinetic energy and potential energy in the case of a pendulum).
However, there are some losses from cycle to cycle, called
damping. When damping is small, the resonant frequency is approximately equal
to the natural frequency of the system, which is a frequency of unforced
vibrations. Some systems have multiple, distinct, resonant frequencies.
Resonance in Closed Ended Pipes
A closed ended instrument has one end closed off, and the
other end open. An example would be an instrument like some organ pipes
(although in some designs they are open), or a flute.
Although you blow in through the mouth piece of a flute, the
opening you’re blowing into isn’t at the end of the pipe, it’s along the side
of the flute. The end of the pipe is closed off near the mouth piece. Remember
that it is actually air that is doing the vibrating as a wave here.
·
The air at the closed end of the pipe must be a
node (not moving), since the air is not free to move there and must be able to
be reflected back.
·
There must also be an antinode where the opening
is, since that is where there is maximum movement of the air.
·
Notice how even though it has been flipped
left-to-right and it looks squished and stretched a bit to fit, this is still ¼
of a wavelength.
Since this is the smallest stable piece of
a wave I can fit in this pipe, this is the Fundamental, or 1st Harmonic.
·
Since the length of the tube is the same as the
length of the ¼ wavelength I know that the length of this tube is ¼ of a
wavelength… this leads to our first formula: L = ¼ λ
·
“L” is the length of the tube in meters. On it’s
own this formula really doesn’t help us much.
Instead, we have to solve this formula for λ
and then combine it with the formula v=fλ to get a more useful formula:
When the wave reaches the closed end it’s going to be
reflected as an inverted wave (going from air to whatever the pipe is made of
is a pretty big change so this is what we would expect). It would look like
Figure above.
This does not change the length of the wave in our formula,
since we are only seeing the reflection of the wave that already exists in the
pipe.
What does the next
harmonic look like? It’s the 3rd Harmonic.
NB:
the harmonics in closed end pipes are going to be odd numbers.
Remember that we have to have an antinode at the opening
(where the air is moving) and a node at the closed end (where the air can’t
move). That means for the 3rd harmonic we get something like Figure above. This
is ¾ of a wavelength fit into the tube, so the length of the tube is… L = ¾ λ
This is the third harmonic of the closed end pipe. The
formula for the frequency of the note we will hear is…
Do you notice a pattern forming in the formulas? Hopefully,
because for both open and closed end pipes, we will only give you the formulas
for the fundamentals lengths. You need to remember how to get the rest.
If we drew in the reflection of the third harmonic it would
look like Figure below.
One more to make sure you see the pattern. The 5th Harmonic
(fig. above)
There is one full wavelength in there (4/4) plus an extra ¼
of a wavelength for a total of 5/4. The length of the pipe is… L = 5/4 λ and
the note produced by the 5th Harmonic is found using the formula… f = 5v/4L
Figure below shows the reflection of a 5th Harmonic for a
closed end pipe.
Open End Pipes
An open ended instrument has both ends open to the air. An
example would be an instrument like a trumpet. You blow in through one end and
the sound comes out the other end of the pipe.
Fundamental
The fundamental
(first harmonic) for an open end pipe needs to be an antinode at both ends,
since the air can move at both ends.
That’s why the smallest wave we can fit in is shown in
Figure 11.
This looks different
than the ½ wavelength that I showed you in.
That means the length of the tube and frequency formulas
are…L = ½ λ
f = v/2L
2nd Harmonic
The next note we can play is the 2nd harmonic.
·
Yep, open end pipes have a 2nd harmonic… they
can have any number harmonic they want, odd or even.
·
Again, it kind of looks weird, but trace it out
and you’ll see that there is exactly one wavelength here.
The length and frequency formulas are…L = 2/2
λ, f= 2v/2L
I’m not going to show you what the 3rd harmonic looks like.
Instead, try drawing it yourself and see what you get.
As a hint to help you, the formulas for the length and
frequency are…L = 3/2 λ, f = 3v/2L
A Simple Musical
Instrument
Construct a simple musical instrument.
A musical instrument is a device constructed for making
music. There are three categories of musical instruments: string, percussion
and wind instruments. The note of a musical instrument depends on five
parameters: pitch, frequency, intensity, loudness and quality of a music note.
Pitch of a note is its position in the musical scale and depends on the frequency
of the vibration of the wave.
Construct a simple musical instrument
Electromagnetic
Spectrum
The Concept of ElectroMagnetic Spectrum
Explain the concept of the electro Magnetic spectrum
The electromagnetic spectrum is the range of all possible
frequencies of electromagnetic radiation. The "electromagnetic
spectrum" of an object has a different meaning, and is instead the
characteristic distribution of electromagnetic radiation emitted or absorbed by
that particular object
The electromagnetic spectrum extends from below the low
frequencies used for modern radio communication to gamma radiation at the
short-wavelength (high-frequency) end, thereby covering wavelengths from
thousands of kilometers down to a fraction of the size of an atom. The limit for
long wavelengths is the size of the universe itself, while it is thought that
the short wavelength limit is in the vicinity of the Planck length.
Properties of
electromagnetic spectrum
·
It is continuous i.e each band merges into the
next and there are no gaps in the frequencies. The different kinds of radiation
gradually change from one to another as their properties also gradually change.
·
In some case, there is an overlap in the range
of wavelength. This is because sometimes the name given to the wave(radiation)
is determined by the source and not the wavelength (or frequency) for example
x-rays and γ -rays.
The Main Bands of the Electro Magnetic Spectrum
Identify the main bands of the electro Magnetic spectrum
Radio waves
Radio waves are a type of electromagnetic radiation with the
longest wavelengths in the electromagnetic spectrum. Radio waves have
frequencies from 300GHz to as low as 3kHz, and corresponding wavelengths
ranging from 1 millimeter (0.039in) to 100 kilometers (62mi).
Like all other electromagnetic waves, they travel at the
speed of light. Naturally occurring radio waves are made by lightning, or by
astronomical objects. Artificially generated radio waves are used for fixed and
mobile radio communication, broadcasting, radar and other navigation systems,
communications satellites, computer networks and innumerable other
applications.
Radio waves are generated by radio transmitters and received
by radio receivers.
Sources of radio
waves
·
alternating electric currents flowing in
antennae
·
oscillators
·
planets, comets, stars and galaxies
Microwaves
Microwaves have a short wavelength of between 10⁻⁴m to about
0.1.
Sources
·
Are produced by oscillation of charges in
special antennae mounted on dishes.
·
Magnetrons.
Infrared waves.
They have the frequency 0f between 10 and 10 Hz. They lie
between the visible light and microwaves. They have the heating effect.
Sources: Vibration of atoms and molecules due to their
thermal energy. All hot bodies emit infrared radiations.
Visible light.
Is the narrow range of EW frequencies to which human eyes
are sensitive.
Detection of Infra-red, Visible and Ultra-Violet Rays
Detect infra-red, visible and ultra-violet rays
Infrared waves
Infrared radiation is invisible to the human eye. However,
any nocturnal animal can “see” infrared radiations. We can sense infrared
radiations as heat. Devices used to detect infrared radiations include black
bulb thermometers, photographic films, thermistors and phototransistors.
Visible light
We see things because they either emit visible light or
because they reflect visible light from another source.
Ultraviolet light.
Ultraviolet is light is detected by Photographic films;
Fluorescent materials
Application of
ElectroMagnetic Wave in daily life
The Application of Micro Waves, Radio-Waves, Gamma Rays and
X-rays
Identify the application of micro Waves, Radio-Waves,
Infra-red, gamma rays and x-rays
Radio waves
. Fixed and mobile radio communication, broadcasting, radar
and other navigation systems, communications satellites, computer networks and
innumerable other applications.
. Astronomers use large radio telescopes to collect and
study radio waves from distant stars and galaxies. This helps them to determine
composition, structure and motion of the celestial bodies.
Microwaves
. In cooking
. Radar systems
Long distance
communication
Gamma rays
. Medical application the same as x-rays
. In agriculture
X-rays
Definition: X –rays is
an elemagnetic wave of high energy and very strong wave length which is able to
pass through many materials opaque to light. Hence, this is termed as among of the form of
radiations which pass through most solid materials. And Most X-rays have
approximately wavelength of ranging from 0.01 to 10 Nanometers, corresponding
to frequencies in range 30 petahertz to 30 exahertz (3Ñ…1016 Hz to 3Ñ…1019 Hz) and
energies in range 100 Ev to 100keV.
Figure: X- ray
. X-ray photography
. Diagnosis and treatment of cancer
The Importance of ElectroMagnetic Waves in Agriculture and
Climate
Explain the importance of electromagnetic waves in agriculture
and climate
Electromagnetic radiations are very useful in agriculture
and climate as they support photosynthesis, the main source of oxygen in the
atmosphere. Gamma rays are used for pest control by sterilizing male pests with
gamma radiation then releasing them into the wild, as is done to control tsetse
flies. Gamma rays are used to obtain disease-resistant
TOPIC
2: ELECTROMAGNETISM
Electromagnetism is
the effect produced by the interaction of an electric current with a magnetic
field. The interaction can result in a force causing the conductor carrying the
current.
If, on the other hand, a force is applied to a conductor
(with no current) in a magnetic field the resulting movement can result in a
current being noticed in the conductor.
When the switch is closed an electric current flows through
the conductor. The electric current generates magnetic field around the
conductor. This will cause a deflection on the compass needle. The magnetic
field around a current-carrying conductor can be shown by means of magnetic
field lines.
The Pattern of the Magnetic Field Lines around a Straight
Conductor
The magnetic field
pattern is usually given in a plan view. In the plan view, the conductor is
represented by a circle. A dot in circle shows that the current is coming out
of the plane. A cross the circle shows that the current is moving into the
plane.
The strength of the magnetic field on the magnitude of the
electric current. The higher the current, the stronger the magnetic field, and
therefore the greater the deflection. The strength of the magnetic field
decreases as you move further from the conductor. There will be less deflection
as the compass is drawn from the current-carrying conductor.
The Direction of
Magnetic Field around a Current-Carrying Conductor
Determine the direction of magnetic field around a
current-carrying conductor
The direction of the field is determined by applying two
rules, these are:
. Right-hand Grip Rule
. Maxwell’s cork screw rule
Right-hand Grip Rule
The Right-hand Grip Rule can be applied to a straight
conductor or a solenoid-carrying an electric current. For a straight conductor,
the Right-hand Grip Rule can be stated as:
“Imagine the wire carrying the current is gripped by the
right hand with the thumb pointing in the direction of the conventional current
(from positive to negative), the fingers will curl around the wire pointing in
the direction of the magnetic field.”
Figure: Right hand grip rule
NB:
For a solenoid, the Right-hand Grip Rule states that:
“When you wrap your right hand around a solenoid with your
fingers pointing in the direction of convectional current, your thumb point in
the direction of the magnetic North pole.”
A solenoid is a long coil containing a large number of close
turns of illustrated copper wire.
Maxwell’s –Right –hand screw rule states that:
“If a right-hand screw advances in the direction of the
current, then the direction of rotation of the screw represents the direction
of the magnetic field due to the current.”
The Presence and Direction of a Force on a Current carrying
Conductor in a Magnetic Field
Determine the presence and direction of a force on a current
carrying Conductor in a magnetic field
The direction of the force on a current-carrying conductor
in a magnetic field can be determined using Fleming’s Left –Hand Rule.
Fleming’s Left –Hand Rule states that:
“If you hold the index finger, the middle finger and the
thumb of your left hand mutually perpendicular to each other so that the index
finger points in the direction of the magnetic field and the middle finger
points in the direction of current in the conductor, then the thumb will point
in the direction of the force acting on the conductor.”
The Direction of Force due to two Current carrying
Conductors when the Current Flowing in the Same or Opposite Direction
Determine the direction of force due to two current
'82air-carrying conductors when the current flowing in the same or opposite
direction
If two current-carrying conductors are placed side by side
close to one another, the currents in the conductors will interact with the
magnetic fields produced by the two conductors. A force may result depending on
the direction of the two currents.
When the currents are flowing in opposite directions, the
conductors repel one another. When the currents are flowing in the same
direction, the conductors attract one another, the conductors attract each
other.
When the currents flow in the same direction, the magnetic
field between the conductors cancel out, thus reducing the net field. However,
on the outside, the magnetic fields add up, thus increasing the net field.
Therefore, the magnetic field is weaker between the conductors that on the
outside. The resultant force pushes the conductor towards each other.
When the currents are in the opposite directions, the fields
between the conductors add up, while they cancel out on the outside. The field
between them is stronger than on the outside. The resultant force is toward the
outside of each conductor, hence repulsion.
ELECTROMAGNETIC
INDUCTION
The Concept of Electromagnetic Induction
Electromagnetic Induction is the production of e. m. f
whenever there is a change in the magnetic flux linking a conductor”. The e. m.
f produced is called induced e. m. f and the resulting current induced current.
The Laws of
ElectroMagnetic Induction
State the laws of electromagnetic induction
Lenz’s Law
This explains the direction of the induced e. m. f and it
states that: “The direction of the induced e. m. f is such that the resulting
induced current flows in such a direction that it opposes the change that
causes it.”
Faraday’s Law.
It relates the magnitude of induced e. m. f and the rate of
change of the magnetic flux linking the conductor. The magnitude of the induced
e. m. f depends on:
·
the area of the conductor that is in the
magnetic field.
·
the strength of the magnetic field.
·
the rate of change of the magnetic flux (speed
of motion)
·
Faraday’s law states that: “The e. m. f induced
in the conductor in a magnetic field is proportional to the rate of change of
magnetic flux linking the conductor.”
THE CONCEPTS OF SELF
AND MUTUAL INDUCTION
Explain the concepts of self and mutual induction
When the current flowing through a conductor varies it
creates a varying magnetic field that cuts across the conductor itself.
This results to self-induced e. m. f in the conductor that
is opposite in direction to the original e. m. f. This voltage, usually
referred to as back e. m. f, tends to limit or reverse the original current.
If the original current is increasing, then the induced
current subtracts from it and then measured current is smaller than it would be
if no self-induced magnetic field was produced in the conductor. If original
current is decreasing, then the original current adds to it and the measured
current is greater than it would be if self-induced magnetic field was produced
in the conductor. This process is called self-induction since the changing
current creates a back e. m. f in itself.
Consider a coil of wire wrapped around a cardboard tube.
When the coil is connected to a battery a current flow in the coil producing a
magnetic field as shown below.
If the current in the coil begins to increase, the magnetic
flux increases. This induces an e. m. f that opposes the battery resulting in a
back current that impedes the increase in current. If the current in the coil
decreases, the magnetic flux decreases. This induces an e. m. f that adds to
the battery resulting in an induced current that impedes the decrease in
current.
If we place two coils near each other, a varying current in
one coil will induce a current in the other. This is called mutual induction.
The coil with a changing current is referred to as the primary coil while that
in which a current is induced is the secondary coil.
The a.m. induced in the secondary coil is proportional to
the rate of change of the current in the primary coil.
The Mode of Action of
Induction Coil
Describe the mode of
action of induction coil
Is an electrical device consisting of two coils, the primary
coil and the secondary coil, wound one over the other on an iron core.
It is used to produce high-voltage alternating current from
low-voltage direct current. The primary coil is made up of tens or hundreds of
turns of coarse wire while the secondary coil consists of thousands of turns of
fine wire. The secondary coil is wound on top of the primary coil.
Mode of action
An induction coil produces high voltage in its secondary
coil by electromagnetic induction. The direct current in the primary is
switched on and off by a make-and-break mechanism. This produces change in
current and magnetic field which is necessary for electromagnetic induction to
occur in the secondary coil.
When the current in the primary coil is switched on, the
induced magnetism in the iron core attracts the soft-iron armature. The moving
iron armature opens a gap between the two contacts which breaks the primary
coil circuit. This switches off the current. As the induced magnetism fades
away, the armature springs back, closes the contacts and completes the circuit
again. This allows the current to flow in the primary coil again. This cycle of
events is repeated automatically.
The induced a.m. is very large, usually in the order of
hundreds of kilovolts (kV). Such a high voltage is achieved because of two
things:
·
The secondary coil has a large number of turns
compared to the primary coil.
·
The rapid change in the primary current when it
is switched on and off causes a rapid in the magnetic field through the
secondary coil.
Applications of the
induction coil
·
It is used in the ignition system of internal
combustion engines.
·
It is used to trigger the flash tubes used in
cameras and strobe lights.
·
It is also used in wireless telegraphy.
The Mode of Action of
a.c and d.c Generator
A generator is
the device which produces electricity on the basis of electromagnetic induction
by the continuous motion of either a coil or a magnet.
A.C Generator or
alternator
An arc generator utilizes Faraday’s law of induction,
spinning a coil at a constant rate in a magnetic field to induce an oscillating
a.m.
The arc generator consists of an armature made up of several
turns of insulated wire wound on a soft-iron core. The armature revolves freely
on an axis between the poles of a powerful magnet, which provides a strong
magnetic field. Two slip rings are connected to the ends of the armature and
two carbon brushes rest on the slip rings.
When the coil is vertical, no cutting of the magnetic lines
of force takes place although the number of lines linking the coil is maximum.
The rate of change magnetic flux is zero and as a result, no a.m. is induced in
the coil.
When the armature is parallel to the magnetic field, the
rate of change of magnetic flux is maximum and the motion of the coil is
perpendicular to the magnetic field, hence an a.m. is induced along the sides
of the coil.
After a 180° turn, starting from the vertical position, the
sides of the loop interchange and the current in the loop is reversed. This
means that the a.m. is positive for one half of the cycle and negative for the
half. The maximum induced a.m. is at 90° rotation from the vertical position
and the minimum is at 270° rotation. If there is an external circuit, the
current through it would also have a maximum value at 90° and minimum at 270°.
This kind of current is called an alternating current and
the corresponding a.m. is the alternating e.m.f.The number of cycles produced
per second is called the
frequency of the arc. The arc obtained is led to an external
circuit through the slip rings and the carbon brushes.
D.C generator
It is made by replacing the slip rings in the arc generator
with a commentator. Each half of the commentator ring is called a commentator
segment and is insulated from the other half. Each end of the rotating loop of
the wires connected to a commentator segment. Two carbon brushes connected to
the outside circuit rest against the rotating commentator.
In the deck generator, the commentator rotates with the loop
of wire, just as the slip rings do with the rotor of an arc generator.
When the loop is rotated in the magnetic field, the induced
e.m.f is still in alternating form. However after rotation of 180° instead of
the current reversing, the connections to the external circuit are reversed so
that the current direction in the external circuit remains the same.
The output of a d.c generator is shown below.
Note: The lower half of the cycle is not cut off but is
reversed.
Simple Step-up and Step-down Transformer
Construct a simple step-up and step-down transformer
A transformer is the device that uses mutual induction
between two coils to convert an a.c across one coil to a larger or smaller a.c
across the other coil.
A transformer is made up of two coils, each with a different
number of loops linked by an iron core so that the magnetic flux from one
passes through the other. When the flux generated by one coil changes the flux
passing through the other will change, inducing a voltage in the second coil.
The coil that provides the flux that is the coil connected
to the a.c power source is known as the primary coil while the coil in which
the voltage is induced is known as the
secondary coil.
When the number of turns in the primary coil (N) is lower
than the number in the secondary coil (N),the secondary voltage will be lower
than the secondary voltage. This is called the step-down transformer. The
opposite of this is called the step-up transformer.
TOPIC 3:
RADIOACTIVITY
The Nucleus of an
Atom
The Structure of the Nucleus of an Atom
Describe the structure of the nucleus of an atom
The word atom is derived from the Greek word atom which
means indivisible. The Greeks concluded that matter could be broken down into
particles to small to be seen. These particles were called atoms.
Atoms are composed of three type of particles: protons,
neutrons, and electron. Protons and neutrons are responsible for most of the
atomic mass e.g in a 150 person 149 lbs, 15 oz are protons and neutrons while
only 1 oz. is electrons. The mass of an electron is very small (9.108 X 10-28
grams).
Both the protons and neutrons reside in the nucleus. Protons
have a positive (+) charge, neutrons have no charge i.e they are neutral.
Electrons reside in orbitals around the nucleus. They have a negative charge
(-).
It is the number of protons that determines the atomic
number, e.g., H = 1. The number of protons in an element is constant (e.g.,
H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons)
may vary.
The same element may contain varying numbers of neutrons;
these forms of an element are called isotopes . The chemical properties of
isotopes are the same, although the physical properties of some isotopes may be
different.
Some isotopes are radioactive-meaning they
"radiate" energy as they decay to a more stable form, perhaps another
element half-life: time required for half of the atoms of an element to decay
into stable form. Another example is oxygen, with atomic number of 8 can have
8, 9, or 10 neutrons.
The Atomic Number, Mass Number and Isotopes of an Element
and their Symbols
Explain the atomic number, mass number and isotopes of an
element and their symbols
The atomic number of a chemical element (also known as its
proton number) is the number of protons found in the nucleus of an atom of that
element. Therefore it is identical to the charge number of the nucleus. It is
conventionally represented by the symbol Z.
The atomic number uniquely identifies a chemical element. In
an uncharged atom, the atomic number is also equal to the number of electrons.
The atomic number, Z, should not be confused with the mass
number, A.
Mass number is the number of nucleons, i. e the total number
of protons and neutrons in the nucleus of an atom. ---The number of neutrons,
N, is known as the neutron number of the atom; thus, A = Z + N (these
quantities are always whole numbers).
Since protons and neutrons have approximately the same mass
(and the mass of the electrons is negligible for many purposes) and the mass
defect of nucleon binding is always small compared to the nucleon mass, the
atomic mass of any atom, when expressed in unified atomic mass units (making a
quantity called the "relative isotopic mass"), is roughly (to within
1%) equal to the whole number A.
Isotopes
Isotopes are atoms with the same atomic number Z but
different neutron numbers N, and hence different atomic masses.
A little more than three-quarters of naturally occurring
elements exist as a mixture of isotopes (see monoisotopic elements), and the
average isotopic mass of an isotopic mixture for an element (called the
relative atomic mass) in a defined environment on Earth, determines the
element's standard atomic weight.
Historically, it was these atomic weights of elements (in
comparison to hydrogen) that were the quantities measurable by chemists in the
19th century. The chemical properties of isotopes are the same, although the
physical properties of some isotopes may be different.
Some isotopes are radioactive-meaning they
"radiate" energy as they decay to a more stable form, perhaps another
element half-life: time required for half of the atoms of an element to decay
into stable form. Another example is oxygen, with atomic number of 8 can have
8, 9, or 10 neutrons.
Forces Holding the
Nucleus
Mention forces holding the nucleus
Stable and unstable
atoms
There are forces within the atom that account for the
behavior of the protons, neutrons, and electrons. Without these forces, an atom
could not stay together.
Recall that protons have a positive charge, electrons a
negative charge, and neutrons are neutral. According to the laws of physics,
like charges repel each other and unlike charges attract each other. A force
called the strong force opposes and overcomes the force of repulsion between
the protons and holds the nucleus together.
The net energy associated with the balance of the strong
force and the force of repulsion is called the binding energy. The electrons
are kept in orbit around the nucleus because there is an electromagnetic field
of attraction between the positive charge of the protons and the negative charge
of the electrons.
In some atoms, the binding energy is great enough to hold
the nucleus together. The nucleus of this kind of atom is said to be stable. In
some atoms the binding energy is not strong enough to hold the nucleus
together, and the nuclei of these atoms are said to be unstable. Unstable atoms
will lose neutrons and protons as they attempt to become stable.
·
Binding energy is the net energy that is the
result of the balance with the strong force and the repulsive force, and this
is the amount of energy that holds the nucleus together.
·
A stable atom is an atom that has enough binding
energy to hold the nucleus together permanently.
·
An unstable atom does not have enough binding
energy to hold the nucleus together permanently and is called a radioactive
atom.
Natural Radioactivity
The Concept of
Radioactivity
Radioactive decay, also known as nuclear decay or
radioactivity, is the process by which a nucleus of an unstable atom loses
energy by emitting ionizing radiation.
A material that spontaneously emits such radiation — which
includes alpha particles, beta particles, gamma rays and conversion electrons —
is considered radioactive.
Radioactive decay is a stochastic (i.e. random) process at
the level of single atoms, in that, according to quantum theory, it is
impossible to predict when a particular atom will decay.
The chance that a given atom will decay never changes, that
is, it does not matter how long the atom has existed. For a large collection of
atoms however, the decay rate for that collection can be calculated from their
measured decay constants or half-lives. The half-lives of radioactive atoms
have no known limits for shortness or length of duration, and range over 55
orders of magnitude in time.
Properties of the
Radiations Emitted by Radio-active Substances
Describe properties of the radiations emitted by
radio-active substances
There are many types of radioactive decay . A decay, or loss
of energy, results when an atom with one type of nucleus, called the parent
radionuclide (or parent radioisotope), transforms into an atom with anucleus in
a different state, or with a nucleus containing a different number of protons
and neutrons. The product is called the daughter nuclide. In some decays, the
parent and the daughter nuclides are different chemical elements, and thus the
decay process results in the creation of an atom of a different element. This
is known as a nuclear transmutation.
The Nuclear Changes due to the Emission of Alpha ('8c'b1), Beta
('8cuc0u8804 ) and Gamma ('8cu8805 ) Radiations
Explain the nuclear changes due to the emission of Alpha
('8c'b1), Beta ('8cuc0u8804 ) and Gamma ('8cu8805 ) radiations
Properties of Alpha
Rays
Alpha rays or alpha
particles are the
·
positively charged particles.
·
Alpha particles have the least penetration
power. They cannot penetrate the skin but this does not mean that they are not
dangerous.
·
Since they have a great ionisation power, so if
they get into the body they can cause serious damage. They have the ability of
ionising numerous atoms a short distance. It is due to this reason that the
radioactive substance that releases alpha particles needs to be handled with
rubber gloves. It should not be inhaled, eaten or allowed near open cuts.
Properties of Beta
Rays.
·
Beta particles are highly energetic electrons
which are released from inside of a nucleus.
·
They are negatively charged and have a
negligible mass.
·
Beta particles have a greater penetration power
than the alpha particles and can easily travel through the skin.
·
Though beta particles have less ionisation power
than the alpha particles but still they are dangerous and so their contact with
the body must be avoided.
Properties of Gamma
Rays
·
They have greatest power of penetration.
·
They are the least ionizing but most penetrating
and it is extremely difficult to stop them from entering the body.
·
These rays carry huge amount of energy and can
even travel through thin lead and thick concrete.
The Detection of '8c'b1, '8cuc0u8804 and '8cu8805 Radiations
Explain the detection of '8c'b1, '8cuc0u8804 and '8cu8805
radiations
Geiger Counter, with Geiger-Mueller (GM) Tube or Probe
A GM tube is a gas-filled device that, when a high voltage
is applied, creates an electrical pulse when radiation interacts with the wall
or gas in the tube. These pulses are converted to a reading on the instrument
meter.
If the instrument has a speaker, the pulses also give an
audible click. Common readout units are roentgens per hour (R/ hr),
milliroentgens per hour (mR/hr), rem per hour (rem/hr), millirem per hour
(mrem/hr), and counts per minute (cpm).
GM probes (e.g., "pancake" type) are most often
used with handheld radiation survey instruments for contamination measurements.
However, energy-compensated GM tubes may be employed for exposure measurements.
Further, often the meters used with a GM probe will also
accommodate other radiation-detection probes. For example, a zinc sulfide (ZnS)
scintillator probe, which is sensitive to just alpha radiation, is often used
for field measurements where alpha-emitting radioactive materials need to be
measured.
Spark counter
This consists of a fine metal gauze mounted about a
millimetre away from a thin wire.A voltage is applied between the two so that
sparking takes place between them - this usually requires some 4000 - 5000 V.
The voltage is then reduced until sparking just stops.
If an alpha-source is brought up close to the gauze it will
ionise the air, and sparks will occur between the gauze and wire. With beta and
gamma sources insufficient ions are usually produced for sparking to take
place.The spark counter can be used to measure the range of alpha-particles.
Cloud chamber
The cloud chamber, also known as the Wilson chamber, is a
particle detector used for detecting ionising radiation.
Rare picture shows in a single shot the 4 particles that we
can detect in a cloud chamber: proton, electron, muon (probably) and alpha. In
its most basic form, a cloud chamber is a sealed environment containing a
supersaturated vapor of water or alcohol.
When a charged particle (for example, an alpha or beta
particle) interacts with the mixture, the fluid is ionized. The resulting ions
act as condensation nuclei, around which a mist will form (because the mixture
is on the point of condensation).
The high energies of alpha and beta particles mean that a
trail is left, due to many ions being produced along the path of the charged
particle. These tracks have distinctive shapes (for example, an alpha
particle's track is broad and shows more evidence of deflection by collisions,
while an electron's is thinner and straight).
When any uniform magnetic field is applied across the cloud
chamber, positively and negatively charged particles will curve in opposite
directions, according to the Lorentz force law with two particles of opposite
charge.
Other devices used to detect radiation include:
·
Photographic film
·
Bubble chamber
·
Gold-leaf
electroscope
Half-Life as Applied to a Radioactive Substance
Describe half-life as applied to a radioactive substance
Half life can be defined as the time taken for the number of
nuclei in a radioactive material to halve. It can also be defined as the time
taken for the count rate of a sample of radioactive material to fall to half of
its starting level.
The count rate is measured by using an instrument called a
Geiger-Muller tube over a period of time. A Geiger-Muller tube detects
radiations by absorbing the radiation and converting it into an electrical
pulse which triggers a counter and is displayed as a count rate.
The release of radiation by unstable nuclei is called
radioactive decay. This process occurs naturally and cannot be influenced by
chemical or physical processes.
The release of radiation is also a random event and overtime
the activity of the radioactive material decreases. It is not possible to
predict when an individual nucleus in a radioactive material will decay.
But it is possible to measure the time taken for half of the
nuclei in a radioactive material to decay. This is called the half life of
radioactive material or radioisotope.
The Half-Life of a
Radioactive Element
Determine the half-life of a radioactive element
An exponential decay process can be described by any of the
following three equivalent formulas:
where
·
N0 is the initial quantity of the substance that
will decay (this quantity may be measured in grams, moles, number of atoms,
etc).
·
N( t) is the quantity that still remains and has
not yet decayed after a time t.
·
t1⁄2 is the half-life of the decaying quantity.
·
Ï„is a positive number called the mean lifetime
of the decaying quantity.
·
λis a positive number called the decay constant
of the decaying quantity.
Where ln (2) is the natural logarithm of 2 (approximately
0.693).
By plugging in and manipulating these relationships, we get
all of the following equivalent descriptions of exponential decay, in terms of
the half-life:
The Application of a Natural Radioactive Substances
Identify the applications of a natural radioactive
Substances
Medical Uses
Hospitals, doctors, and dentists use a variety of nuclear
materials and procedures to diagnose, monitor, and treat a wide assortment of
metabolic processes and medical conditions in humans. In fact, diagnostic
x-rays or radiation therapy have been administered to about 7 out of every 10
Americans. As a result, medical procedures using radiation have saved thousands
of lives through the detection and treatment of conditions ranging from
hyperthyroidism to bone cancer.
The most common of these medical procedures involves the use
of x-rays — a type of radiation that can pass through our skin. When x-rayed,
our bones and other structures cast shadows because they are denser than our
skin, and those shadows can be detected on photographic film. The effect is
similar to placing a pencil behind a piece of paper and holding the pencil and
paper in front of a light. The shadow of the pencil is revealed because most
light has enough energy to pass through the paper, but the denser pencil stops
all the light. The difference is that x-rays are invisible, so we need
photographic film to "see" them for us. This allows doctors and
dentists to spot broken bones and dental problems.
X-rays and other forms of radiation also have a variety of therapeutic
uses. When used in this way, they are most often intended to kill cancerous
tissue, reduce the size of a tumor, or reduce pain. For example, radioactive
iodine (specifically iodine-131) is frequently used to treat thyroid cancer, a
disease that strikes about 11,000 Americans every year.
X-ray machines have also been connected to computers in
machines called computerized axial tomography (CAT) or computed tomography (CT)
scanners. These instruments provide doctors with color images that show the shapes
and details of internal organs. This helps physicians locate and identify
tumors, size anomalies, or other physiological or functional organ problems.
In addition, hospitals and radiology centers perform
approximately 10 million nuclear medicine procedures in the United States each
year. In such procedures, doctors administer slightly radioactive substances to
patients, which are attracted to certain internal organs such as the pancreas,
kidney, thyroid, liver, or brain, to diagnose clinical conditions.
Academic and
Scientific Applications
Universities, colleges, high schools, and other academic and
scientific institutions use nuclear materials in course work, laboratory
demonstrations, experimental research, and a variety of health physics
applications. For example, just as doctors can label substances inside people's
bodies, scientists can label substances that pass through plants, animals, or
our world. This allows researchers to study such things as the paths that
different types of air and water pollution take through the environment.
Similarly, radiation has helped us learn more about the types of soil that
different plants need to grow, the sizes of newly discovered oil fields, and
the tracks of ocean currents.
In addition, researchers use low-energy radioactive sources
in gas chromatography to identify the components of petroleum products, smog
and cigarette smoke, and even complex proteins and enzymes used in medical
research.
Archaeologists also use radioactive substances to determine
the ages of fossils and other objects through a process called carbon dating.
For example, in the upper levels of our atmosphere, cosmic rays strike nitrogen
atoms and form a naturally radioactive isotope called carbon-14. Carbon is
found in all living things, and a small percentage of this is carbon-14. When a
plant or animal dies, it no longer takes in new carbon and the carbon-14 that
it accumulated throughout its life begins the process of radioactive decay. As
a result, after a few years, an old object has a lower percent of radioactivity
than a newer object. By measuring this difference, archaeologists are able to
determine the object's approximate age.
Industrial Uses
We could talk all day about the many and varied uses of
radiation in industry and not complete the list, but a few examples illustrate
the point. In irradiation, for instance, foods, medical equipment, and other
substances are exposed to certain types of radiation (such as x-rays) to kill
germs without harming the substance that is being disinfected — and without
making it radioactive. When treated in this manner, foods take much longer to
spoil, and medical equipment (such as bandages, hypodermic syringes, and
surgical instruments) are sterilized without being exposed to toxic chemicals
or extreme heat. As a result, where we now use chlorine — a chemical that is
toxic and difficult-to-handle — we may someday use radiation to disinfect our
drinking water and kill the germs in our sewage. In fact, ultraviolet light (a
form of radiation) is already used to disinfect drinking water in some homes.
Similarly, radiation is used to help remove toxic
pollutants, such as exhaust gases from coal-fired power stations and industry.
For example, electron beam radiation can remove dangerous sulphur dioxides and
nitrogen oxides from our environment. Closer to home, many of the fabrics used
to make our clothing have been irradiated (treated with radiation) before being
exposed to a soil-releasing or wrinkle-resistant chemical. This treatment makes
the chemicals bind to the fabric, to keep our clothing fresh and wrinkle-free
all day, yet our clothing does not become radioactive. Similarly, nonstick
cookware is treated with gamma rays to keep food from sticking to the metal
surface.
The agricultural industry makes use of radiation to improve
food production and packaging. Plant seeds, for example, have been exposed to
radiation to bring about new and better types of plants. Besides making plants
stronger, radiation can be used to control insect populations, thereby decreasing
the use of dangerous pesticides. Radioactive material is also used in gauges
that measure the thickness of eggshells to screen out thin, breakable eggs
before they are packaged in egg cartons. In addition, many of our foods are
packaged in polyethylene shrink-wrap that has been irradiated so that it can be
heated above its usual melting point and wrapped around the foods to provide an
airtight protective covering.
All around us, we see reflective signs that have been
treated with radioactive tritium and phosphorescent paint. Ionizing smoke
detectors, using a tiny bit of americium-241, keep watch while we sleep. Gauges
containing radioisotopes measure the amount of air whipped into our ice cream,
while others prevent spillover as our soda bottles are carefully filled at the
factory.
Engineers also use gauges containing radioactive substances
to measure the thickness of paper products, fluid levels in oil and chemical
tanks, and the moisture and density of soils and material at construction
sites. They also use an x-ray process, called radiography, to find otherwise
imperceptible defects in metallic castings and welds. Radiography is also used
to check the flow of oil in sealed engines and the rate and way that various
materials wear out. Well-logging devices use a radioactive source and detection
equipment to identify and record formations deep within a bore hole (or well)
for oil, gas, mineral, groundwater, or geological exploration. Radioactive
materials also power our dreams of outer space, as they fuel our spacecraft and
supply electricity to satellites that are sent on missions to the outermost
regions of our solar system.
Nuclear Power Plants
Electricity produced by nuclear fission — splitting the atom
— is one of the greatest uses of radiation. As our country becomes a nation of
electricity users, we need a reliable, abundant, clean, and affordable source
of electricity. We depend on it to give us light, to help us groom and feed
ourselves, to keep our homes and businesses running, and to power the many
machines we use. As a result, we use about one-third of our energy resources to
produce electricity.
Electricity can be produced in many ways — using generators
powered by the sun, wind, water, coal, oil, gas, or nuclear fission. In
America, nuclear power plants are the second largest source of electricity
(after coal-fired plants) — producing approximately 21 percent of our Nation's
electricity.
The purpose of a nuclear power plant is to boil water to
produce steam to power a generator to produce electricity . While nuclear power
plants have many similarities to other types of plants that generate
electricity, there are some significant differences. With the exception of
solar, wind, and hydroelectric plants, power plants (including those that use
nuclear fission) boil water to produce steam that spins the propeller-like
blades of a turbine that turns the shaft of a generator. Inside the generator,
coils of wire and magnetic fields interact to create electricity. In these
plants, the energy needed to boil water into steam is produced either by
burning coal, oil, or gas (fossil fuels) in a furnace, or by splitting atoms of
uranium in a nuclear power plant. Nothing is burned or exploded in a nuclear
power plant. Rather, the uranium fuel generates heat through a process called
fission.
Nuclear power plants are fueled by uranium, which emits
radioactive substances. Most of these substances are trapped in uranium fuel
pellets or in sealed metal fuel rods. However, small amounts of these
radioactive substances (mostly gases) become mixed with the water that is used
to cool the reactor. Other impurities in the water are also made radioactive as
they pass through the reactor. The water that passes through a reactor is
processed and filtered to remove these radioactive impurities before being
returned to the environment. Nonetheless, minute quantities of radioactive
gases and liquids are ultimately released to the environment under controlled
and monitored conditions
The U.S. Nuclear Regulatory Commission (NRC) has established
limits for the release of radioactivity from nuclear power plants. Although the
effects of very low levels of radiation are difficult to detect, the NRC's
limits are based on the assumption that the public's exposure to man-made
sources of radiation should be only a small fraction of the exposure that
people receive from natural background sources.
Experience has shown that, during normal operations, nuclear
power plants typically release only a small fraction of the radiation allowed
by the NRC's established limits. In fact, a person who spends a full year at
the boundary of a nuclear power plant site would receive an additional
radiation exposure of less than 1 percent of the radiation that everyone
receives from natural background sources . This additional exposure, totaling
about 1 millirem (a unit used in measuring radiation absorption and its
effects), has not been shown to cause any harm to human beings.
In agriculture
Radioisotopes are used to induce mutations in plants in
order to develop superior varieties that are harder and more resistant to
diseases.
Artificial
Radioactivity
Artificial
radioactivity is the phenomenon by which even light elements are made
radioactive by artificial or induced methods.
Artificial radioactivity occurs when a previously stable
material has been made radioactive by exposure to specific radiation. Most
radioactivity does not induce other material to become radioactive. This
Induced radioactivity was discovered by Irène Curie and F. Joliot in 1934. This
is also known as man-made radioactivity. The phenomenon by which even light
elements are made radioactive by artificial or induced methods is called artificial
radioactivity.
Curie and Joliot showed that when lighter elements such as
boron and aluminium were bombarded with α-particles, there was a continuous
emission of radioactive radiations, even after the α−source had been removed.
They showed that the radiation was due to the emission of a particle carrying
one unit positive charge with mass equal to that of an electron.
Neutron activation
is the main form of induced radioactivity, which happens when free neutrons are
captured by nuclei. This new heavier isotope can be stable or unstable
(radioactive) depending on the chemical element involved.
Because free neutrons disintegrate within minutes outside of
an atomic nucleus, neutron radiation can be obtained only from nuclear
disintegrations, nuclear reactions, and high-energy reactions (such as in
cosmic radiation showers or particle accelerator collisions). Neutrons that
have been slowed down through a neutron moderator (thermal neutrons) are more
likely to be captured by nuclei than fast neutrons.
Methods of Producing
Artificial Radioactive Isotopes
Describe methods of producing artificial radioactive
isotopes
Methods of inducing
radioactivity
Nuclear activation: Neutron activation is the process in which
neutron radiation induces radioactivity in materials, and occurs when atomic
nuclei capture free neutrons, becoming heavier and entering excited states. The
excited nucleus often decays immediately by emitting gamma rays, or particles
such as beta particles, alpha particles, fission products and neutrons (in
nuclear fission). Thus, the process of neutron capture, even after any
intermediate decay, often results in the formation of an unstable activation
product. Such radioactive nuclei can exhibit half-lives ranging from small
fractions of a second to many years.
Photonuclear reactions:
A photonuclear reaction is a reaction resulting from an interaction between a
photon and a nucleus.-During a photonuclear reaction energy of a gamma-ray
photon is fully or partially absorbed by the nucleus forcing it into and
excited state. From this excited state the nucleus can emit any particle,
provided it has enough energy for such a process to occur. Most commonly it
will emit a photon, but also a neutron (n), a proton (p) or an alpha (α)
particle can be emitted.
Applications of
Artificial Radioactivity
Application of
artificial radioactivity include:
Radiation safety: For physicians and radiation safety officers,
activation of sodium in the human body to sodium-24, and phosphorus to phosphorus-32,
can give a good immediate estimate of acute accidental neutron exposure.
Neutron detection: One way to demonstrate that nuclear fusion has
occurred inside a fusor device is to use a Geiger counter to measure the gamma
ray radioactivity that is produced from a sheet of aluminum foil.In the ICF
fusion approach, the fusion yield of the experiment (directly proportional to
neutron production) is usually determined by measuring the gamma-ray emissions
of aluminum or copper neutron activation targets. Aluminum can capture a
neutron and generate radioactive sodium-24, which has a half-life of 15
hours[7][8] and a beta decay energy of 5.514 MeV.The activation of a number of
test target elements such as sulfur, copper, tantalum and gold have been used
to determine the yield of both pure fissionand thermonuclear weapons.
Materials analysis: Main article: neutron activation analysis.
Neutron activation analysis is one of the most sensitive and accurate methods
of trace element analysis. It requires no sample preparation or solubilization
and can therefore be applied to objects that need to be kept intact such as a
valuable piece of art. Although the activation induces radioactivity in the
object, its level is typically low and its lifetime may be short, so that its
effects soon disappear. In this sense, neutron activation is a non-destructive
analysis method.
The potential use of
photonuclear reactions for a range of applications is described. These are:
photonuclear transmutation doping of semiconductors, neutron production from
electron linacs, quality checking of radioactive waste, fission product
incineration, photoexcitation of isomers for dosimetry, and nuclear resonance
fluorescence for materials analysis. Initial brief descriptions of atomic and nuclear
interactions of photons and of bremsstrahlung are given.
Radiation Hazards and
Safety
Explain the effects of nuclear radiation on human body
Certain body parts are more specifically affected by
exposure to different types of radiation sources. Several factors are involved
in determining the potential health effects of exposure to radiation. These
include:
·
The size of the dose (amount of energy deposited
in the body)
·
The ability of the radiation to harm human
tissue
Which organs are affected
The most important factor is the amount of the dose - the
amount of energy actually deposited in your body. The more energy absorbed by
cells, the greater the biological damage. Health physicists refer to the amount
of energy absorbed by the body as the radiation dose. The absorbed dose, the
amount of energy absorbed per gram of body tissue, is usually measured in units
called rads. Another unit of radation is the rem, or roentgen equivalent in
man. To convert rads to rems, the number of rads is multiplied by a number that
reflects the potential for damage caused by a type of radiation. For beta,
gamma and X-ray radiation, this number is generally one. For some neutrons,
protons, or alpha particles, the number is twenty.
Hair: The losing
of hair quickly and in clumps occurs with radiation exposure at 200 rems or
higher.
Brain: Since
brain cells do not reproduce, they won't be damaged directly unless the
exposure is 5,000 rems or greater. Like the heart, radiation kills nerve cells
and small blood vessels, and can cause seizures and immediate death.
Thyroid: The
certain body parts are more specifically affected by exposure to different
types of radiation sources. The thyroid gland is susceptible to radioactive iodine.
In sufficient amounts, radioactive iodine can destroy all or part of the
thyroid. By taking potassium iodide can reduce the effects of exposure.
Blood System:
When a person is exposed to around 100 rems, the blood's lymphocyte cell count
will be reduced, leaving the victim more susceptible to infection. This is
often refered to as mild radiation sickness. Early symptoms of radiation
sickness mimic those of flu and may go unnoticed unless a blood count is done.
According to data from Hiroshima and Nagaski, show that symptoms may persist
for up to 10 years and may also have an increased long-term risk for leukemia
and lymphoma. For more information, visit Radiation Effects Research
Foundation.
Heart: Intense
exposure to radioactive material at 1,000 to 5,000 rems would do immediate
damage to small blood vessels and probably cause heart failure and death
directly.
Gastrointestinal
Tract: Radiation damage to the intestinal tract lining will cause nausea,
bloody vomiting and diarrhea. This is occurs when the victim's exposure is 200
rems or more. The radiation will begin to destroy the cells in the body that
divide rapidly. These including blood, GI tract, reproductive and hair cells,
and harms their DNA and RNA of surviving cells.
Reproductive Tract: Because reproductive tract cells divide
rapidly, these areas of the body can be damaged at rem levels as low as 200.
Long-term, some radiation sickness victims will become sterile.
Radiation sickness
Radiation sickness results when humans (or other animals)
are exposed to very large doses of ionizing radiation. Radiation exposure can
occur as a single large exposure (acute), or a series of small exposures spread
over time (chronic). Exposure may be accidental or intentional (as in radiation
therapy).
Causes
·
Accidental exposure to high doses of radiation
such as a nuclear power plant accidents.
·
Exposure to excessive radiation for medical
treatments.
Symptoms
·
Bleeding from the nose, mouth, gums, and rectum,
Bloody stool, Bruising, Confusion, Dehydration, Diarrhea, Fainting, Fatigue, Fever,
Hair loss, Inflammation of exposed areas (redness, tenderness, swelling,
bleeding), Mouth ulcers, Nausea and vomiting
·
Open sores on the skin, Skin burns (redness,
blistering)Sloughing of skin
·
Ulcers in the esophagus, stomach or intestines
·
Vomiting blood
·
Weakness
First Aid
·
Check the person's breathing and pulse.
·
Start CPR, if necessary.
·
Remove the person's clothing and place the items
in a sealed container. This stops ongoing contamination.
·
Vigorously wash body with soap and water.
·
Dry the body and wrap with soft, clean blanket.
·
Call for emergency medical help or take the
person to nearest emergency medical facility if you can do so safely.
PROTECT
HIMSELF/HERSELF FROM NUCLEAR RADIATION HAZARDS
PRECAUTIONS
Distance: Increasing
the distance from the radiation beam decreases the risk of exposure. doubling
the distance between the primary beam and operator, reduces the exposure by a
factor of four. In addition, the radiation exposure varies according to the
angle at which the camera is projected Oblique views (left and right anterior
oblique) and steep angulations increase radiation exposure but are often
employed to improve visualisation. 60-degree angulations give up to three times
the operator dose than 30-degree angulations (11). The second operator or
assistant is generally less exposed to radiation compared to the first operator
but certainly more at risk than the other staff in the room.
Shielding: Lead
shields and shielding will significantly reduce the risk of exposure but only
if appropriately used and in proper working order. Protective equipment
includes lead aprons, thyroid collars and leaded glasses. With the newly
designed frames and ultra light lenses, protective leaded eyewear is now used
by more of the cardiologists and staff in cardiac cath lab. Some cath labs also
use overhanging lead screens to prevent radiation exposure to brain. The staff
should wear a protective apron of at least 0.25 mm lead equivalent. Protective
gloves should be of at least 0.35 mm lead equivalent. All such protective
clothing should bear an identifying mark and should be examined at yearly
intervals. Defective items should be withdrawn from use.
Adhering to guideline
and protocols: Every unit or work place that deals with ionising radiation
should have their own local guidelines and rules for radiation safety. These
must be read, understood and strictly adhered to in daily practice. Staff must
comply with these local rules in order to insure that the Trust and all their
employees do not contravene statutory requirements of the ionising radiation
regulations and other relevant legislation.
Minimising risk of
exposure to staff and patients: The occupational limit of radiation
exposure in the UK currently is estimated at 20 mSv per year averaged over five
consecutive years (5). Every operator who undertakes a cardiovascular procedure
in the cath lab is responsible for the amount of radiation exposure to the
patient, his or her co-staff and to themselves. In the event of an incident
where the patient might have been exposed to inadvertent excess radiation
either due to clinical circumstances, malfunctioning of the equipment or
operation errors, the radiation protection adviser should be informed of the
incident. It is their duty to estimate the radiation dose received by the
patient and also advise whether the incident is to be reported.Only essential
staff shall be in the cath lab during radiation exposure. All persons not
required in the room should leave the room during serial radiographic exposure.
The operator shall stand behind a barrier if possible. People who must move
around the room during the procedure should wear a wraparound protective
garment. When possible, the cardiologist and all other personnel required in
the room should step back from the table and behind portable shields during
cine and serial radiography procedures.
Time: An average
the procedure time for a diagnostic coronary angiogram is approximately 30
minutes and an interventional procedure PCI or EPS/pacing would take between 90
to 120 minutes. However the fluoroscopic and the cine screening time are highly
variable depending on the nature of the procedure and the experience of the
operator. The lower the amount of time spent in a radiation area, the lower the
exposure will be. Significant reductions can be achieved when an activity is
delayed until after cine imaging is completed. Every effort should be made by
the operating cardiologist in the cath lab to minimise fluoroscopy and cine
screening time.
Nuclear Fission and
Fusion
The Nuclear Fission and Fusion
Nuclear fission
Nuclear fission is either a nuclear reaction or a
radioactive decay process in which the nucleus of an atom splits into smaller
parts (lighter nuclei).
The fission process often produces free neutrons and photons
(in the form of gamma rays), and releases a very large amount of energy even by
the energetic standards of radioactive decay. It is an exothermic reaction
which can release large amounts of energy both as electromagnetic radiation and
as kinetic energy of the fragments (heating the bulk material where fission
takes place).
In order for fission to produce energy, the total binding
energy of the resulting elements must be less negative (higher energy) than
that of the starting element.
Nuclear fusion
Nuclear fusion is a nuclear reaction in which two or more
atomic nuclei come very close and then collide at a very high speed and join to
form a new type of atomic nucleus.
During this process, matter is not conserved because some of
the matter of the fusing nuclei is converted to photons (energy).
The fusion of two nuclei with lower masses than iron (which,
along with nickel, has the largest binding energy per nucleon) generally
releases energy, while the fusion of nuclei heavier than iron absorbs energy.
The opposite is true for the reverse process, nuclear
fission. This means that fusion generally occurs for lighter elements only, and
likewise, that fission normally occurs only for heavier elements.
Application of
Nuclear Fission and Fusion
Nuclear fission is used in:
·
Nuclear
power plants to generate electricity for domestic and industrial use.
·
In making
nuclear bombs.
·
In fusion
power plants to make electricity.
·
To make
nuclear weapons such as the hydrogen bombs.
TOPIC 4: THERMIONIC EMISSION
Thermionic Emission
Concept.
Thermionic emission is the discharge of electrons from
heated materials, widely used as a source of electrons in conventional electron
tubes (e.g., television picture tubes) in the fields of electronics and
communications. The phenomenon was first observed (1883) by Thomas A. Edison as
a passage of electricity from a filament to a plate of metal inside an
incandescent lamp. The classical example of thermionic emission is the emission
of electrons from a hot cathode into a vacuum (also known as thermal electron
emission or the Edison effect) in a vacuum tube. The hot cathode can be a metal
filament, a coated metal filament, or a separate structure of metal or carbides
or borides of transition metals. Vacuum emission from metals tends to become
significant only for temperatures over 1000 K. The science dealing with this
phenomenon has been known as "thermionic," but this name seems to be
gradually falling into disuse.
Cathode Rays
Cathode rays (also called an electron beam or e-beam) are
streams of electrons observed in vacuum tubes. Electrons were first discovered
as the constituents of cathode rays. In 1897 British physicist J. J. Thomson
showed the rays were composed of a previously unknown negatively charged
particle, which was later named the electron. Cathode ray tubes (CRTs) use a
focused beam of electrons deflected by electric or magnetic fields to create
the image in a classic television set.
The Production of
Cathode Rays
Cathode rays are so named because they are emitted by the
negative electrode, or cathode, in a vacuum tube. To release electrons into the
tube, they first must be detached from the atoms of the cathode.
Modern vacuum tubes use thermionic emission, in which the
cathode is made of a thin wire filament which is heated by a separate electric
current passing through it. The increased random heat motion of the filament
atoms knocks electrons out of the atoms at the surface of the filament, into
the evacuated space of the tube.
Since the electrons have a negative charge, they are
repelled by the cathode and attracted to the anode. They travel in straight
lines through the empty tube. The voltage applied between the electrodes
accelerates these low mass particles to high velocities. Cathode rays are
invisible, but their presence was first detected in early vacuum tubes when
they struck the glass wall of the tube, exciting the atoms of the glass and
causing them to emit light, a glow called fluorescence.
Researchers noticed that objects placed in the tube in front
of the cathode could cast a shadow on the glowing wall, and realized that
something must be travelling in straight lines from the cathode.
After the electrons reach the anode, they travel through the
anode wire to the power supply and back to the cathode, so cathode rays carry
electric current through the tube. The current in a beam of cathode rays
through a tube can be controlled by passing it through a metal screen of wires
(a grid) to which a small voltage is applied.
The electric field of the wires deflects some of the
electrons, preventing them from reaching the anode. Thus a small voltage on the
grid can be made to control a much larger voltage on the anode. This is the
principle used in vacuum tubes to amplify electrical signals.
High speed beams of cathode rays can also be steered and
manipulated by electric fields created by additional metal plates in the tube
to which voltage is applied, or magnetic fields created by coils of wire
(electromagnets). These are used in cathode ray tubes, found in televisions and
computer monitors, and in electron microscopes.
THE PROPERTIES OF
CATHODE RAYS
Properties of Cathode
Rays Include:
·
Cathode rays travel in straight lines. That is
why, cathode rays cast shadow of any solid object placed in their path. The
path cathode rays travel is not affected by the position of the anode.
·
Cathode rays consist of matter particles, and
posses energy by the virtue of its mass and velocity. Cathode rays set a paddle
wheel into motion when it is placed in the path of these rays one the bladder
of the paddle wheel.
·
Cathode rays consist of negatively charged
particles. When cathode rays are subjected to an electrical field, these get
deflected towards the positively charge plate (Anode).We know that a positively
charged body would attract only a negatively charged body, therefore the
particles of cathode rays carry negative charge. Cathode rays also get
deflected when these are subjected to a strong magnetic field.
·
Cathode rays heat the object only which they
fall. The cathode ray particles possess kinetic energy. When these particles
strike an object, a part of the kinetic energy is transferred to the object.
The causes a rise in the temperature of the object.
·
Cathode rays cause green fluorescence on glass
surface, i.e., the glass surface only which the cathode rays strike show a
colored shine.
·
Cathode rays can penetrate through thin metallic
sheets.
·
Cathode rays ionize the gases through which they
travel.
Cathode rays when fall only certain metals
such as copper, but rays produced. The X-rays are not deflected by electrical
or magnetic fields. X-rays pass through opaque materials such as black paper,
but stopped by solid objects such as bones.
·
Cathode rays travel with speed nearly equal to
that of light.
Application
of cathode ray tube includes:
·
Televisions
·
Before LCD or Plasma television, the CRT was
used to create a moving image. It used the same principle as a CRT, and for
Black and White televisions, that worked fine. B&W TVs were essentially the
same thing as a CRT, as all that's needed is the control of the brightness of
the beam.
·
A CRT TV works by having the electron beam
"scan" the screen at an rate faster than our eyes can perceive. This
means that it shoots across the screen like a machine gun, and the images we
see are actually made from many fluorescent dots.
·
The fluorescence caused by the beam striking the
screen lasts a bit longer so that the next scan can be made without the
previous image disappearing. It scans twice each time, first filling in the odd
"holes" then the even ones. Each scan is about 1/50 of a second.
·
Colour CRT TVs had 3 electron guns rather than a
single one, a shadow mask, and a modified fluorescent screen. The 3 electron
guns were needed as there were three primary colours (Red, Green and Blue) that
could be adjusted in different amounts to create any colour.
·
The colours are formed as a result of the shadow
mask, which is a layer with holes in it that controls the angle of the incoming
electron beams. This is because the fluorescent screen is separated into multi-colored
phosphors that are placed adjacent to each other at small intervals.
Thus it isn't actually a single coloured pixel, but rather 3
very small pixels that join together to form a larger dot.
Cathode Ray
Oscilloscopes
A Cathode Ray Oscilloscope (CRO) is a diagnostic device that
allows one to "see" voltage. It is essential a Cathode Ray Tube with
two perpendicular sets of deflecting electric plates. The vertical set is where
an input voltage is plugged in for the oscilloscope to display.
However, the horizontal set is connected to a "sweep generator”.
This is what provides a constant, but adjustable, time base for the sweeping.
It essentially creates a "saw tooth voltage. “This is what causes the
image to be animated, and measured with a linear scale.
X-Rays
The Structure and Mode of Action of the X-ray Tube
Describe the
structure and mode of action of the x-ray tube
X-radiation (composed of X-rays) is a form of
electromagnetic radiation. Electromagnetic radiation (EM radiation or EMR) is a
form of radiant energy released by certain electromagnetic processes. Visible
light is one type of electromagnetic radiation, other familiar forms are
invisible electromagnetic radiations such as X-rays and radio waves.
Most X-rays have a wavelength ranging from 0.01 to 10
nanometers, corresponding to frequencies in the range 30 pent hertz to 30 megahertz
(3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV.X-ray
wavelengths are shorter than those of UV rays and typically longer than those
of gamma rays.
In many languages, X-radiation is referred to with terms
meaning Rontgen radiation, after Wilhelm Rontgen, who is usually credited as
its discoverer, and who had named it X-radiation to signify an unknown type of
radiation.
X-ray tube
The x-ray tube consists of an emitter (either a filament or
a cathode), which emits electrons into a vacuum and an anode to accelerate the
electrons. This establishes a flow of electrons through the tube.
These electrons are referred to as a beam. The cathode is in
the form of a filament which emits electrons when heated. The anode is made of
copper and also carries the target.
A high p.d between the anode and the cathode is maintained
by an external high-voltage source. A battery that supplies high current is
used to heat the cathode filament, which in many cases is made of tungsten. The
cathode is in the form of a coil to provide high resistance to the passing
current.
Production of X-rays
The electrons from the filament experiences the p. d and
accelerated towards the anode. When they hit the anode, they are stopped and
thereby transfer their energy to the electrons of the anode material. This
gives rise to x-rays.
It is only a very small percentage of their energy that is
converted to x-rays, with the rest of it being transformed to heat.
NB:
X-rays may be classified as hard or soft depending on their
wavelengths, which give rise to different properties.
Differences between hard and soft
x-rays
Hard x-rays
|
Soft x-rays
|
They have shorter wavelength (high frequency)
|
They have longer wavelength
|
They have higher energy
|
Have less energy
|
Thigher penetrating power
|
Lower penetrating power
|
Are produced by higher accelerating potential
potential
|
Produced by lower accelerating
|
Have higher velocity
|
Have lower velocity
|
PROPERTIES OF X-RAYS
INCLUDE:
·
They travel in straight lines.
·
They readily penetrate matter.
·
They are not affected by electric or magnetic fields
(they have no charge).
·
They cause fluorescence in certain substances.
·
They can be detected by photographic emulsion.
·
They ionize gases causing the gases to conduct
electricity.
The Application of
X-Rays in Daily Life
The following are
some uses of x-rays:
·
Astronomy
·
X-ray microscopic analysis
·
X-ray fluorescence
·
Security installations
·
Industries In the medical field
·
Crystallography
TOPIC 5: ELECTRONIC
Semi-Conductors
The Concept of Energy
Band in Solids
In solid-state physics, the electronic band structure (or
simply band structure) of a solid describes those ranges of energy that an
electron within the solid may have (called energy bands, allowed bands , or
simply bands) and ranges of energy that it may not have (called band gaps or
forbidden bands ).
Band theory derives these bands and band gaps by examining
the allowed quantum mechanical wave functions for an electron in a large,
periodic lattice of atoms or molecules. Band theory has been successfully used
to explain many physical properties of solids, such as electrical resistivity
and optical absorption, and forms the foundation of the understanding of all
solid-state devices (transistors, solar cells, etc.).
Insulators
An electrical insulator is a material whose internal
electric charges do not flow freely, and therefore make it impossible to
conduct an electric current under the influence of an electric field. This
contrasts with other materials, semiconductors and conductors, which conduct
electric current more easily.
The property that distinguishes an insulator is its
resistivity; insulators have higher resistivity than semiconductors or
conductors.
A perfect insulator does not exist, because even insulators
contain small numbers of mobile charges (charge carriers) which can carry
current. In addition, all insulators become electrically conductive when a
sufficiently large voltage is applied that the electric field tears electrons
away from the atoms. This is known as the breakdown voltage of an insulator.
Some materials such as glass, paper and Teflon, which have
high resistivity, are very good electrical insulators. A much larger class of
materials, even though they may have lower bulk resistivity, are still good
enough to prevent significant current from flowing at normally used voltages,
and thus are employed as insulation for electrical wiring and cables. Examples
include rubber-like polymers and most plastics.
Conductors
A conductor is an object or type of material that allows the
flow of electrical current in one or more directions. For example, a wire is an
electrical conductor that can carry electricity along its length.
In metals such as copper or aluminum, the movable charged
particles are electrons. Positive charges may also be mobile, such as the
cationic electrolyte(s) of a battery, or the mobile protons of the proton
conductor of a fuel cell. Insulators are non-conducting materials with few
mobile charges and support only insignificant electric currents.
Semiconductors
A semiconductor material has an electrical conductivity
value falling between that of a conductor, such as copper, and an insulator,
such as glass. Semiconductors are the foundation of modern electronics.
Semiconducting materials exist in two types: elemental materials and compound
materials.
The modern understanding of the properties of a
semiconductor relies on quantum physics to explain the movement of electrons
and holes in a crystal lattice. The unique arrangement of the crystal lattice
makes silicon and germanium the most commonly used elements in the preparation
of semiconducting materials.
An increased knowledge of semiconductor materials and
fabrication processes has made possible continuing increases in the complexity
and speed of microprocessors and memory devices. Some of the information on
this page may be outdated within a year because new discoveries are made in the
field frequently.
Examples of semiconductors are Silicon, Germanium.
The Effects of Temperature on the Conductivity of
Conductors, Semiconductors and Insulators
Describe the effect of temperature on the conductivity of
conductors, semiconductors and insulators
The conductivity of pure defect free metal decreases with
increase in temperature. With increased temperature in a metal, thermal energy
causes atoms in metal to vibrate, in this excited state atoms interact with and
scatter electrons.
Thus decreasing the mean free path, and hence the mobility
of electrons too decreases, and resistivity increases.
Since, resistivity = 1/conductivity
The electrical conductivity of a semiconductor will increase
exponentially with an increase in temperature, as temperature increases the
electrons in the valance band will gain energy and go into the higher energy
levels in the conduction band where they become charge carriers.
The increase in conduction can also be explained, I guess,
due to the formation of Cooper pairs and hence the creation of Phonon field.
Types of
Semiconductors
There are two types of semiconductors
·
Intrinsic semiconductors
·
Extrinsic semiconductors
Intrinsic
semiconductors
An intrinsic semiconductor material is chemically very pure
and possesses poor conductivity. It has equal numbers of negative carriers
(electrons) and positive carriers (holes). Examples are Silicon and Germanium.
A silicon crystal
is different from an insulator because at any temperature above absolute zero
temperature, there is a finite probability that an electron in the lattice will
be knocked loose from its position, leaving behind an electron deficiency
called a "hole."
If a voltage is applied, then both the electron and the hole
can contribute to a small current flow. The conductivity of a semiconductor can
be modeled in terms of the band theory of solids.
The band model of a semiconductor suggests that at ordinary
temperatures there is a finite possibility that electrons can reach the
conduction band and contribute to electrical conduction. The term intrinsic here
distinguishes between the properties of pure "intrinsic" silicon and
the dramatically different properties of doped n-type or p-type semiconductors.
The current flow in an intrinsic semiconductor is influenced
by the density of energy states which in turn influences the electron density
in the conduction band. This current is highly temperature dependent. The
electrical conductivity of intrinsic semiconductors increases with increasing
temperature.
Extrinsic
semiconductors
Extrinsic semiconductor is an improved intrinsic
semiconductor with a small amount of impurities added by a process,known as
doping, which alters the electrical properties of the semiconductor and
improves its conductivity.
Introducing impurities into the semiconductor materials (
doping process ) can control their conductivity.Doping process produces two
groups of semiconductors:
The negative charge conductor
( n-type ).
The positive charge
conductor ( p-type ).
Semiconductors are available as either elements or
compounds. Silicon and Germanium are the most common elemental semiconductors.
Compound Semiconductors include InSb, InAs, GaP, GaSb, GaAs, SiC, GaN. Si and
Geboth have a crystalline structure called the diamond lattice. That is, each
atom has its four nearestneighbors at the corners of a regular tetrahedron with
the atom itself being at the center.
In addition to the pure element semiconductors, many alloys
and compounds are semiconductors. The advantage of compound semiconductor is
that they provide the device engineer with a wide range of energy gapsand
mobilities, so that materials are available with properties that meet specific
requirements. Some of these semiconductors are therefore called wide band gap
semiconductors.
The Mechanism of
Doping Intrinsic Semiconductors
Describe the mechanism of doping intrinsic semiconductors
The addition of a small percentage of foreign atoms in the
regular crystal lattice of silicon or germanium produces dramatic changes in
their electrical properties, producing n-type and p-type semiconductors.
Pentavalent
impurities
The addition of pentavalent impurities such as antimony,
arsenic or phosphorous contributes free electrons, greatly increasing the
conductivity of the intrinsic semiconductor. Phosphorous may be added by
diffusion of phosphine gas (PH3).(5 valence electrons) produce n-type
semiconductors by contributing extra electrons.
Trivalent impurities
(3 valence electrons) produce p-type semiconductors by
producing a "hole" or electron deficiency.
N-Type Semiconductor
The addition of pentavalent impurities such as antimony,
arsenic or phosphorous contributes free electrons, greatly increasing the
conductivity of the intrinsic semiconductor. Phosphorous may be added by
diffusion of phosphine gas (PH3).
P-Type Semiconductor
The addition of trivalent impurities such as boron, aluminum
or gallium to an intrinsic semiconductor creates deficiencies of valence electrons,
called "holes". It is typical to use B2H6 diborane gas to diffuse
boron into the silicon material.
P-n junctions
P-n junctions are formed by joining n-type and p-type
semiconductor materials.
Since the n-type region has a high electron concentration
and the p type a high hole concentration, electrons diffuse from the n-type
side to the p-type side. Similarly, holes flow by diffusion from the p -type
side to the
n -type side.
If the electrons and holes were not charged, this diffusion
process would continue until the concentration of electrons and holes on the
two sides were the same, as happens if two gasses come into contact with each
other. However, in a p-n junction, when the electrons and holes move to the
other side of the junction, they leave behind exposed charges on dopant atom
sites, which are fixed in the crystal lattice and are unable to move.
On the n -type side, positive ion cores are exposed. On the
p -type side, negative ion cores are exposed. An electric field Ê forms between
the positive ion cores in the n -type material and negative ion cores in the
p-type material. This region is called the "depletion region" since
the electric field quickly sweeps free carriers out, hence the region is
depleted of free carriers.
Diodes
A semiconductor diode is a crystalline piece of
semiconductor material with a p–n junction connected to two electrical terminals.
In electronics, a diode is a two-terminal electronic component with asymmetric
conductance; it has low (ideally zero) resistance to current in one direction,
and high (ideally infinite) resistance in the other
The Construction of
P-N Junction
Describe the construction of P-N junction
When the polarity of the battery is such that electrons are
allowed to flow through the diode, the diode is said to be forward-biased.
Conversely, when the battery is “backward” and the diode blocks current, the
diode is said to be reverse-biased. A diode may be thought of as like a switch:
“closed” when forward-biased and “open” when reverse-biased.
Diode behavior is analogous to the behavior of a hydraulic
device called a check valve. A check valve allows fluid flow through it in only
one direction.
The Mode of Action a
P-N Junction
In a p-n junction, electrons cross over the boundary from
the n-type material to holes in the p-type material. At the same time, holes
cross over from the p-side to the n-side and capture electrons. This movement
of holes and electrons causes the n-side to become positively charged and the
p-side to become negatively charged. A p.d is created across the junction to
stop further electron flow.
THE TYPES OF DIODES
THERE ARE DIFFERENT
TYPES OF DIODES, THE FOLLOWING ARE THE MOST COMMON ONES:
Laser diode: This
type of diode is not the same as the ordinary light emitting diode because it
produces coherent light. Laser diodes are widely used in many applications from
DVD and CD drives to laser light pointers for presentations. Although laser
diodes are much cheaper than other forms of laser generator, they are considerably
more expensive than LEDs. They also have a limited life.
Light emitting
diodes: The light emitting diode or LED is one of the most popular types of
diode. When forward biased with current flowing through the junction, light is
produced. The diodes use components miconductors, and can produce a variety of
colours, although the original colour was red. There are also very many new LED
developments that are changing the way displays can be used and manufactured.
High output LEDs and OLEDs are two examples.
Photodiode: The photo-diode is used for detecting light. It is
found that when light strikes a PN junction it can create electrons and holes.
Typically photo-diodes are operated under reverse bias conditions where even
small amounts of current flow resulting from the light can be easily detected.
Photo-diodes can also be used to generate electricity. For some applications,
PIN diodes work very well as photodetectors.
PIN diode: This
type of diode is typified by its construction. It has the standard P type and
N-type areas, but between them there is an area of Intrinsic semiconductor
which has no doping. The area of the intrinsicse miconductor has the effect of
increasing the area of the depletion region which can be useful for switching
applications as well as for use in photodiodes, etc.
PN Junction: The
standard PN junction may be thought of as the normal or standard type of diode
in use today. These diodes can come as small signal types for use in radio
frequency, or other low current applications which may be termed as signal
diodes. Other types may be intended for high current and high voltage
applications and are normally termed rectifier diodes.
Schottky diodes:
This type of diode has a lower forward voltage drop than ordinary silicon PN
junction diodes. At low currents the drop may be somewhere between 0.15 and 0.4
volts as opposed to 0.6 volts for a silicon diode. To achieve this performance,
they are constructed in a different way to normal diodes having a metal to
semiconductor contact. They are widely used as clamping diodes, in RF
applications, and also for rectifier applications.
Step recovery diode: A form of microwave
diode used for generating and shaping pulses at very high frequencies. These
diodes rely on a very fast turn off characteristic of the diode for their
operation.
Tunnel diode: Although not widely used today, the tunnel diode was
used for microwave applications where its performance exceeded that of other
devices of the day.
Varactor diode or
varicap diode: This type of diode is used in many radio frequency (RF)
applications. The diode has a reverse bias placed upon it and this varies the
width of the depletion layer according to the voltage placed across the diode.
In this configuration the reactor or varicap diode acts like a capacitor with
the depletion region being the insulating dielectric and the capacitor plates
formed by the extent of the conduction regions. The capacitance can be varied
by changing the bias on the diode as this will vary the width of the depletion
region which will accordingly change the capacitance.
Zener diode: The Zener diode is a very useful type of diode as it
provides a stable reference voltage. As a result, it is used in vast
quantities. It is run under reverse bias conditions and it is found that when a
certain voltage is reached it breaks down. If the current is limited through a
resistor, it enables a stable voltage to be produced. This type of diode is
therefore widely used to provide a reference voltage in power supplies. Two
types of reverse breakdown are apparent in these diodes: Zener breakdown and
Impact Ionization. However, the name Zener diode is used for the reference
diodes regardless of the form of breakdown that is employed.
Backward diode:
This type of diode is sometimes also called the back diode. Although not widely
used, it is a form of PN junction diode that is very similar to the tunnel
diode in its operation. It finds a few specialist applications where its
particular properties can be used.
BARITT diode:
This form of diode gains its name from the words Barrier Injection Transit Time
diode. It is used in microwave applications and bears many similarities to the
more widely used IMPATT diode.
Gunn Diode:
Although not a diode in the form of a PN junction, this type of diode is a
semiconductor device that has two terminals. It is generally used for
generating microwave signals. Gunn diode
The Construction of a Half-wave and Full-Wave Rectifier
Construct a half-wave and Full-Wave rectifier
A rectifier is an
electrical device that converts alternating current (AC), which periodically
reverses direction, to direct current (DC), which flows in only one direction.
The process is known as
rectification.
Transistor
The Construction of a PNP Transistor
Describe the
construction of a PNP transistor
A transistor is a semiconductor device used to amplify and
switch electronic signals and electrical power. It is composed of semiconductor
material with at least three terminals for connection to an external circuit.
The Mode of Action of
a PNP Transistor
Explain the mode of action of a PNP transistor
The transistor is
the fundamental building block of modern electronic devices, and is ubiquitous
in modern electronic systems.
A voltage or
current applied to one pair of the transistor's terminals changes the current
through another pair of terminals. Because the controlled (output) power can be
higher than the controlling (input) power, a transistor can amplify a signal.
The Types of Transistors
Identify the types of
transistors
There are two types
of transistors, which have slight differences in how they are used in a
circuit:
·
Bipolar transistor
·
Field-effect transistor
Bipolar transistor
A bipolar transistor has three terminals labeled base, collector,
and emitter. A small current at the base terminal (that is, flowing between the
base and the emitter) can control or switch a much larger current between the
collector and emitter terminals. There are two types of bipolar transistors:
·
n-p-n transistors
·
p-n-p transistors
Field-effect
transistor
For a field-effect transistor, the terminals are labeled
gate, source , and drain, and a voltage at the gate can control a current
between source and drain.
N-P-N transistor
(Note: Arrow defines the emitter and conventional current
flow, “out” for a Bipolar NPN Transistor).
The construction and terminal voltages for a Bipolar NPN
Transistor are shown above. The voltage between the Base and Emitter ( VBE ),
is positive at the Base and negative at the Emitter because for an NPN
transistor, the Base terminal is always positive with respect to the Emitter.
Also the Collector supply voltage is positive with respect to the Emitter ( VCE
). So for a bipolar NPN transistor to conduct the Collector is always more
positive with respect to both the Base and the Emitter.
NPN Transistor
Connection
Then the voltage sources are connected to an NPN transistor
as shown. The Collector is connected to the supply voltage VCC via the load resistor,
RL which also acts to limit the maximum current flowing through the device. The
Base supply voltage VB is connected to the Base resistor RB, which again is
used to limit the maximum Base current.
So in a NPN Transistor it is the movement of negative
current carriers (electrons) through the Base region that constitutes
transistor action, since these mobile electrons provide the link between the
Collector and Emitter circuits. This link between the input and output circuits
is the main feature of transistor action because the transistors amplifying
properties come from the consequent control which the Base exerts upon the
Collector to Emitter current.
Then we can see that the transistor is a current operated
device (Beta model) and that a large current ( Ic ) flows freely through the
device between the collector and the emitter terminals when the transistor is
switched “fully-ON”. However, this only happens when a small biasing current (
Ib ) is flowing into the base terminal of the transistor at the same time thus
allowing the Base to act as a sort of current control input.
The transistor current in a bipolar NPN transistor is the
ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device
and is given the symbol of hfe or nowadays Beta, ( β ). The value of β can be
large up to 200 for standard transistors, and it is this large ratio between Ic
and Ib that makes the bipolar NPN transistor a useful amplifying device when
used in its active region as Ib provides the input and Ic provides the output.
Note that Beta has no units as it is a ratio.
Also, the current gain of the transistor from the Collector
terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ), and is a
function of the transistor itself (electrons diffusing across the junction). As
the emitter current Ie is the sum of a very small base current plus a very
large collector current, the value of alpha α, is very close to unity, and for
a typical low-power signal transistor this value ranges from about 0.950 to
0.999.
α and β Relationship in a NPN Transistor
By combining the two parameters α and β we can produce two
mathematical expressions that gives the relationship between the different
currents flowing in the transistor.
The values of Beta vary from about 20 for high current power
transistors to well over 1000 for high frequency low power type bipolar
transistors. The value of Beta for most standard NPN transistors can be found
in the manufactures data sheets but generally range between 50 – 200.
The equation above for Beta can also be re-arranged to make
Ic as the subject, and with a zero base current ( Ib = 0 ) the resultant
collector current Ic will also be zero, ( β x 0 ). Also when the base current
is high the corresponding collector current will also be high resulting in the
base current controlling the collector current. One of the most important
properties of the
Bipolar Junction Transistor is that a small base current can
control a much larger collector current. Consider the following example.
P-N-P transistor
The PNP Transistor is the exact opposite to the NPN
Transistor device we looked at in the previous tutorial. Basically, in this
type of transistor construction the two diodes are reversed with respect to the
NPN type giving a Positive-Negative-Positive type of configuration, with the
arrow which also defines the Emitter terminal this time pointing inwards in the
transistor symbol.
Also, all the polarities for a PNP transistor are reversed
which means that it “sinks” current into its Base as opposed to the NPN
Transistor which “sources” current through its Base. The main difference
between the two types of transistors is that holes are the more important
carriers for PNP transistors, whereas electrons are the important carriers for
NPN transistors.
Then, PNP transistors use a small base current and a negative
base voltage to control a much larger emitter-collector current. In other words
for a PNP transistor, the Emitter is more positive with respect to the Base and
also with respect to the Collector. The construction of a “PNP transistor”
consists of two P-type semiconductor materials either side of an N-type
material as shown below.
(Note: Arrow defines the emitter and conventional current
flow, “in” for a PNP transistor).
The construction and terminal voltages for an NPN transistor
are shown above. The
PNP Transistor has very similar characteristics to their NPN
bipolar cousins, except that the polarities (or biasing) of the current and
voltage directions are reversed for any one of the possible three
configurations looked at in the first tutorial, Common Base, Common Emitter and
Common Collector.
PNP Transistor Connection
The voltage between the Base and Emitter ( VBE ), is now
negative at the Base and positive at the Emitter because for a PNP transistor,
the Base terminal is always biased negative with respect to the Emitter. Also
the Emitter supply voltage is positive with respect to the Collector ( VCE ).
So for a PNP transistor to conduct the Emitter is always more positive with
respect to both the Base and the Collector.
The voltage sources are connected to a PNP transistor are as
shown. This time the Emitter is connected to the supply voltage VCC with the
load resistor, RL which limits the maximum current flowing through the device
connected to the Collector terminal. The Base voltage VB which is biased
negative with respect to the Emitter and is connected to the Base resistor RB,
which again is used to limit the maximum Base current.
To cause the Base current to flow in a PNP transistor the
Base needs to be more negative than the Emitter (current must leave the base)
by approx. 0.7 volts for a silicon device or 0.3 volts for a germanium device
with the formulas used to calculate the Base resistor, Base current or
Collector current are the same as those used for an equivalent NPN transistor
and is given as.
We can see that the fundamental differences between a NPN
Transistor and a PNP Transistor is the proper biasing of the transistors
junctions as the current directions and voltage polarities are always opposite
to each other. So for the circuit above: Ic = Ie – Ib as current must leave the
Base.
Generally, the PNP transistor can replace NPN transistors in
most electronic circuits, the only difference is the polarities of the
voltages, and the directions of the current flow. PNP transistors can also be
used as switching devices and an example of a PNP transistor switch is shown
below.
The Output Characteristics Curves for a PNP transistor look
very similar to those for an equivalent NPN transistor except that they are
rotated by 180o to take account of the reverse polarity voltages and currents,
(the currents flowing out of the Base and Collector in a PNP transistor are
negative). The same dynamic load line can be drawn onto the I-V curves to find
the PNP transistors operating points.
The Application of
Transistors in Daily Life
Outline the
applications of transistors in daily life
Transistors are used in all electronic devices such as
calculators, tv, radios, computers etc.
They are used in switching circuits, amplifier circuits,
oscillator circuits, current source circuits, voltage regulator circuits, power
supply circuits, digital logic integrated circuits and in any circuit that uses
small control signals to control larger currents.
Single Stage
Amplifier
The Concept of
Analogue Signals
An Analog signal is any continuous signal for which the time
varying feature (variable) of the signal is a representation of some other time
varying quantity, i.e., analogous to another time varying signal. It differs
from a digital signal in terms of small fluctuations in the signal which are
meaningful.
The Concept of
Digital Signal
A digital signal uses discrete (discontinuous) values. By
contrast, non-digital (or analog) systems use a continuous range of values to
represent information. Although digital representations are discrete, the
information represented can be either discrete, such as numbers or letters, or
continuous, such as sounds, images, and other measurements of continuous
systems.
Properties of Digital
vs Analog signals
Digital information
has certain properties that distinguish it from analog communication methods.
These include:
Errors: disturbances
in analog communication causes errors in actual intended communication but
disturbances in digital communication does not cause errors enabling error free
communication. Errors should be able to substitute, insert or delete symbols to
be expressed.
Copying: analog
communication copies are quality wise not as good as their originals while due
to error free digital communication, copies can be made indefinitely.
Granularity: for a
continuously variable analog value to be represented in digital form there
occur quantization error which is difference in actual analog value and digital
representation and this property of digital communication is known as
granularity.
Synchronization: digital
communication uses specific synchronization sequences for determining synchronization.
Language: digital communications requires a language
which should be possessed by both sender and receiver and should specify
meaning of symbol sequences.
Differences in Usage
in Equipment
Many devices come with built in translation facilities from
analog to digital. Microphones and speaker are perfect examples of analog
devices. Analog technology is cheaper but there is a limitation of size of data
that can be transmitted at a given time.
Digital technology has revolutionized the way most of the
equipments work. Data is converted into binary code and then reassembled back
into original form at reception point. Since these can be easily manipulated,
it offers a wider range of options. Digital equipment is more expensive than
analog equipment.
Comparison of Analog vs Digital Quality: Digital devices
translate and reassemble data and in the process are more prone to loss of
quality as compared to analog devices. Computer advancement has enabled use of
error detection and error correction techniques to remove disturbances
artificially from digital signals and improve quality
Differences in
Applications: Digital technology has been most efficient in cellular phone
industry. Analog phones have become redundant even though sound clarity and
quality was good.
Analog technology comprises of natural signals like human speech.
With digital technology this human speech can be saved and stored in a
computer. Thus digital technology opens up the horizon for endless possible
uses.
A Single-Stage
Amplifier
Design a single-stage
amplifier
Single-stage amplifier have only one amplifying device. It
consists of amplification stage that includes a transistor. The transistor is
connected to a load resistor through which a load current flows. The value of
the load resistor together with the trans conductance value affects the
amplifier’s voltage gain.
Single-stage
amplifiers include:
·
Common-emitter (CE) amplifier
·
Common-collector (CC) amplifier
·
Common-base (CB) amplifier
It is called the common-emitter configuration because
(ignoring the power supply battery) both the signal source and the load share
the emitter lead as a common connection point
Common-emitter
amplifier
It is called the common-emitter configuration because
(ignoring the power supply battery) both the signal source and the load share
the emitter lead as a common connection point
Common-emitter amplifier: The input and output signals both
share a connection to the emitter
Before, a small solar cell current saturated a transistor,
illuminating a lamp. Knowing now that transistors are able to “throttle” their
collector currents according to the amount of base current supplied by an input
signal source, we should see that the brightness of the lamp in this circuit is
controllable by the solar cell’s light exposure. When there is just a little
light shone on the solar cell, the lamp will glow dimly. The lamp’s brightness
will steadily increase as more light falls on the solar cell.
Common collector
amplifier
It is called the common-collector configuration because
(ignoring the power supply battery) both the signal source and the load share
the collector lead as a common connection point
Common collector: Input is applied to base and collector.
Output is from emitter-collector circuit.
It should be apparent that the load resistor in the
common-collector amplifier circuit receives both the base and collector
currents, being placed in series with the emitter. Since the emitter lead of a
transistor is the one handling the most current (the sum of base and collector
currents, since base and collector currents always mesh together to form the
emitter current), it would be reasonable to presume that this amplifier will
have a very large current gain. This presumption is indeed correct: the current
gain for a common-collector amplifier is quite large, larger than any other
transistor amplifier configuration. However, this is not necessarily what sets
it apart from other amplifier designs.
Common-base amplifier
It is called the common-base configuration because (DC power
source aside), the signal source and the load share the base of the transistor
as a common connection point shown in.
Common-base amplifier: Input between emitter and base,
output between collector and base.
Perhaps the most striking characteristic of this
configuration is that the input signal source must carry the full emitter
current of the transistor, as indicated by the heavy arrows in the first
illustration. As we know, the emitter current is greater than any other current
in the transistor, being the sum of base and collector currents. In the last
two amplifier configurations, the signal source was connected to the base lead
of the transistor, thus handling the least current possible.
Because the input current exceeds all other currents in the
circuit, including the output current, the current gain of this amplifier is
actually less than 1 (notice how Road is connected to the collector, thus
carrying slightly less current than the signal source). In other words, it
attenuates current rather than amplifying it. With common-emitter and
common-collector amplifier configurations, the transistor parameter most
closely associated with gain was β. In the common-base circuit, we follow
another basic transistor parameter: the ratio between collector current and
emitter current, which is a fraction always less than 1. This fractional value
for any transistor is called the alpha ratio, or α ratio.
6: ELEMENTARY
ASTRONOMY
Introduction to Astronomy
The Concept of
Astronomy
Astronomy is a
branch of science which deals with the study of origin, evolution, composition,
distance and the motion of all bodies and scattered matter in the universe.
·
Universe is the totality of space and time
together with matter and energy.
·
Astronomers are the people who deals astronomy.
Importance of
astronomy include:
- It presents a new
frontier for exploration.
·
It was the earliest method of measuring time.
·
It was used to develop calendars that made it
possible to predict the seasons.
·
It is used in navigation. -Helps us to
understand the earth and the life it supports originated from and how it
evolved.
Solar System
The solar system is made up of the sun and the celestial
objects bound to it by gravity. These objects include the eight planets and
their known moons and billions of small bodies that include asteroids, comets,
meteoroids and interplanetary dust.
Stars and planets
A star is a large
celestial body made up of hot gases known as plasma.
Plasma is an
ionized gas in which a certain proportion of electrons are free rather than
bound to an atom or molecule.
The sun is a
large star. The sun is also the closest star to the earth. Astronomical unit is
the distance between the earth and the sun which is used to measure distances
across the solar system (its value is approximately 149.60 million kilometers).
A Galaxy is a
giant collection of stars, gas and dust.
Most stars in the universe are in the galaxies. Nearly all
of the stars visible in the night sky are within our own galaxy, sometimes
called the Milky Way Galaxy.
Planet is a major
(large) object which is in orbit around a star. There are eight planets which
are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Characteristics of a
planet
·
It has cleared the neighborhood around its
orbit.
·
It is a celestial body that orbits a star.
·
It is massive enough so that its own gravity causes
it to assume a spherical shape.
Pluto is not considered as a planet because it resides in an
area of space populated by numerous other objects. It is now designated a dwarf
planet. The dwarf planet does not meet the third characteristic i.e has not
cleared the neighborhood around its orbit.
Differences between
stars and planets
Stars
|
Planets
|
Emit their own light
|
Do not emit their own light
|
Twinkle at night
|
Do not twinkle at night
|
Appear to be moving from east to west.
|
Planets move around the sun from west to east.
|
Their temperatures are usually very high
|
Their temperature depends on their distances from the sun
|
Countless in number
|
There are eight in the solar system
|
Very big in size but they appear small because of the farthest from
the earth
|
small in size as compared to stars.
|
Asteroids (minor
planets) are small solar system bodies in orbit around the sun, especially
in the inner solar system. Asteroids are smaller than planets but larger than a
speck of dust.
A comet is a solid body orbiting the sun typically composed
of rock dust or ice. Most comets were formed from condensed interstellar gas
and dust clouds in the early stages of the creation of the universe.
The Force of
Gravitation Which Maintains Celestial Bodies in their Orbits
Explain the force of gravitation which maintains celestial
bodies in their Orbits
Gravitation force is the attractive force existing between
any two objects that have mass. It pulls objects together and acts on all
matter on the universe, hence it is sometimes referred to as universal
gravitation.
Newton’s law of
universal gravitation
It states that: ‘Every single point mass attracts every
other point by a force directed along the line joining the two masses.'The
force is proportional to the product of the two masses and inversely
proportional to the square of the distance between the point masses.
Where:
·
F is the magnitude of the attractive force
between the two point masses.
·
G is the universal gravitation constant.m₁ is
the mass of the first point mass.
·
m₂ is the mass of the second point mass.
·
r is the distance between the centers of the two
point masses.
Gravitation force is actually very weak force. The huge
gravitational force of the nearest star, the sun, holds together the eight
planets of the solar system. The planets move round the space at speeds that
just balance the sun’s gravitational pull, so they are locked into a permanent
path (orbit) around the sun.
Natural satellites (moons) orbits planets while artificial
satellites orbit the earth in the same way as the moon orbits the planets.
Gravity is the gravitational force that occurs between the
earth and the other objects. It pulls the objects towards the center of the earth.
It holds us on the ground and causes objects to fall back to the ground after
being thrown uo in the air.
The earth gravitational pull extends out into space in all
directions. The further you move away from the center of the earth, the weaker
the force becomes. The measure of the force of gravity on an object on the
earth’s surface is the weight of that object and is measured in newton (N).
The weight of an object changes depending on its location in
the universe.
Constellations
The Concept of Constellation
Explain the concept of constellation
Constellation is
a group of stars that form a definite shape or pattern when viewed from the
earth.
Constellations are usually named after mythological
characters, people, animals and things. There are about 88 known
constellations. The various constellations are visible during a particular
period of the year.
Identify
constellation
·
The Uses of Constellation in Everyday Life
·
Uses of constellation in everyday life
Use of knowledge
about constellations include:
·
Religious-In early days, people thought that the
gods lived in the heavens and that the gods created the constellations. Many
cultures believed that the position of the stars were their god’s way of
telling stories.
·
Agricultural. -Before there were proper calendars,
people had no way of determining when to sow or harvest except by the stars.
Constellations made the patterns of the stars easy to remember
·
Navigation. One can figure out his or her
latitude (North or South) just by looking how high Polaris (The North Star)
appear in the night sky. This allowed the ships to travel across the globe.
The Earth and the
Moon
The Surface Features
and Temperature of the Moon
Describe the surface features and temperature of the moon
The moon of the earth is the sixth largest in the solar
system. It has a diameter of 3,476km and a mass of 7.35 x 10²²kg. Like the
earth, the moon has an iron core surrounded by a rocky mantle and crust.Unlike
the earth, no part of the moon’s iron core is molten so it does not have a
magnetic field.Surface gravity on the moon is 1/6 that of the earth.
The moon revolves in a anticlockwise direction around the
earth in an elliptical orbit. The moon’s orbit is tilted at 5° relative to the
earth’s orbit around the sun.The distance between the earth and the moon varies
from perigee (nearest the earth) where it is 356,000km to apogee (furthest from
the earth) where it is 406,000km. The average distance is 384,000km.
It take the moon 27.3 earth days to complete one orbit, a
period of time called the Sidereal month.The moon also rotates about its axis
at a rate equal to its rate of revolution. The result of this is that one side
of the moon face the earth. The side which faces the earth is called the near
side while the side which faces away is called the far side.
The spinning of the earth causes the moon to rise and set
each day ,just like the sun. However, because of moons’ orbital motion around
the earth ,it(the moon) rises about 50 minutes later each day .As a result, the
moon can be seen at different times of the day and night during a month.
The temperatures on the surface of the moon are on average
107°C during the day and 53°C during the night.
Surface features of the moon.
There are two primary types of terrain on the moon. These
are;
·
Heavily cratered very old lunar highlands.
·
Relatively smooth and younger Maria.
From the surface of the earth, the moon’s surface appears to
have bright and dark regions when viewed with the unaided eye.
The bright areas are the lunar highlands that have many
craters and covered with a highly reflective layer of fine dust. The highlands
are geologically the oldest parts of the moon’s surface.
The dark areas are low areas similar to ocean basins on the
earth. They are with dark solidified lava and are less cratered than the
highlands. Galileo called these areas matia, Italian word for seas, because
their dark smooth surface appears to be large bodies of water.
The Maria which makes 16% of the moon’s surface, are huge
impact craters that were later flooded with molten lava. Most of the Maria is
covered with regolith, a mixture of fine dust and rocky debris produced by
meteor impact.
The Causes of Ocean
Tides
Explain the causes of
ocean tides
Tides are periodic rises and falls of large bodies of water
caused mainly by gravitational interaction between the earth and the moon.
The earth and the moon are attracted each other, just like
magnets are attracted to each other. The moon rises to pull at anything on the
earth to bring it closer. But the earth is able to hold onto everything except
the water. Since the water always moving, the earth can't hold onto it and the
moon is able to pull at it. This results into ocean tides.
Each day, there are two high tides and two low tides. The
ocean constantly moves from high tide to low tide, and then back to high tide.
There is a time interval of about 12 hours and 25 minutes between the two high
tides.
How tides occur
Gravitational attraction of the moon causes the oceans to
bulge out in the direction of the moon. Another bulge occurs on the opposite
side since the earth is also being pulled towards the moon (and away from the
water on the far side).
Ocean levels fluctuate daily as the sun, moon and earth
interact .As the moon travels around the earth, and as they together travel
around the sun, the combined gravitational forces cause the world ocean water
levels to rise and fall. Since the earth is rotating while this is happening,
two tides occur each day.
Types of tides
There are two types
of tides:
1.
Spring tides
They occur during the full moon and the new moon. During
this time, the earth, the sun and the moon are in a line. The gravitational
forces of the moon and the sun both contribute to the tides.
At these times, the high tides are very high and the low
tides are very low. These are known as spring high tines and spring low tides
respectively.
Spring tides are especially strong tides. Proxigen spring
tide is a rare unusually high tide. It occurs when the moon is both unusually
close to the earth (at its closest perigee, called the proxigee) and in the new
moon phase (when the moon is between the earth and the sun).
The proxigen spring tide occurs at most once every 1.5
years.
2.
Neap tide
When the sun and the moon are not aligned, the gravitational
forces cancel each other out, and the tides are not very high or very low.
These are called neap tides.
They occur during quarter moons. During this time, the
gravitational forces of the moon and the sun are perpendicular to one another
(with respect to the earth).
This causes the bulges to cancel each other. The result is a
smaller difference between high and low
TOPIC 7: GEOPHYSICS
Geophysics is a
branch of science that deals with the physical, chemical, geological,
astronomical and other characteristic properties of the earth. It deals with geological
phenomena such as the temperature distribution of the earth’s interior, the
source, configuration and the geomagnetic field.
Structure and
Composition of the Earth
The Structure of the Earth
The structure of the earth is composed of three major zones
arranged in concentric manner. These are crust, mantle and core.
The crust
Is the outer solid layer o the earth. It is extremely thin (5
to 15km) compared to the radius of the earth (6,371km). There are two types of
crust, namely:
. Continental crust: This is heterogeneous and of relatively
low density (2 to 2.8 tonnes per cubic meter). -It is composed mainly of
granites and sedimentary rocks.
. Oceanic crust: This is basaltic and denser (3.0 to 3.1
tonnes per cubic meter). Both the continental and the oceanic crusts floats on
the denser mantle. Because of its low density, the continental crust floats on
the mantle at a higher elevation, forming the land masses and mountains. The
continental crust is 30 to 70 km thick. The denser oceanic crust floats at a
lower elevation forming oceanic basins. It is about 8km thick. The boundary
between the crust and the mantle is called Mohorocivic discontinuity or simply Moho.
It is a zone between one and several kilometers thick.
The mantle
It begins from the Moho and extends to a depth of 2,900km
below the earth’s surface, up to its boundary with the earth’s core. This
boundary is called the Gutenberg discontinuity.
The mantle contains about 70% of the earth’s mass. It is
composed of rocks, both in solid and mountain states. The upper surface of the
mantle has the temperature of about 870°C, and this temperature increases
downwards through the mantle to about 2,200°C near the core.
The core
Is the innermost part of the earth. It extends from the
Gutenberg discontinuity to the earth’s geometric centre. The core consists of
two distinct regions namely:
·
The inner core: It is composed of solid material
because the high pressure at this depth. -It is composed of iron-nickel alloys.
·
The outer core: Is composed of liquid of molten
nickel and iron known as magma. It extends from the mantle to a depth of about
5,000km below the earth’s surface.
The Composition of
the Layers of the Earth
Describe the composition of the layers of the earth
Continental crust is made of granite and sedimentary rocks
forming the lands and the mountains while the oceanic crust forms oceanic
basins. Mantle is made of solids and molten rocks. The outer core is made of
molten nickel and iron called magma while the inner core is solid because of
the high pressure. The crust and the mantle are separated by the mohorovicic
discontinuity.
The Importance of the
Layers of the Earth
Explain the importance of the layers of the earth
Continental crust forms the land and mountains of the earth
on which all human activities are carried out e.g farming, housing etc. Oceanic
crust forms the base of the oceans and seas on which oceanic water rests and
all aquatic organisms like fishes live. The mantle provides the heat transfer
from the core to the outer layers a process which causes the volcanic actions
and earthquakes.
Earthquake and Volcanoes
Both of them, volcano and earthquake are caused by the
movement of molten rock and heat deep inside the earth. These movements are
referred to as subterranean movements. Most earthquakes and volcanic activity
happen near tectonic boundaries.
The Origin of Volcanoes
Explain the origin of volcanoes
Volcanoes are places where molten rock called magma leaks
out through a hole or a crack in the earth’s crust.Magma originates from the mantle,where
high temperature and pressure cause the rock to melt.When a large pool of magma
if formed,it rises through the denser rock layer towards the earth’s surface.
Magma that has reached the earth’s surface is called lava.
Most volcanoes form along constructive and destructive boundaries between
tectonic plates.However a few form plate boundaries.
Types of volcanoes
There are two main types of volcanoes, namely:
. Fissure volcanoes: These occur along the cracks in and
between tectonic plates. They can be many kilometers long. Lava is usually
ejected quietly and continuously,forming enormous plains or plateaus of
basaltic volcanic rock.
. Central volcanoes: These have a single vertical main vent
through which mzgm reaches the earth’s surface. They usually develop a cone
shape that builds up from successive layers of lava and ash.
Classification of volcanoes
Volcanoes are classified into three categories based on
their frequency of eruption, namely:
. Active volcanoes: Are those that either erupt constantly
or have erupted in recent times. Eg; Oldonyo Lengai.
. Dormant volcanoes: Are those that have been inactive for
some time(a few thousand years) but can erupt again. Eg; Mt Kilimanjaroc.
3. Extinct volcanoes : They have not erupted in recorded
history. They will probably never erupt again.
Effects of Volcanoes
Describe effects of volcanoes
Effects of volcanoes include:
. Landscape: Most of the earth’s surface is covered with
volcanic rocks.Volcanoes are also responsible for the formation of many
mountains and islands.
. Vegetation and wildlife: Volcanic eruption sometimes set
the surrounding vegetation into fire.Wild animals are also killed by being
buried into the lava or being burnt by the forest fires.
3. Environment: Volcanic eruptions emit harmful gases into
the environment. Such gases include sulphur dioxide. Some of the gases
contribute to global warming and climate change.
. Human life and property: Volcanic eruptions sometimes kill
people and destroy property.
5. Soil: Volcanoes help in soil formation by bringing
important minerals from from deep underground onto the earth’s surface.
6. Minerals: Volcanoes also bring valuable minerals to the
earth’s surface. The minerals are important economic resources.
THE ORIGIN OF
EARTHQUAKE
The origin of earthquake:
-
An earthquake is defined as a sudden motion or shaking of
the earth caused by a sudden release of energy that has accumulated within or
along the edges of the earth’s tectonic plates.
Earthquakes happen when rocks in the earth’s crust move
suddenly, shaking the earth. Earthquake also occur as a result of movement of
magma at constructive boundaries under volcanoes and where continental plates
collide and push mountain ranges.
Occurrence of
earthquake
Earthquakes mostly occur on or near the boundaries between
tectonic plates. However, earthquakes can also occur far from plate boundaries.
Such earthquakes probably occur as a result of faults formed millions of years
ago.
Pressure liquid builds between them until the friction force
holding the plates together gives way. The plates move suddenly, releasing the
pressure or energy and then holds together again. This sudden jerk is what is
felt as an earthquake.
The point within the earth where an earthquake begins is
called the hypocenter or the focus of the earthquake. Earthquake rarely occur
along constructive plate boundaries.
Seismic waves
This refers to the energy released by an earthquake. They
are grouped into three categories:
·
Primary waves or p-waves: Are the first waves
released from the hypocenter. They are felt as a sudden jolt.
·
Secondary waves or s-waves: These arrive a few seconds
later after p-waves. They are felt as a series of side-to-side tremors.
·
Surface waves. -They radiate outward from the
point on the earth’s surface directly above the hypocenter. This point is
called the epicenter of the earthquake.
NB:
Seismic Focus: The point above the ground where seismic waves occur.
There are two types
of surface waves:
·
Rayleigh waves- create a rolling movement that
makes the land surface move up and down.
·
Love waves- make the ground shift from side to
side. It is the surface waves that damage to surface structure such as
buildings and hydroelectric power plants.
The Principle of
Measurement
The nature of an earthquake is usually described by measuring
two properties, namely the magnitude and intensity.
The magnitude of an earthquake is a measure of the energy it
releases. It is usually measured on the Richter scale.
The Richter scale magnitudes are based on a logarithmic scale
(base 10). The intensity of an earthquake is a measure of its strength based on
the changes it causes to the landscape. The intensity is usually measured on
the Modified Mercalli scale. The scale is calibrated 1 to 12.
Note: An earthquake can have only one magnitude. However,
its intensity reduces as the seismic waves spread out from the hypocenter, just
the same way the loudness of a sound changes as you move away from the source.
The Seismograph: Is
an instrument used to record ground movements caused by earthquakes. It measures
ground oscillations by recording the relative motion between a pendulum and the
ground. It is also possible to use the ratio between the deflection and the of
the pendulum and the acceleration of the of the ground to record an earthquake.
Precaution against
Earthquake Hazards
Earthquakes hazards
The following are some of the hazards associated with
earthquakes:
·
Landslides
·
Tsunamis
·
Collapsing buildings
·
Fire outbreak
·
Backward rivers
Earthquake warning
signs
The following are important signs that are observed before
an earthquake occurs:
·
Thermal indicator
·
Water indicator
·
Seismo electromagnetic indicator
·
Animal indicator
Human indicator
Precautions to be taken during an earthquake
The following are some precautions that can be taken to minimize
injuries or death of human beings in the event of an earthquake:
·
If you are indoors during an earthquake, drop,
cover and hold on. Get under a desk, table or a bench. Hold on to one of the
legs and cover your eyes. If there is no desk or table nearby, sit down against
an interior wall.
·
Pick a safe place where things will not fall on
you-away from windows or tall heavy furniture.
·
Do not run outside when the earthquake happens
because bricks, roofing and other materials may fall from buildings during and
immediately after an earthquake, injuring persons near the building.
·
Wait in your safe place until the shaking stops,
then check to see if you are hurt. You will be better able to help others if
you take care of yourself first,then check on the people around you.
·
Move carefully and watch out for things that
have fallen or broken creating hazards. Be ready for additional earthquakes
called after shocks.
·
Be on the lookout for fires. Fire is the most
common earthquake-related hazard due to damaged gas and electrical lines.
·
If you must leave a building after the shaking
stops, use the stairs and not elevator. Earthquakes can cause fire alarms and
fire sprinklers to go off. You will not be certain whether there is a real
threat of fire. As a precaution, use the stairs.
·
If you are outside during an earthquake, stay
outside. Move away from buildings, trees, streetlights and power lines. Crouch
down and cover your head. Bricks, roofing and other materials can fall from
buildings, injuring persons nearby. Trees, streetlights and poor lines may also
fall, causing damage or injury.
Structure and
Composition of the Atmosphere
The Vertical Structure of the Atmosphere
The atmosphere is a layer of gases containing numerous small
suspended solid and liquid particles surrounding the earth. It has no outer boundary;
it just fades into space. The dense part of the atmosphere lies within 30km
above the earth’s surface.
The atmosphere is divided into regions based on its thermal characteristics
(temperature changes),chemical composition, movement and density. It is divided
into five regions, which are:
·
Troposphere
·
Stratosphere
·
Mesosphere
·
Thermosphere
·
Exosphere
THE COMPOSITION OF
THE ATMOSPHERE
Troposphere
This is the region nearest to the earth’s surface which
extends to an altitude up to 10 km above the poles and 20km above the equator.
It is the most dense part of the atmosphere (80% by mass of the atmosphere)
which contains most of the atmosphere's water vapour.
The temperature in this region decreases with altitude at an
average rate of 6°C/km. It encourages the change of weather(most of weather
phenomenon occur in the troposphere).Clouds and rain are formed within this
region.
The boundary which separates the troposphere from the
stratosphere is called the tropopause. At the tropopause, the temperature stops
decreasing with altitude and becomes constant. The tropopause has an average
height of about 10km.
Stratosphere
It starts from the tropopause and extends to 50km high. It
is more stable, drier and less dense compared to troposphere.
The temperature slowly increases with altitude due to the
presence of ozone layer which absorbs ultraviolet rays from the sun. The ozone
layer lies in the middle of the stratosphere between 20 and 30km. Ozone is
triatomic(three-molecules) form of oxygen.
The stratosphere together with troposphere are collectively
known as the lower atmosphere. The boundary which separates the stratosphere
from the outer layer is called the stratopause.
Advantages of
stratosphere
It absorbs the ultraviolet radiations which would otherwise
reach the earth’s surface which is harmful to both plants and animals.
It prevents large storms from extending much beyond the
troposphere due to its stability. Planes also fly within this layer because it
has strong steady horizontal winds which are above the stormy weather of the
troposphere.
Mesosphere
It starts just above the stratosphere and extends to 85km high.
The temperature at this layer decreases with altitude. The lowest temperature
of the atmosphere occurs within this region(-90°C).
Is the layer where most meteors burn while entering the
earth’s atmosphere. The boundary which separates the mesosphere from the
thermosphere is called the mesopause.
Thermosphere
It is just above the mesopause and extends up to 690km high.
The temperature increases with altitude due to the sun’s heat. The temperature
in this region can go as high as1727°C and chemical reactions occur faster in
this region than on the earth’s surface.
This layer is also known as the upper atmosphere. The lower
part of the thermosphere, from 80 to 550km above the earth’s surface, contains
the ionosphere. This is a region containing a high concentration of charged
particles called ions and free electrons.
Importance of
ionosphere
The large number of free electrons in the ionosphere allows
the propagation of electromagnetic waves. It absorbs the dangerous radiations
like X-rays and extreme ultraviolet(EUV) radiation. It plays an important role
in communition of radio waves.
Exosphere
Is the outermost region of the atmosphere. In this region,
the atmospheric gas pressure is very low such that light atoms such as hydrogen
and helium may acquire sufficient energy to escape the earth’s gravitational
pull.
The upper part of the exosphere is called magnetosphere. The
motion of ions in this region is strongly constrained by the presence of the
earth’s magnetic field. This is the region where satellites orbit the earth.
The Importance of
Various Layers of the Atmosphere
The importance of the atmosphere includes the following:
·
The troposphere controls the climate and
ultimately determines the quality of life on the earth.
·
The troposphere is important for life on earth.
The layer contains gases which include oxygen which is used for respiration by
animals and carbon dioxide which is used by plants in photosynthesis. The
nitrogen found in this laye also provides an inactive environment for many
chemical processes to take place. The gases also support many important
chemical processes such as combustion, weathering and oxidation.
·
The stratosphere prevents harmful ultraviolet
radiation from reaching the earth.
·
The mesosphere, thermosphere and exosphere also
prevent harmful radiation such as cosmic rays from reaching the earth’s
surface.
·
Communication is also made possible by some
layers of the atmosphere, specifically the ionosphere.
The Greenhouse Effect
and Global Warming
The Greenhouse Effect
Explain the
greenhouse effect
Global warming is the increase of the average temperatures
near or on the surface of the earth as a result of what is known as the
greenhouse effect. The effect is caused by greenhouse effect. These gases are
produced from natural and industrial processes.
The greenhouse effect
Refers to the process in which the emission of radiation by the atmosphere
warms the earth’s surface.
When heat from the sun reaches the earth’s surface in form
of sunlight, some of it is absorbed by the earth. The rest is radiated back to
the atmosphere at a longer wavelength than the incoming sunlight. Some of these
longer wavelengths are absorbed by greenhouse gases in the atmosphere before
they are lost to space. The absorption of this long-wave radiant energy warms
the atmosphere.
The greenhouse gases act like a mirror, reflecting back to
the earth some of the heat energy which would otherwise be lost to space.
SOURCES OF GREENHOUSE
Sources of greenhouse effect include:
·
Carbon dioxide
·
Clearing and burning of vegetation
·
Burning of fossil fuel
·
Methane
·
Dinitrogen oxide
·
Chlorofluorocarbons(CFCs)
The Occurrence of
Global Warming
Occurrence of global warming
Global warming is primarily a problem of too much carbon
dioxide (CO2) in the atmosphere—which acts as a blanket, trapping heat and
warming the planet. As we burn fossil fuels like coal, oil and natural gas for
energy or cut down and burn forests to create pastures and plantations, carbon
accumulates and overloads our atmosphere. Certain waste management and
agricultural practices aggravate the problem by releasing other potent global
warming gases, such as methane and nitrous oxide.
The Consequences of Global Warming
State the
consequences of global warming
·
Change in world’s climatic pattern.
·
Acidification of the oceans.
·
Extreme weather events.
·
Effects of Global Warming
·
Increase in the temperature of the oceans.
·
Rise in sea levels.
·
Higher or lower agricultural yields.
·
Melting of Arctic ice and snowcaps. This cause
landslides, flash floods and glacial lake overflow.
·
Increase in the range of disease vectors, that is,
organisms that transmit diseases
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