Friday, March 24, 2017

Class XII, PHYSICS, Chapter # 16 ""

Tuesday, July 23, 2013


Qs. What are the basic postulates of Bohr’s Atomic Thoery?
Neil Bohr studied the spectrum of hydrogen atom. On the basis of his study, he proposed a theory, which is known as Bohr’s Atomic theory.
The important Postulates of Bohr’s Atomic Theory are as follows:
Angular Momentum
Electrons revolve only in those orbits for which its orbital angular momentum is an integral multiple of h/2π, i.e.
L = mvr(n) = nh / 2π
m = mass of electron
V = velocity of electron
r(n) = radius of nth orbit
n = Principal quantum number
h = Plank’s Constant
1. Energy
The total energy of an electron remains constant as long as it remains in the same orbit. i.e. it does not radiate energy while revolving around the nucleus.
2. Energy Release
When an electron jumps from a higher orbit having energy ‘En’ to a lower orbit having energy ‘Ep’ then energy is released in the form of energy ‘hv’ i.e.
Eo – Ep = hv = hc / λ
v = Frequency of Photon
λ = Wavelength of Photon
c = Speed of light
h = Plank’s constant
Qs. Find out the radius, energy and wave number of hydrogen atom with the help of Bohr’s Atomic theory.
A hydrogen atom is the simplest of all atoms. It consist of a proton in the nucleus and an electron revolving around the nucleus.
Consider an electron of charge ‘-e’ revolving in a hydrogen atom around a proton of charge ‘+e’ with constant speed v.
When the electron revolves around the nucleus, then two forces balance its motion.
Coulomb’s Force = F = ke² / r² ——– (I)
Centrifugal Force = F = mv² / r ——- (II)
Comparing eq (I) and (II)
ke² / r² = mv² / r
=> ke² / mv² = r² / r
=> r = ke² / mv² ——– (III)
According to Bohr’s theory, angular momentum is an integral multiple of h/2π
mvr = nh / 2π
=> v = nh / 2π mr
=> 1/v = 2π mr / nh
Taking square of both sides
=> 1/v² = 4π² m² r² / n²h²
Substituting the above value in eq (III)
r = Ke² / m = 4π² m² r² / n²h²
=> r / r² = 4π² m k e² / n²h²
=> 1 / r = 4π² m k e² / n²h²
=> r = n²h² / 4π²m k e²
We know that,
k = 1 /4π Єo => 1 / k = 4π Єo
=> r = n²h² / 4 π² m e² x 4π Єo
=> r = n²h² Єo / π e²
The above equation gives the radius of hydrogen atom.
Radii of Various Orbits
Radius of first orbit of hydrogen atom is calculated by substituting the following values in the equation of radius.
n = 1
h = 6.25 x 10(-34) J.sec
m = 9.1 x 10(-31) kg
k = 9 x 10(9) Nm²/col²
e = 1.6 x 10(-19) col
r = (1)² (6.625 x 10(-34)² / 4² (9.1 x 10(-31) (9 x 10(9)) (1.6 x 10-19)²
=> r = 0.53 x 10(-10)m
=> r1 = 0.53 Aº
For other orbits radius is given by
r2 = (2)² x 0.53 Aº
r3 = (3)² x 0.53 Aº
rn = n² x 0.53 Aº
An electron revolving in the orbit of hydrogen atom possesses kinetic energy as well as Potential Energy. Therefore, total energy is given by
E = K.E + P.E —— (I)
Kinetic Energy
When an electron revolves in the orbit, then coulomb’s force is balanced by centrifugal force
ke²/r² = mv²/r
=> mv² = ke²/r
=> 1/2 mv² = ke²/2r
=> K.E = ke²/2r
Potential Energy
Potential energy is given by
P.E = F.dr
=> P.E = Ke² / r² dr
=> P.E = ke² 1 / r² dr
=> P.E = ke² |-1/r|
=> P.E = -ke² [1/r - 1/∞]
=> P.E = -ke² (1/r – 0)
=> P.E = -ke² / r
Total Energy
Substituting the values of K.E and P.E in eq (I)
E = ke² / 2 – ke² / r
=> E = k2² / 2r
r = n² h²/ 4π² m k e²
=> E = ke² / 2 4π² m k e² / n² h²
=> |E = 2π² m k² e² / n² h²|
The above equation gives the energy of the orbits of hydrogen atom. Negative sign shows that the electron is bound with the nucleus. When energy of the electron becomes positive, then electron will leave the nucleus.
When art electron jumps from higher orbit to inner orbit, then it radiates energy in the form of photons.
Qs. Explain the spectrum of hydrogen atom.
When an electron jumps from a higher orbit to a lower orbit, it radiates energy which appears in the form of a spectral line. A set of such spectral lines is known as hydrogen spectrum. Hydrogen spectrum is the simplest one which consists of five series.
1. Layman Series
When an electron jumps from a higher orbit to the first orbit, Laymen Series (ultra violet region) is obtained.
The wavelength and wave number of Laymen Series can be calculated by
v = R(H) (1/1² – 1/n²)
Where n = 2, 3, 4, ……
2. Balmer Series
When an electron jumps from a higher orbit to the second orbit then Balmer Series (visible region) is obtained.
The wavelength and wave number of Balmer Series can be calculated by
v = R(H) (1/2² – 1/n²)
Where n = 3, 4, 5, ……
3. Paschen Series
When an electron jumps from a higher orbit to the third orbit then Paschen Series (infra red region) is obtained.
The wavelength and wave number of Paschen Series can be calculated by
v = R(H) (1/3² – 1/n²)
Where n = 4, 5, 6, ……
4. Bracket Series
When an electron jumps from a higher orbit to the fourth orbit then Bracket Series (infra red region) is obtained.
The wavelength and wave number of Bracket Series can be calculated by
v = R(H) (1/4² – 1/n²)
Where n = 5, 6, 7, ……
5. Pfund Series
When an electron jumps from a higher orbit to the fifth orbit then Pfund Series (infra red region) is obtained.
The wavelength and wave number of Pfund Series can be calculated by
v = R(H) (1/5² – 1/n²)
Where n = 6, 7, …..
Qs. Write a note on spectra of X-rays. Also write down the properties.
X-Rays were discovered by W.K. Roentgen are also known as Roentgen rays. These rays of shorter wavelength, ranging from 0.1 nm to i nm. X-rays are produced if heavier atoms are bombarded by energetic electrons.
A Filament F and target T are produced in a vacuum chamber and voltage V is applied across the ends. Electrons are produced by heating the filament. These electrons are accelerated towards the metal by applying very high voltage (several thousands volts). When electrons hit the target, then X-rays are produced. There are two types of spectra obtained from this experiment.
1. A continuous spectrum of frequencies or X-rays Brems Strahlung.
2. Characteristics spectrum or a line spectrum of a limited number of fairly definite frequencies.
1. Continuous Spectra
When electrons hit the metal target, a continuous spectrum of frequencies of X-rays is emitted. The frequencies depend upon the accelerating voltage and are very nearly independent of the material of target.
Continuous spectrum is produced when electrons pass close to the atomic nuclei. The are deflected and slowed down due to which they lose their energy. The energy lost by decelerating electrons appears in the form of photon in the X-ray range. The process is represented as
Atoms + e(Fast) —–> Atom + e(Slow) + hv
2. Characteristic Spectra
In the heavy atoms, electrons are assumed to be arranged in concentric shells at increasing distance from the nucleus. The electrons of inner shell are much tightly bound as compared to the electrons of outer shells. Therefore, a large amount of energy is required to displace them Consequently photons of larger energy are emitted when atoms are stabilized. Thus the transition of inner shell electrons gives rise to high-energy spectra or Characteristic spectra. To obtain characteristic spectra, target metal of higher atomic number is used.
The process of emission of characteristic spectra takes place as follows. When a highly energetic incident electrons knocks an electron from the k-shell, a vacancy occurs in that shell. This vacancy is filled by the arrival of an electron from outside the k-shell, emitting excess amount of energy in the form of photon.
If the electrons jumps only one shell and returns with the emission of X-rays to Y shell, then X-rays are termed as ‘Yα’ X-rays. If the electron jumps two shells and returns with emission of X-rays to suppose ‘Y’ shell, then X-rays are termed as ‘Yβ’ rays and so on, where Y may be K, L, M, ……


The nucleus consists of protons and neutrons. A protonis a positively charged particle having mass 1.6726 x 10(-27) kg and charge 1.6 x 10(-19) coulomb. The charge of the proton is equal in magnitude of the charge of an electron, but opposite to it in sign. Neutrons have no charge. Its mass is 1.6750 x 10(-31). The mass of proton is 1836 times the mass of an electron.
The sum of the number of protons and neutrons in a nucleus is called Mass Number.
It is denoted by ‘A’. This number is also called Nucleus Number.
The number of protons in a nucleus is called Atomic Number or proton number or charge number.
It is denoted by ‘Z’.
The difference between mass number and atomic number is called Neutron Number.
It is denoted by ‘N’ and is given by
N = A – Z
An element X having mass number A and atomic number Z is represented by the symbol zXA.
Where X is the chemical abbreviation for the particular element.
The elements having same atomic number but different mass number or neutrons number are called isotopes.
For example hydrogen deuterium and tritium
Hydrogen A = 1, Z = 1, N = 0
Deuterium A = 2, Z = 1, N = 1
Tritium A = 3, Z = 1, N = 2
Qs. Explain the phenomenon of radioactivity.
Henri Bacqural discovered that Uranium atoms (z = 92) emit highly penetrating radiations that could penetrate paper, glass and even aluminium. On the basis of his experimental results, he explained the phenomenon of radiation.
The phenomenon of spontaneous disintegration of nucleus of atoms is known as radioactivity.
Radioactivity is a self-disrupting activity exhibited by some naturally occurring elements. It has been found, that the elements with atomic number greater than 83 are unstable and emit certain type of radiations. Such substances (e.g. Uranium, Radium, Thorium) are called Radio-active substances and the radiations emitted from their nuclei are called radio active radiations and the phenomenon is known as Radioactivity. Rutherford and his co-workers proved that the radiations emitted by a radio active substance are of three different types.
Radio Active radiations can be separated by applying electric or magnetic field to the element. A small amount of radioactive substance is placed at the bottom of a cavity drilled in a block of lead. When the narrow beam of radioactive rays is allowed to pass through the space between the two charged plates, the path of some rays bend. A similar effect is observed in the presence of magnetic field.
Results Obtained
The conclusion that were made fro the experiment are
1. α – Particles
The rays towards the negative plate indicate that they consist of positively charged particles. These were named as α-rays.
2. β – Particles
The rays bending towards the positive plate indicate that they consist of negatively charged particles. These were named as β (beta) rays.
3. γ – Rays
The rays that go undeflected indicate no charge and are therefore energetic photons or γ (gamma) rays.
Properties of α – Particles
1. α – Particles are Helium nuclei. The charge of a α-particle is twice the charge of a proton and its mass is four times than that of a-proton.
2. The speed of α-particles is 1/100 times the speed of light.
3. They produce fluorescence and effect the photographic plate.
4. α – Particles have low penetrating power.
5. They have high ionization power.
6. When a nucleus zXA disintegrates by the emission of an α-particles, its charge number (z) decreases by 2 and mass number (A) decreases by 4.
zXA —-> Z2 (VA.4) + α – Particle
Properties of β – Particles
1. β – Particles are electrons with more energy as compared to ordinary electrons because their origin is nucleus and not the atomic orbits.
2. The speed of β – particles is 1/10 times the speed of light.
3. They produce fluorescence and affect the photographic plate.
4. β – particles have greater penetrating power then α-particles.
5. They have low ionizing power.
6. When a nucleus zXA disintegrates by the emission of β – particle, its charge number (Z) decreases or increases by 1, while mass number remains same.
zXA —-> z+1 γA + -1βº (electron)
zXA —-> z-1 γA + +1βº (positron)
Properties of γ – Rays
1. γ – Rays are energetic photons and have no charge. They are similar to X – rays but more energetic.
2. They travel with the speed of light.
3. The produce fluorescence and affect the photographic plate.
4. Their penetrating power is very high.
5. They do not have any ionization power.
6. When γ – Rays emit out from the nucleus of a radio active substance, then the mass number (A) and charge number (Z) remain same
zXA —-> zXA + γ – Rays
Where zXA represents the nucleus in excited state.
Qs. Define and explain the law of radioactive decay. How can you determine the half life of a radioactive substance with the help of this law?
The rate of decay in a radioactive process is directly proportional to the number of parent nuclides, present in the unstable nuclides of the given species.
Mathematical Form
If ΔN be the number of nuclides disintegrated in time Δt and N be the number of Nuclides at time t, then:
ΔN ∞ N
ΔN ∞ Δt
=> ΔN ∞ NΔt
=> ΔN = -λNΔt
=> ΔN / Δt = -λN
Where is the decay constant and negative sign shows that number of atoms decrease w.r.t
=> 1/N ΔN = – λΔt
=> ΔN / N= – λΔt
It is the time in which half of radioactive elements decays from paront element to daughter element.
It is denoted by T 1/2.
Suppose we have 10,000 radioactive atoms. If in 10 seconds, 50,000 of them decay, then this time is called the half life that radioactivity element.
Qs. Explain nuclear fission reaction also discuss its type.
In 1943, Fermi, Serge and their co-workers studied the phenomenon of nuclear studies the phenomenon of nuclear reactions. On the basis of their experimental results they proposed a remarkable reaction. This was advanced by many scientist and fission reaction was discovered.
The process in which a heavy nucleus breaks up into two lighter nuclei of nearly equal masses after bombardment by a slow neutron is known as nuclear fission.
When an isotope of uranium of 92U235 is bombarded with slow moving neutrons, then fission reactions takes place. During this process two new elements three neutrons and a large amount of energy is released. The two nuclei of new elements produced are Barium and Krypton. The nuclear fission reaction.
Barium and Krypton are known as Fission pigments, which are radio active. A large amount of heat energy is also liberated, which may be produced.
Fission reaction is a chain reaction that has been classified into the following two types.
1. Controlled Fission Chain Reaction.
2. Uncontrolled Fission Chain Reaction.
1. Controlled Fission Chain Reaction
In a fission reaction for one atom of uranium, three neutrons are produced, which may give rise to fission reaction in other uranium atoms. If two neutrons out of three are stopped then chain reaction takes place at uniform rate and a fixed amount of energy is obtained. This is done by usually Cadmium or graphite rods. In a nuclear reactor controlled chain reaction takes place.
2. Uncontrolled Fission Chain Reaction
If in a fission reaction, the number of neutrons is not controlled, then the reaction will build up at a very fast rate and in only few seconds, an explosion occurs. In an atom bomb, uncontrolled fission chain reaction takes place.
Qs. Define and explain the phenomena of Nuclear Fusion.
A process in which two light nuclei combine (or fuse together) to form a heavy nucleus and energy is released is called Nuclear Fusion.
The energy released is called Thermo-Nucleus Fusion Energy.
For example when light nuclei of hydrogen are combined to form a heavier nucleus of helium energy is liberated. The final mass is smaller than the initial mass and the deficit of mass is comparatively greater than in fission. For this reason the energy liberated in the process of fission.
It is very difficult to produce fusion reaction due to the fact that when two positively charged nuclei are bought closer and closer and then fused together. Work has to be done against the electrostatic force of repulsion. This requires a great deal of energy.
Fusion reaction can produce great amount of energy. The raw material 1 the reaction is deuteron, which is found in abundance in world oceans as heavy water.
The fusion reaction is possible in sun and stars because of very high temperature. The fusion reactions are also the basic source of energy in stars including the sun.
This process is called Proton-Proton cycle. In this fusion process the amount of energy released is of the order of 25 MeV.
Another fusion process is suggested by Bethe. It is called Carbon-Nitrogen cycle or simply Carbon Cycle. This process is assumed to occurs in the sun. In this process four protons are converted into an alpha particle with carbon acting as a catalyst in the reaction


Qs. What do you know about Wilson and Cloud Chamber? WILSON CLOUD CHAMBER
Wilson Cloud Chamber is used to observe the path of ionizing particles. It helps to examine the mechanism of ionization of various ionizing radiations and the product of their interaction with material inside the chamber.
It consist of a closed cylindrical chamber with transparent glass top “I” and a movable piston on the bottom. On the sides near the top the cylindrical is provided with a glass window for light and for the ionizing particles or radiations. The piston can be moved up or down by a lever attached to it. Before making the enclosed space above the piston arright, enough quantity of a low boiling point liquid such as water or alcohol is introduced in the space to produce its saturated vapours. A small quantity of the liquid stay on the piston.
The vapours of the liquid usually condense at its dew point but the condensation never takes place in the absence of some particles, dust particles or ions, which are essential to form the nuclei (centres) of condensation. In particle free space the saturated vapour may cool much below the dew point. Then they are called Super Saturated Vapours. Paths, additional information about the charged and uncharged nature, the magnitude of the charge, the charge to mass ration (e/m), etc of the incident particle or the particle found by their interaction with the atoms can be obtained. By this very method a number of particles have been discovered.
Qs. Explain the construction and working of Gelger Counter.
Gelger counter is a portable device which is widely used for the detaction of ionizing particles or radiations.
It consists of a hollow metal cylinder, one end of which is closed by an insulating cap. At the centre of the cap is fixed a stiff straight wire along the axis of the cylinder. A thin mica or glass disc closes the other end which also serves as all entrance window for the ionizing particles or radiations. The sealed tube usually contains a special mixture (air, argon, alcohol etc) at a low pressure of 50 to 100 millimetres of mercury. A potential difference of the order of one thousand volts is applied between the metal cylinder and difference is only slightly less than than, necessary to start a discharge between the wire and a cylinder.
When an ionizing particle enter the tube under this condition if a charged particle pass through the chamber it produces ionization along its track. The condensation of vapours takes place on ion in the form of tiny droplets of fog, which can be photographed.
1. α-Particle
An α-particle is highly ionizing the ions produced are so numerous that its trade is a thick and continuous line.
2. β-Particle
β-Particle is much less ionizing its track is therfore, a thin and broken line.
3. γ – Rays
γ – Rays are photons emitted in a widening cone of some angle. They produce ionization by photoelectric effect distributed on a wide space. Some of the photoelectrons ejected by them give tiny line tracks in directions like the β- Particles and scattered dots are produced. The γ – rays not produce well-defined line track.

Wednesday, July 17, 2013


Qs. What are the basic postulates of Einstein’s Special theory of relativity. Also give the consequences of the theory.
Einstein examine the motion of objects in frames of references moving relative to one another. On the basis of his experimental results he proposed a special theory of relativity in the year 1905. This theory is valid specially for inertial frames and is to be modified into a general theory for accelerated frames of reference.
The Einstein’s special theory of relativity is based on two assumptions known as the postulates of special relativity. The two postulates are states as follows.
First Postulate
The speed of light was regarded as the universal constant. It means that the speed of light in vacuum is the same for all observers in uniform transnational motion and is independent of the motion of the observer and the source.
Second Postulate
According to this postulate the laws of physics in the frame moving with uniform velocity can be expressed by a single set of mathematical expression.
This postulate points out if some event takes place in any of the frame and the frames are moving with uniform velocity the result of the two frames will be identical. Conversely if the frames are in accelerated motion then the result will not be identical.
Einstein proved that energy has inertia, which is the property of matter and associated with mass. Thus mass is simply a property attributed to the total energy of the body and only total energy is required to know total mass of the body. Hence in special theory of relativity total energy and mass are related by the famous Einstein’s equation.
E = mc(2)
From this relation between mass and energy it has been predicted that any process that changed the mass by a detectable amount of energy.
Qs. Write a note on Compton Effect
In 1926, Arthur Compton studies this phenomenon of change in wavelength. On the basis of his experimental results he proposed a theory based on the idea of photon theory of radiation. Since the detailed study of phenomenon was made by Compton, the effect is now known as the Compton’s Effect.
The phenomenon in which a photon (hv) strike with stationary electron and after collision both scattered in different direction in such a way v > v is known as Compton Effect.
In order to explain this phenomenon we assume that photon strike with a stationary electron and after collision both makes an angle θ and ф with respect to their initial line of motion.
Qs. Write note on Pair Production and Annihilation of Matter.

The phenomenon in which photon collides with heavy nucleus then two material particles, electron and positron are produced, is called Pair Production.
The positron produced during pair production has been identified to be identical with an electron in mass and carries an equal positive charge and is called the anti particle of electron. Since the process of pair production involves the creation of particle and its anti particle, therefore it is also known as materialization of energy. This phenomenon is the practical proof of Einstein’s mass energy equivalence, in which mass and energy of the system remains constant.
For the production of electron and positron 1.02 MeV energy is required. I can be calculated by the following equation
Eo = 2moC(2)
=> Eo = 2 x 9.1 x 10(-33) x (3 x 10(8))2 / 1.6 x 10(-19)
=> Eo = 1.02 x 10(6) cV
=> Eo = 1.02 MeV
If energy of photon is less than 1.02 MeV then this phenomenon cannot produce. If energy of photon is greater than 1.02 MeV then rest of energy is used to accelerate the electron and Positron. The energy conservation in Pair Production demands.
hv = e+ + e + K.E + K.E+
=> hv = moc² + moc² + K.E + K.E+
=> hv = moc² + K.E + K.E+
The phenomenon in which electron and positron fuse together to form at least two photons, is known as Annihilation of matter.
Annihilation is the reverse process of pair production. In Pair Annihilation a particle and one of its anti particle come close enough to be converted completely into radiation energy of the two photons moving in opposite direction conserving the total momentum of the creation and annihilation process. Each photon will have an energy equal to rest mass energy moc of an electron that is equal to 0.51 MeV.
The energy conservation equation for the process will be
(mo)e + c² + K.Ee + (mo)e-c² + (K.E_e = 2hv
The phenomenon of Pair Production and annihilation helps us to conclude that energy and mass are inter changeable.
Qs. Write note on Uncertainty Principle
In classical physics we can easily determine the momentum and position of moving body simultaneously with accuracy, that no uncertainties are involved in it. But for a light particle is found that however refined we make our instruments there is a fundamental limitation to the accuracy with which the positron and momentum can be known simultaneously.
This limitation was first expressed by Hersenberg in 1927 and is known as Uncertainty Principle.
It is impossible to measure with accuracy both positron and momentum of a particle simultaneously.
Consider a slit of thickness Δy placed near to a screen. Now a particle bean strikes the slit then after diffraction at very small angle, it reaches at points A.
As we know that momentum is a vector quantity, therefore, it can be resolved into two components. Consider ΔOAB.
tan θ = Perpendicular / Base
=> tan θ P(y) / P(x)
Since θ is very small, therefore
tan θ ≈ θ
=> θ P(y) / P(x)
=> Py = Px θ ——- (I)
From the condition of interference,
mλ = d sin θ
For first maximum,
m = 1
=> λ = Δy sin θ
sin θ ≈ θ
=> λ = Δy θ
=> θ = λ / Δy
Substituting the value of θ in eq (I)
=> P(y) = P(x) λ / Δy
=> P(y)Δy = P(x) λ
From Debroglie’s wave equation
λ = h / P(x)
=> λP(y)Δy = P(x) h / P(x)
=> P(y)Δy = h
P(x)Δx = h
P(z)Δz = h
The uncertainty principle is of no importance in our daily life because plank’s constant h is very small and so the uncertainties in position and momentum of even light objects are far too small to be experimentally observed.
Qs. State and explain Debroglie’s Hypothesis.
In 1924, Debroglie proposed an idea called Debroglie’s Hypothesis.
If light can have particle behaviour then material particles such as electrons and protons etc can also behave in a wave like manner.
Mathematical Form
According to Debroglie’s Hypothesis a particle like electron can possesses a momentum given by
P = mv = h / λ
Where m is the mass of particle. This relation has related the electron a particle and the wave character of a frequency. Thus we can write down the wave length associated with the particle i.e.
λ = h / mv
The Debroglie’s relation was initially developed for the electron but it is valid for all material objects including particles. However for massive materials the associated wavelength is too small to be measured.
Qs. Define and Explain Photoelectric Effect
In 1887, Hertz discovered the phenomenon of emission of electrons. When ultra violet light falls on certain metals. On the basis of his experimental results, he proposed the phenomenon of photoelectric effect.
The emission of electrons from a solid or liquid surface when it is subjected to electromagnetic radiation is known as Photo-electric effect.
Consider a glass tube in which two electrodes are suspended connected to a positive and negative terminal of a battery. A milliammeter is connected in series with the circuit to detect the flow of current.
When ultra violet rays strike the negative plate, then electrons emit. These electrons are repelled by the negative (-) plate and attracted by the positive plate. Hence, current flows in the circuit. The effect is known as Photoelectric effect.
Maximum K.E of Electrons
The maximum K.E. of electrons can be achieved by reversing the polarity of the circuit. When ultra violet rays strike the positive (+) electrode. The kinetic energy possessed by the electrons can be achieved if it is balanced by the voltage. So we increase the voltage to such an extent that no electrons emit out. At this stage K.E. is maximum and can be calculated by
K.E(MAX) = Voe
=> 1/2 mv² = Voe
m = mass of electron
e = charge of electron
v = velocity of electron
Vo = voltage of circuit
Results Obtained
The conclusions that were made from the experiment on Photoelectric effect are
1. Increasing the intensity of source of light increases the number of photoelectrons but not the velocity with which it leaves the metallic surface.
2. For each substance, there is a certain frequency called the threshold frequency below which the effect does not occur.
3. The higher the frequency of incident ray, the greater the K.E of electrons.
Photoelectric effect could not be explained on the basis of classical wave theory, because according to the theory:
There should be no threshold frequency because by that time electrons might escape from the metallic surface by absorbing enough energy.
The velocity of photelectrons should depend upon the intensity of the incident ray rather than the frequency.
Qs. Give Einstein’s explanation of the photoelectrons effect on the basis of quantum theory of radiation.
Albert Einstein was successful in providing an explanation of the photoelectric effect. He proposed his description on the basis of quantum theory of radiation.
Einstein explained the photoelectric effect on the basis of following postulates.
1. An electron absorbs neither one whole photon or it absorbs none.
2. An electron cannot absorb more than one photon.
3. After absorbing a photon, it acquires energy (hv) equal to photon. The energy is either used up in ejecting the electrons or it dissipates within the metal surface.
4. An electron may lose some of its energy before leaving the metal surface and is ejected with a kinetic energy less than hv.
5. If the energy of the photon is less than the energy required to overcome the forces then the electron will not emit.
Mathematical Expression
The energy of the electron is given as
Total Energy = Work Function + K.E
=> hv = фo + 1/2 mv²
фo = hvo
=> hv = hvo + 1/2 mv²
=> hv – hvo = 1/2 mv²
=> h(v-vo) = 1/2 mv²
Since, K.E = 1/2 mv² = Voe and v = c/λ
=> h [c/λ - c/λo] = Voc
=> hc [1/λ - 1/λ] = Voc
The above equation is known as Einstein’s Photoelectric Equation.
Qs. What is a Photo Cell? Also Write its Uses.
The Photocell or photo tube consist of an evacuated glass tube fitted with an anode and a concave metallic cathode of an appropriate surface.
The material of the cathode can be choosen to respond to the frequency range over which the photocell operates.
When light of suitable frequency fall on the cathode photoelectrons are emitted which are attracted by the positive anode and current flows in the external circuit. The current would cease to flow if the light beam is interrupted.
1. Simple Photo Cell
A simple photo cell can be used in any situation where beam of light falling on a cell is interrupted or broken. Examples are given below.
To count vehicles passing a road or items running on a conveyer belt.
To open door automatically.
To operate burglar alarm etc.
2. Photo Conducting Cell
In this cell internal photoelectric effect liberates free charge carrier in a material and its electrical conductivity increases as much as 10,000 times, Its uses are
For detection and measurement of infrared radiations where the wavelength is of the order of 10(-6) m.
As relays for switching on artificial lighting, such as streetlights.
3. Photo Voltaic Cell
Such cells are used as exposure meters to set the aperture of the camera.
4. Other Uses
Photocells are used for the production of pictures in television cameras and the sound tracks on motion pictures. The sound information is stored on the film in the form
Radiant energy comes out in discreat amounts or guanta of energy. The energy E content of each quantum was directly proportional to the frequency v.
Mathematical Form
E ∞ v
=> E = hv ——– (I)
Where h = Plank’s constant = 6.63 x 10(-34) Js. Since,
c = vλ
=> v = c / λ
Thus equation (I) becomes
E = hc / λ
c = velocity of light = 3 x 10(8) m/s.
λ = wavelength of radiation
The energy of ‘n’ photons is given by
E = nλy
n = 0, 1, 2, 3 ………


The moving coil galvanometer is a basic electrical instrument. It is used for the detection or measurement of small currents.
When current flows in a rectangular coil placed in a magnetic field it experience a magnetic torque due to which it rotates through an angle proportional to the current flowing through it.
The essential parts of a moving coil galvanometer are
1. A U-shaped permanent magnet with cylindrical concave pole-pieces.
2. A flat coil of thin enamel Insulated wire (usually rectangular)
3. A soft iron cylinder
4. A scalar lamp and scale arrangement.
In suspended type or D Arsonvals galvanometer the flat rectangular coil of thin enamel insulated wire of suitable number of terms wound on a light non-metallic or non-magnetic (brass or aluminum) frame is suspected between the cylindrical concave pole pieces of the permanent U-shaped magnet by a thin phosphor-bronze strip. One end of the wire of the soil is soldered to strip. The other end of the strip is fixed to the frame of the galvanometer and connected to an external terminal. It serves as one current lead. The other end of the wire of the coil is soldered to a loose and soft spiral of wire connected to another external terminal. The soft spiral of wire serves as the other current lead. A soft iron cylinder, coaxial with the pole-pieces is placed within the frame of the coil but quite detached from it and is fixed to the body of the galvanometer. In the space between it and the pole pieces, where the coil moves freely, the soft iron cylinder makes the field stronger and radial so that the magnetic field is always parallel to the plane of the coil. To note the deflection a concave mirror along with lamp and scale arrangement is used.
When a current passes through the galvanometer coil, it experiences a magnetic deflecting torque, which tends to rotate it from its rest position. As the coil rotates it produces a twist in the suspension strip. The coil rotate until the elastic restoring torque due to which the strip does not equalize and cancel the deflecting magnetic torque and then it attains equilibrium and stops rotating further.
i.e. Deflecting torque = Restoring torque
But deflecting torque = BINA Cos α
BINA Cos α = c0
Where B = strength of the magnetic field
I = current in the coil
A = Area of the coil
N = Number of turns in the coil
θ = Angle of twist of the suspension strip
c = torque per unit twist of the suspension strip for the equilibrium
As c/BNA is constant.
In pivoted type or Weston galvanometer the coil instead or being suspended by a strip is pivoted between two jeweled bearings. The restoring torque is provided two hair springs one on either side of the coil and curling on the opposite sense are connected one to each spring. The hairspring thus also serve as current leads to the coil. A light aluminum pointer is fixed to the coil, which moves over a calibrated circular scale with equal divisions, which measures the deflection directly.
Current Sensitivity of a Galvanometer
A galvanometer is said to be sensitive if for a small current the deflection “θ” is sufficiently large. A galvanometer can be made more sensitive if c/BNA is made small. Thus to increase sensitivity “c” may be decreased or B, N and A may be increased “C” can be decreased by increasing the length of suspension wire or by decreasing its can be decreased by increasing the length of suspension wire or by decreasing its diameter, but this process cannot be taken too far < as the suspension must be strong enough to carry the coil. “N” or “A” cannot be increased because it makes the coil heavy. The loss of sensitivity due to the use of fewer turns is however made us by the very high value of the magnetic field employed.
The current sensitivity of a galvanometer is usually defined as the current in microampere required to cause one-millimeter deflection on a scale place 1.0 meter from the mirror of the galvanometer.
Ammeter is an instrument, which is used for measuring electric current. A galvanometer can measure small current if its scale is calibrated for the current. For the measurement of large current a bypass resistance called a shunt, of appropriate small value is connected in parallel with the galvanometer coil. This resistance allows the large excess current through itself while a fraction of the current passes through the galvanometer coil. The scale of the instrument is so calibrated that it can measure the main current directly.
Consider a galvanometer “G” whose resistance is “Rg” and which gives full scale deflection when current “Ig” flows through it to convert the galvanometer into an ammeter which can measure a maximum current “I” a shunt “Rs” of appropriate small resistance should be connected in parallel with the galvanometer such that the current “Ig” must flow through the galvanometer coil.
The potential difference “Vg” across the galvanometer is given by
Vg = Ig Rg
The potential difference “Vs” across the shunt is given by
Vs = Is Rs = (I – Ig) Rs
Where Is = I – Ig = current through the shunt.
As “Rg” and “Rs” are connected in parallel to each other therefore potential difference across them will be equal i.e.
Vs = Vg
(I – Ig) Rs = Ig Rg
Rs = Ig Rg / I – Ig
Ammeter is always connected in series with the circuit.
Multi Range Ammeter
Sometimes an ammeter has more than one range, which means that it has as many different shunts as the ranges. The desired range is selected by insertion the proper shunt in position. In one type, one end of each shunt is permanently connected to a common terminal while the other end of each is connected through a range to a second common terminal.
Voltmeter is an instrument which is used for measuring potential difference between any two points of a current carrying (or between the two terminals of a source of emf). A galvanometer can be used for measuring a very small Potential Difference. If its scale is calibrated for voltage. For the measurement of large potential difference. A high resistance of the order of Kilo-ohms is connected in series with it. This resistance is commonly known as “Multiplier Resistance”.
Consider a galvanometer “G” where resistance is “Rg” and which deflects full scale for the current “Ig” to convert this galvanometer into a voltmeter measuring a Potential difference upto “V” volts. An appropriate high resistance “Rn” must be connected in series with it such that for the potential difference “V” applied between the ends of the above combination. The current “Ig” must flow through the galvanometer. However the total resistance between the terminal a & b is Rn + Rg.
(Rn + Rg+ Ig = V
Rn + Rg = V / Ig
Rn = V / Ig – Rg
Voltmeter is always connected in parallel with the circuit.
Multirange Voltmeter
Sometimes a voltmeter has more than one range, which means it has as many different resistance as the ranges. The desired range is selected by inserting the proper resistance in position.
We have a common terminal marked (+) and as many other terminals as the ranges. In the other type one terminal is common marked (+) while the different range terminals can be connected by a range switch to the other common terminals.
If four resistances R1, R2, R3 and R4 are connected end to end in order to form a closed mesh ABCDA and between one pair of opposite junctions. A and C cell is connected through a key K1 while between the pair of opposite junctions B and D a sensitive galvanometer “G” is connected through another key K2. The circuit so formed is called a “Wheatstone-bridge”.
In the above bridge if the key is closed first, some current flows through the cell and the resistance R1, R2, R3 and R4. If the key K2 is also closed the current will usually be found to flow through the galvanometer indicated by its deflection. However if the resistance R1, R2, R3 and R4 (or at least one of them) are adjusted, a condition can always be attached in which the galvanometer show no deflection at all i.e. no current passes through it. Then the potential difference between B & D must be zero i.e. B & D must be at the same potential. This implies that
P.d. between A and B = p.d. between A and D
V(AB) = V(AD)
P.d. between B and C = p.d. between D and C
V(bt) = V(Dt)
Since no current flows through the galvanometer the current R1 equals that in R2, say II and the current in R3 equals that in R4 say 12
V(AB) = V(AD)
1(1) R(1) = 1(2) R(3) ——– (I)
V(BC) = V(DC)
1(1) R(2) = 1(2) R(4) ——— (II)
Dividing eq 1 by eq 2
1(1) R(1) / 1(1) R(1) = 1(1) R(1) / 1(1) R(1)
R(1) / R(1) = R(1) / R(1)
Under balanced condition if any three resistance are known then the fourth can be found easily (i.e. wheatstone principle).
The wheatstone principle is used in Meter Bridge, the P.O. box Carey Foster’s Bridge, Callender and Graffite’s Bridge etc.
The Meter Bridge also called slide – Wire Bridge is an instrument based on wheatstone principle. It consists of a long thick copper strip bent twice at right angles. Two small portions are cut off from it near the bends to provide the gaps across which two resistances are known one and an unknown may be connected. Each of the three pieces of the strip is provided with binding screws. A uniform wire (of magnetic or other) one meter long and of fairly high resistance is stretched, along the side a meter scale is connected to the ends of the strip.
For measuring an unknown resistance “X” it is connected in one gap of the Meter Bridge and a standard resistance box “R” is connected in the other gap. A cell and a galvanometer are connected. The jockey “J” is moved along the wire to obtain the balance point D. Under balanced condition if the length of the wire segment. A D toward X is Lx and the length of the wire segment CD towards R is L(R) then their resistances are ρL(R) respectively.
Where ρ = resistance per unit length of the wire.
Post Office Box is an instrument, which is based on wheatstone principle. It was first introduced for finding resistance of telegraph wires and for fault – findings work in the post and telegraph office that’s why it is called “Post Office Box”. It is more compact and easier to use.
It consists of three sets of resistances P, Q and R. The arms P and Q called the ratio arms, usually consists of three resistances each viz. 10, 100 and 1000 ohms so that any decimal ratio from 1:100 to 100:1 may be used. The third arm “R” is an ordinary set of resistances. The unknown resistances “X” to be measured forms the fourth arm. Introducing the ratios 1:1, 10:1, 100:1 in turn the balance or null position is traced by adjusting “R”. Balance is usually obtained at the ratio 100:1 for some value of “R”. With this value of “R” the value of X can be easily be calculated using relation of Wheatstone bridge i.e.
P/Q = R/X
X = R Q/P
The ohmmeter is a device used for the measurement of resistance. It consists of a sensitive galvanometer “G”, adjustable resister “R” and a torch cell “E” connected in series between two terminals A and B. The unknown resistance “X” to be measured is connected between the terminals A and B. The resistance R is so chosen that when the terminals A and B are short circuited (i.e. X = 0). The galvanometer gives full-scale deflection when no connection is between A and B (i.e. X = ∞). The galvanometer shows zero deflection for the value of X between = and ∞. The deflection is small or large depending on the value of X. The scale of the galvanometer is calibrated with different known values of X and there the circuit serves as an ohmmeter to measure any unknown resistance approx. The scale of the ohmmeter however is not linear.
Using different conditions of R is series and different shunts across the galvanometer worked by range switches, the ohmmeter can be adopted for different accessories for e.g. 1 Ω accuracy in tens of ohms, in hundreds of ohms, in thousands of ohms, in mega ohms etc. Ohmmeter is not a very accurate instrument.
Potentiometer is device for measuring the p.d (voltage) between two points of a circuit or the e.m.f of a current source. It consists of a uniform wire stretched on a wooden board along a meter scale.
Consider a uniform resistance wire AB of length L and Resistance R, across which is connected to a source of constant EMF (e.g. an accumulator) through a key and a rheostat to adjust and maintain a constant current 1 through it.
As the current flows, the P.d. between A and B = V(AB) = IR
If one terminal of a wire is connected to A while other is moved on the wire AB then instrument acts as a Potential Divider.
To find an unknown EMF of a cell or some other potential difference or the ratio of the emf of two cells consider the circuit. The positive terminals of a cell of unknown EMF “E(N)” and a standard cell of Emf E(N) are connected to the terminal A. The negative terminals of both the cells are joint to the jockey through a two way key and a sensitive galvanometer. Using the two-way key first cell E(N) only is introduced into the galvanometer branch and balanced point C and length L are found for it.
An Avo-meter is an apparatus which is used to measure current, voltage and resistance in other words it is an ampere, volts and ohms. It can measure direct as well as alternative voltage and currents. It consists of a galvanometer with different scales graduated in such a way that all the three quantities can be measured. A selector-cum-range switch is provided. Its has its own battery. A rectifier is also included in the instrument to convert A.C. into D.C. before they pass through the Galvanometer.


It was discovered by Oersted that when current masses through a conductor, magnetic field is produced. This field is known as “Magnetic Field of Induction” and is denoted bu “B”.
Ampere found that when two current carrying conductors are near each other, they experience force at each other. If the current is in the same direction the force is attractive and if the current is in opposite direction.
When electric charges are at rest they exert electrostatic force of attraction or repulsion on each other. When the charges are in motion they exert electric as well as magnetic force on each other because and isolated moving positive and negative charge create both electric and magnetic field.
Magnetic Field is a space or region around a magnet or current carrying coil of wire where its effect can be felt by small compass needle. Magnetic field of induction can be visualized by magnetic lines of induction.
A line of induction is an endless curve, which can be traced by a compass needle.
The total number of magnetic lines of induction passing through a surface is called magnetic flux.
The charge to mass into of an electron was determined by Sir J.J. Thomson by an apparatus which consists of a highly evacuated pear shaped glass pulls into which several metallic electrodes are sealed.
Electrons are produced by heating a tungsten flament F by passing a current through it. The electrons moving sideways are also directed towards the screen by applying negative potential on a hollow cylinder C open on both the sides surrounding the filament. Electrons are accelerated by applying positive potential to discs A and B. If V be the total total P.d between the disc Band the filament F taken then Kinetic Energy.
The beam strikes the screen coated with zinc sulphide after passing through the middle of the two horizontal moetal P’P and a spot of the light produced at O on the screen where the beam strikes and its position is noted.
A magnetic field of induction B is produced in between the plate directed into the paper. The magnetic field is produced by two identical current carrying coils placed on either side of the tube at the position of plates.
The force due to the magnetic field on the moving electron makes them move in a curved path and the light spot shifts from O to O on the screen there from of magnetic field acts as centripetal force
e. V B = mv2 / r
e/m = V/Br ——– I
e/m can be computed if the radius r and the expression of the circular path are in which the beam moves in the field region is determined. The radius r is calculated from the shift of the light spot i.e. r = 3.
A better method of determined V is as under. An electric field E is produced between the plates by applying suitable potential difference to exert a force “Be” on the electron opposite to that due to the magnetic field.
The potential diff. VI is so adjusted that two fields neutralize each other effects and the spot come back to its initial position O. Thus each other effects and the spot come back to its initial position O. Thus
Ee = Be V
V = E/B —– (II)
Where E = V1 / d
d = distance between the plates.
Putting the value of V from eq 2 in 1
e/m = E/B2r
e/m = K75888 x 10(11) e/kg
According to this law the sum of the product of the tangential component of the magnetic field of indaction and te length of an element of a closed curve taken in a magnetic field is μo times the current which passes through that area bounded by the curve.
Consider a long straight wire carrying a current 1 in the direction. The lines of force are concentric circles with their common centre on the wire. From these circles consider a circle of radius r. The magnitude of the magnetic field at all points on this circle and inside the circle is same.
Biot and Savart experimentally found that the magnitude of the field depends directly on twice the current and inversely proportional to the distance r from the conductor.
A solenoid is a coil of an insulated copper wire wound on a circular cylinder with closed turns. When current passes through it, magnetic field is produced with is uniform and strong inside the solenoid while outside it the field is negligibly weak.
Consider a solenoid through which the current 1 is passing in order to determine the magnetic field of induction B at any point inside the solenoid imagine a closed path “abcda” on the form of a rectangular. The rectangular is divided into four elements of length L1, L2, L3, L4. L1 is along the axis inside the solenoid and L3 is far from the solenoid.
By applying amperes circuital law
B L1 + B. L2 + L2 + B. L3 + B. L4 = μo x current enclosed —– (I)
Since B. L1 is parallel inside the solenoid
B. L1 = BL4 cos 0 = BL4
The field is very weak outside the solenoid is very weak and therefore it can be negnected thus
B. L3 = 0
As B is perpendicular to L2 and L4 inside the solenoid therefore
B. L2 = BL2 cos 90 = 0
B. L4 = BL4 cos 90 = 0
substitute the above values is eq 1
B. L1 + O + 0 + 0 = μo x current closed
B. L1 = μo x current enclosed ——- (II)
If there are n turns per unit length of the solenoid and each turn carries a current I will be “n L1I”
A Toroid or a circular solenoid is a coil of insulated copper wire wound on a circular core with close turn. When the current passes through the toroid, magnetic field is produced which is strong enough inside while outside it is almost zero.
Consider a toroid that consists of N closely packed turns that carry a current I. Imagine a circular curve of concentric the core.
It is evident form of the symmetry at all points of the curve must have the same magnitude an should be tangential to the curve at all points. Divide the circle into small elements each of length ΔL is so small that B and ΔL are parallel to each other.
By amperes law
Σ B : ΔL = μo x current enclosed
ΣB ΔL Cos 0 = μo x current enclosed
ΣB ΔL = μo x current enclosed
BΣ ΔL = μo x current enclosed
Σ ΔL = 2 π r
B 2 π r = μo x current enclosed ——– (I)
If the circular path 1 is outside the core on the inner side of the toroid if enclose no current. Thus eq 1 become
B 2 π r = μo x 0 = 0
B = 0
If the circular path 2 is outside the core on the outer side of the toroid each turn of the winding passes twice through the area bounded by this path carrying equal currents in opposite directions thus the net current through the area is zero hence eq 1 becomes
B 2 π r = μo x 0 = 0
B = 0
If the circular path 3 is within the core the area bounded by the curve will be threaded by N turns each carrying 1. Thus Current enclosed = NI
Therefore eq 1 becomes
B 2 π r = μo NI
B = μo NI / 2 π r
The phenomenon in which an Emf is set up in a coil placed in a magnetic field whenever the flux through it is changing is called ELECTROMAGNETIC INDUCTION. If the coil forms a part of a closed circuit the induced Emf cases a current to flow in the circuit. This current is called INDUCED CURRENCY.
The magnitude of induced emf depends upon the rate at which the flux through the coil charges. It also depends on the number of turns on the coil.
The magnetic flux through a circuit can be changed in a number of different ways. By changing the relative position of the coil w.r.t to a magnetic field or current bearing solenoid.
By changing current in the neighbouring coil or by changing current in the coil itself.
By moving a straight conductor in the magnetic field in such a way that it cut the magnetic lines of force.
The product of number of turns N and the flux ф through each turn of the coil is called flux linkage i.e.
Flux Linkage = N ф
A Emf is induced in a coil through which the magnetic flux is changing. The Emf lasts so long as the change of flux is in progress and becomes zero as soon as the flux through the coil becomes constant or zero.
Consider a coil through which an electric current is flowing. Due to this current magnetic field will be produced which links with the coil itself. If for any reason the current changes the magnetic flux also changes and hence an Emf is induced in the coil this phenomenon is known as SELF INDUCTANCE. In accordance with Lenz Law, the emf posses the change that has induced it and it is therefore known as back emf.
If the current is increasing the back emf opposes the increase. If the current decreasing it opposes the decrease.
The back emf is directly proportional to the rate of change of current. If ΔL change in current Δ t then back emf E is given.
e = L Δl / Δt ——- (I)
Where L = self inductance of the coil.
The measure of the ability of a coil to give rise to a back emf is called the Self inductance. Its value depends on the dimensions of the coil, the number of turns and the permeability of the core material. Its unit is henry.
The self inductance of a coil is 1 Henry if the current varying through is at the rate of 1 amp/sec, induces a back emf of 1 volt.
If N be the number of turns in the coil and Δ φ be the change of flux in time Δ t then by Faraday’s Law.
Є = -N Δφ / Δt —– (II)
-N Δφ / Δt = – Δl / Δt
N Δφ = L Δl
Δ (Nφ) = Δ (Ll)
Nφ = L1
Consider two coils close to each other. One coil is connected to a source of emf and the other with a galvanometer. The coil which is connected to the emf is called the primary coil and the other is called secondary coil. Some of the magnetic flux produced by the current in the primary coil is changed the magnetic flux in the secondary coil also changes and hence an emf is induced in the secondary this phenomenon is called mutual induction.
The back emf “ξ” induced in the secondary coil is directly proportional to the rate of change of current Δ1 / Δt in primary coil and is given by
Є2 = -M ΔI / Δt ——– (I)
Where M is the mutual inductance of the pair of coils. Its value depends upon the number of turns of the coil, their cross-sectional area, their closeness and core material. Its unit is Henry.
If N2 be the number of turns in the secondary and Δф / Δt be the rate of change of flux in it then by faraday’s law.
Є2 = -N2 Δφ2 / Δt —— (II)
Comparing 1 and 2
-N2 Δφ2 / Δt = – M Δ1 / Δt
N2 Δφ2 = M Δ1
Δ(N2 φ2) = Δ(M 1)
N2 φ2 = M 1
Non-Inductive Winding
In bridge circuits such as used for resistance measurements self inductance is a nuisance.
When the galvanometer key of bridge is closed the current in the arms of bridge are re-distributed unless the bridge happens to the balanced. When the currents are being re-distributed these are changing and self induction delays the reading of new equilibrium. Thus the galvanometer key thus not corresponds to steady state which the bridge will eventually reach. Its me therefore be misleading.
To minimize their self inductance coils of the bridge and re-resistance boxes are so wound as to setup extremely small magnetic field.
The wire is doubled back on itself before being coiled.
In this type of winding current flows in opposite direction in the double wires and consequently the magnetic field and hence the magnetic flux setup by one wire in neutralized by that due the other wire. Hence self induced emf will not be produced when the current through the circuit changes.