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Nuclear Radiations - Question Answers - Physics XII

Q.1: Explain how you would test whether the radiation from a radioactive source is α, β or Gamma radiation?

Ans: When radiations are allowed to pass through a magnetic field, the α and β particles are deflected while γ-rays pass through undeflected. This technique helps to identify the radiation.

Q.2: A particle which produces more ionization is less penetrating. Why?
Ans: When a particle ionizes an atom, it loses a part of its energy. Since the greater the ionizing power, the greater is the loss of energy; and hence, the smaller is its penetrating power.

Q.3: It is said that α or β particles carry an atom without colliding with its electrons. How can each do so?
Ans: An α-particle is positively charged and a β particle is negatively charged. So an α particle ionizes an atom by attraction while a β particle ionizes an atom by repulsion.

Q.4: In how many ways can Gamma rays produce ionization of the atom?
Ans: Gamma rays only ionize an atom by collision. Being a high-energy photon, it can produce ionization in three ways:
i. it may lose all its energy in a single collision with the electron of an atom (photoelectric effect);
ii. it may lose only a part of its energy in a collision (Compton effect);
iii. it may be stopped by a heavy nucleus giving rise to electron-position pair (materialization of energy).

Q.5: In what way does a neutron produce ionization of an atom?
Ans: A neutron collides with a substance containing a large number of hydrogen atoms and knocks out a proton. In this way, it causes ionization.

Q.6: Name different electromagnetic radiations that are capable of producing ionization of atoms. By what process do they ionize?
Ans:
i. Ultraviolet rays
ii. X-rays
iii. Gamma rays

The rays interact with matter inelastically. They remove electrons from the atoms of the target material.

Q.7: Why is lead a better shield against α, β, and gamma radiations than an equal thickness of a water column?
Ans: α and β particles do not travel far enough in water due to intense ionization they produce. Reduction of gamma rays' beam intensity is a measure of its range, which is considerably more. However, materials having large numbers of electrons per unit volume are more effective absorbers of gamma radiations. When gamma rays are incident on lead, then, because of the photoelectric effect, they lose their energy in a single encounter and travel only a small distance. But as water has fewer electrons than lead, so gamma rays lose less energy and penetrate through a larger distance in water. Hence, lead is a better shield against gamma rays than water.

Q.8: Lead is heavier and denser than water. Yet water is more effective as a shield against neutrons?
Ans: To be stopped or slowed down, a neutron must undergo a direct collision (elastic) with a nucleus or some other particle that has a mass comparable to that of the neutron. Water contains hydrogen. Thus nuclear protons of hydrogen atoms, after collision, move; while the neutron is slowed down. But when neutrons collide with the nucleus of lead, it bounces neutrons back almost with the same speed. Hence, water is a better shield against neutrons than lead.

Q.9: In an X-ray photograph, bones show up very clearly, but the fleshy part shows very faintly. Why?
Ans: X-rays can be stopped by bones, but they can penetrate flesh.

Q.10: In a cloud chamber photograph, the path of an α particle is a thick and continuous line, whereas that of a β particle is a thin and broken line. Why?
Ans: An α-particle is highly ionizing than a β-particle.

Q.11: Why do gamma rays not give line tracks in the cloud chamber photograph?
Ans: Gamma rays do not produce ionization directly. They interact with atoms to eject electrons. These electrons, like β particles, produce irregular cloud tracks of their own, which branch out from the direction of gamma rays.

Q.12: A neutron can produce little ionization. Is there any sure chance of getting a cloud chamber track for it to count in the Geiger counter?
Ans: Neutrons are unable to ionize a gas. However, ionization is only produced when a neutron strikes directly a nucleus or a hydrogenous material, e.g., body tissues. The knocked-out proton produces ionization in the Geiger counter.

Q.13: A cloud chamber track of an α particle sometimes shows an abrupt bend accompanied by a small branched track. What could possibly be the cause of this forked track?
Ans: When an α-particle strikes a nucleus, the recoiling nucleus leaves a track. This is the cause of a forked track.

Q.14: Why is the recommended maximum dose for radiation a bit higher for women beyond the childbearing age than for young women?
Ans: It has been found that ovary and grown follicular cells are most sensitive cells for radiation. But primordial follicles and oocytes are more radiation repellent, and they grow even after irradiation. Also, the fertility of ovary is much affected when the whole body is irradiated by a specific dose of radiation (e.g., 200 RAD) than when ovary alone is irradiated by the same dose.

Q.15: It is possible for a man to burn his hand with x- or γ-rays so seriously that he must have it amputated and yet may suffer no other consequence. However, a whole-body x- or γ-ray overexposure so slight as to cause no detectable damage might cause birth deformity in one of its subsequent children. Explain. Why?
Ans: The damage to body cells, caused by very high doses of radiation, can be as serious as to stop them from working and multiplying. Widespread damage of cells may kill people. Delayed effects, such as cancer, leukemia, deformity, and mental retardation in children and grandchildren, may take place due to genetic syndromes.

Q.16: Which of α, β, and γ rays would you advise for the treatment of (i) skin cancer (ii) the cancer of flesh just under the skin (iii) a cancerous tumor deep inside the body?
Ans:
i. For the treatment of skin cancer, we use α-particles, as their penetration is small.
ii. For the treatment of cancer of flesh just under the skin, β-particles should be used because of their medium penetration power.
iii. For the treatment of deep infection in the body, γ rays should be used, as they are highly penetrating.

Q.17: Two radioisotopes of an element are available: one of long half-life and the other of short half-life. Which isotope is advisable for the treatment of a patient and why?
Ans: For the treatment, radioisotopes of short half-life should be used so that any material remaining in the body quickly decays away.

Q.18: Why are many artificially prepared radioisotopes of elements rare in nature?
Ans: Many artificially prepared radioisotopes of elements are rare in nature because of their extremely small half-life.

Q.19: Can radiocarbon dating be used to measure the age of stone walls of ancient civilizations?
Ans: No, radiocarbon dating cannot be used to measure the age of stone walls. "Carbon-14 clock" can be used for organic archaeological samples (i.e., matter that was once living). However, a "uranium clock" can be used for this purpose.

Q.20: How can a radioisotope be used to determine the effectiveness of a fertilizer?
Ans: When P-32 is given to a plant mixed with water, the amount of the chemical absorbed by various parts of the plant is checked by a G.M. counter. This technique helps to find the exact amount of the fertilizer required.

THE ATOMIC NUCLEUS

Chapter – 19

Q.1: How many neutrons and protons do the following nuclei contain?

Nuclide	Protons	Neutrons
$^{27}_{13}Al$	13	$27 - 13 = 14$
$^{40}_{18}Ar$	18	$40 - 18 = 22$
$^{138}_{56}Ba$	56	$138 - 56 = 82$
$^{207}_{82}Pb$	82	$207 - 82 = 125$
$^{28}_{14}Si$	14	$28 - 14 = 14$
$^{238}_{92}U$	92	$238 - 92 = 146$

Q.2: Do $\alpha$ , $\beta$ , and gamma rays come from the same element? Why do we find all three in many radioactive samples?
Ans: A radioactive element either emits $\alpha$ -particles or $\beta$ -particles, but never both. Gamma radiations generally accompany $\beta$ -emission and, in some cases, with $\alpha$ -emission.

A radioactive element (or sample) is a mixture of various nuclides of different relative abundances and with different modes of disintegration. Hence, we can find all the three types of radiations in a radioactive sample at the same time. For example, R-226 is an $\alpha$ -emitter, but Ra-25 is a $\beta$ -emitter.

Q.3: It is more difficult to start fusion reaction than fission. Why?
Ans: Fission is caused by captured neutrons by heavy nuclei. Neutrons, being electrically neutral, are highly penetrating particles for nuclei. But in fusion of two light nuclei, the positively charged nuclei are repelled by the repulsive forces. So work has to be done against the repulsive forces of the two nuclei.

Q.4: Is it possible that fusion of two small nuclei may occur without collision at extremely high energy?
Ans: No. Two nuclei must collide with sufficient kinetic energy to penetrate their mutual “Coulomb Barrier” and come within the range of the nuclear forces.

Q.5: Explain how a nuclear reactor produces heat as a result of fission?
Ans: In fission, the difference of binding energies of reactants and products is converted into energy. The difference of mass (0.22u) appears as energy (200 MeV). If fission takes place in a bulk solid, most of the disintegration energy appears as an increase in the internal energy of the solid, which shows a corresponding rise in temperature. This thermal energy is carried away to the heat exchanger by circulating the coolant through the reactor.

Q.6: What are the benefits and risks of nuclear reactors? Which reactor is relatively better from the point of safety?
Ans: Nuclear reactors are used to produce (i) electricity, (ii) nuclear fuels, and (iii) radioisotopes. These are peaceful uses of nuclear energy; the reactor fuel is clean burning and relatively easy to transport.

The risks of reactors include the possibility of safety hazards for the workers, environmental damage near the plant, the problem of storing highly radioactive wastes, and a limited supply of raw materials. Nuclear reactors have built-in safety devices. The accidental problems, such as leakage of radioactive substances, could occur if safety features malfunctioned. Pressurized water reactors, using water as a moderator and coolant, are safer with shut-off control rods and liquid "poison."

Q.7: Both fission and fusion apparently produce energy. How can you reconcile this with the law of conservation of energy?
Ans: In fission of U-235 with thermal neutrons, the loss of mass (0.2153 u) is converted into energy, producing about 200 MeV per fission.

In fusion, four protons may be combined to produce one helium nucleus and two positrons. Here, the losses of mass (0.027 u) are converted into energy, producing about 26 MeV. Thus, in both cases, the total "mass-energy" remains conserved.

Q.8: When a photon disappears in producing an electron and a positron, is the energy of a photon equivalent to that of the particles produced? Explain.
Ans: No, the energy of the photon is always greater than the rest mass energy of elements (electron and positron pair, 1.02 MeV). The surplus energy is taken by the two particles as their kinetic energy.

Q.9: When a neutron decays into a proton and an electron, there would be a loss of mass. What would be the energies of the products and their relative directions of motion?
Ans: Neutron is not a stable particle outside nuclei. It decays into a proton, an electron, and an antineutrino. The half-life of the free neutron is 10.8 min.

$^0n^1 \rightarrow ^1p^1 + e^0 + \nu$

Mass of neutron = 1.008649 u = 939.58 MeV
Mass of proton = 1.0072766 u = 938.23 MeV
Mass of electron = 0.000549 u = 0.511 MeV
Mass of proton + electron = 1.0078256 u
Loss of mass = 1.0086469 - 1.0078256 = 0.0008393 u = 0.78 x 18 MeV

These would be the energies of the products. Due to their kinetic energies, the two particles will move apart (and not be attracted).

Q.10: Why do most moderators, used in nuclear reactors, are light atoms like $H^1, H^2,$ $C^{12}$ slow down the neutrons, and hence they are slowed?
Ans: Fast moving neutrons can be stopped when they make elastic collisions with stationary particles of the same mass. Since the mass of protons, deuterons, or graphite nuclei is comparable to the mass of neutrons, hence they are slowed.

Q.11: Can a conventional fission reactor ever explode like a bomb does? Why?
Ans: In a nuclear reactor, a fission explosion is not possible because the amount of fuel (e.g., U-235 or Pu-239) is of sub-critical mass and it can shut off control rods in emergencies. Also, liquid "poison" can be inserted directly into the moderator if other safety devices fail.

Q.12: In LMFBR, would you expect the radioactivity of the sodium coolant to include the life time of the reactor?
Ans: Yes, because sodium can capture neutrons.

$^{11}\text{Na}^{12} + ^0\text{n}^1 \rightarrow ^{11}\text{Na}^{24} + \gamma$

Here, Na-24 is radioactive (beta and gamma emitter) with a half-life of 15.0 h.

Q.13: Consider a sample of 1000 radioactive nuclei with a half-life T. Approximately, how many will be left after a time 3T?
Ans: The number of nuclei decayed in one half-life (T = T) are 500. Also, the number of nuclei that decay in three periods of half-life are $1000/2^3$ . Hence, the number of nuclei left undecided is 125.

Q.14: What is the condition for “critical mass?”
Ans: If the mass of fissile material is such that the multiplication factor k > 1, then fission is said to occur in a critical mass. The multiplication factor is the ratio of the number of neutrons in any particular generation to the number of neutrons in the preceding generation. In a reactor, it is slightly above 1; but in a fission bomb, it is about 2.5.

Q.15: Why is heavy water more efficient as a moderator than ordinary water?
Ans: Heavy water (D₂O) has a much lower probability of capturing neutrons, but it can slow down neutrons. In fact, heavy water is 1600 times more efficient as a moderator than ordinary water (H₂O).

Q.16: In LMFBR, why is water not used as a coolant instead of liquid metal?
Ans: If water is used as a coolant in LMFBR, it slows down the neutrons through collisions and hinders the process of breeding (which requires less neutrons to convert U-238 into Pu-239). Also, the probability of capturing neutrons for water is high. Moreover, high pressure is needed to stop vaporization of water, and the core is heated up.

Sodium is a solid at room temperature but becomes liquid at 98°C. Hence, there is no need to pressurize the reactor to keep the sodium from vaporizing. Sodium is highly valued for thermal conductivity and heat transfer coefficient.

Q.17: Why are breeder reactors a necessity?
Ans: The world’s deposit of fossil fuels may not last more than 500 years, and nuclear fuels may not last for more than 5000 years. So, reactors that generate more nuclear fuels than they consume—breeder reactors—are a necessity.

THE ATOMIC SPECTRA

Q.1: The Bohr’s theory of hydrogen atom is based upon several assumptions. Do any of these assumptions contradict classical physics?
Ans: The assumption in Bohr’s theory that an electron moving around the nucleus in a certain orbit does not radiate energy is contrary to classical electrodynamics.

Q.2: Why does the hydrogen gas produced in the laboratory not glow and emit radiations?
Ans: A spectrum is given by the light emitted from an incandescent gas or vapour e.g., electric discharge through a gas or hydrogen-filled discharge tube.

Q.3: Why are the energy levels of the hydrogen atom less than zero?
Ans: The energy levels of the hydrogen atom are negative. This shows that the electron is bound (not free). Thus, one must do work (or expend energy) to remove it from the atom.

Q.4: If the hydrogen gas is bombarded by electrons of energy 13.6 eV, would you expect to observe all the lines of the hydrogen spectrum?
Ans: If a hydrogen atom is bombarded by electrons of energy 13.6 eV, it gets ionized; because 13.6 eV is the ground state energy, which is equivalent to the ionization energy. As such, no spectral lines of hydrogen will be observed.

Q.5: Hydrogen gas at room temperature absorbs light of wavelengths equal to the lines in the Lyman series but not those in the Balmer series. Explain?
Ans: Hydrogen gas at room temperature contains electrons in the ground state (p=1). If the energy supplied to the electron is such that the electron is lifted from its ground state to one of the higher allowed orbits, the atom will be excited, and it will absorb energy equal to the difference of the energies of the electron in the two states. Thus, light of wavelength equal to the lines in the Lyman series will be absorbed.

Q.6: How are x-rays different from visible radiations?
Ans: Both x-rays and visible radiations are electromagnetic waves, but x-rays differ from the visible radiations in the following features:
i. X-rays are highly penetrating. They can pass through many opaque solids such as wood or flesh but are stopped by bones and metals. Hence x-ray photographs are used in medicine.
ii. They cause ionization in gases.
iii. They can eject photoelectrons on striking some metals.
iv. They produce fluorescence in many substances like zinc sulphide, barium platinocyanide, etc.
v. They can damage living tissues if exposed to them for a longer duration.

Q.7: What property of x-rays makes them so useful in seeing otherwise invisible internal structures?
Ans: In solids, the atoms are grouped together in a regular manner. The interatomic distance in a crystal is of the order of the wavelength of x-rays. Hence a crystal is used as a 'transmission grating' to produce diffraction of x-rays. This x-ray crystallography has helped to locate the internal structure of crystal systems (called basic unit cells). Recently developed internal imaging devices (for the human body) include CT (computerized tomography) scanning, MRI (Magnetic Resonance Imaging), and PET (Positron Emission Tomography).

Q.8: Explain the difference between laser light and light from an incandescent lamp (or bulb)?
Ans:

Laser Light	Incandescent Light
i. Laser light is highly monochromatic.	i. Light from an incandescent bulb is a mixture of several wavelengths.
ii. It consists of parallel waves in a narrow beam and is highly directional (i.e., moves straight without spreading).	ii. It is emitted in all directions and spreads out.
iii. It is produced due to stimulated emission of radiation.	iii. It is produced due to spontaneous emission of radiation.

Q.9: Does the light emitted by a neon sign constitute a spectrum or only a few colors? Explain?
Ans: The luminous neon in a discharge tube has a reddish color. Its spectrum is composed of a few colors (line spectrum) of wavelength, very close to each other. So the spectral lines are closely spaced to form a band spectrum.

Q.10: Suppose you are given a glass tube having two electrodes sealed on both ends. The inside is either hydrogen or helium. How can you tell which one it is without breaking the tube?
Ans: The electrodes are connected across a voltage source. If the voltage is gradually increased, then the hydrogen-filled tube will become luminous first because its ionization potential is four times less than that of helium.

The gases can also be differentiated by taking the spectrum of each other.

Q.11: The hydrogen atom contains only a single electron and yet the hydrogen spectrum contains many lines. Why is this so?
Ans: The atoms of hydrogen can be excited to different energy levels. The excited electrons will not stay there. These will jump to the inner orbits. On de-excitation, an electron does not necessarily return to the ground state in a single jump. Rather, it may return by several jumps. Thus, several spectral lines of different frequencies are emitted, depending upon the differences of energies between the levels for the transitions. So, the spectrum of hydrogen contains many lines.

Q.12: The electron in a hydrogen atom requires energy of 10.2eV for the excitation to a higher energy level. A photon and an electron, each of energy 10.5eV, are incident on the atom. Which of these can excite the atom? Give an explanation in support of your answer.
Ans: To excite an electron, energy can be supplied to the electron by direct collision with accelerated particles as well as by the photons of energy hv. The energy of a photon must be exactly equal to the excitation energy (10.2 eV) for the bound electron; otherwise, it will not be absorbed (since it cannot transfer its energy in parts).

On the other hand, accelerated particles can give energy to the bound electron in full as well as in part. Hence an electron 10.5eV (a little higher than the excitation energy of 10.2 eV) can excite the hydrogen atom.

Q.13: Describe the atomic processes in the target of an x-ray tube whereby x-ray continuous spectra and characteristic spectra are produced?
Ans: The x-rays produced by an x-ray tube consist of two parts:
i. A series of un-interrupted wavelengths having a short cut-off wavelength (Xm) are produced when high-velocity electrons are decelerated by a heavy nucleus. This constitutes a continuous spectrum of photons, including x-rays. This process is called 'bremsstrahlung' (German for breaking radiation).
ii. A number of distinct and discreet wavelengths which constitute line (or discontinuous) spectrum of the x-rays are produced when electrons are dislodged from the inner most orbits, followed by electron jumps from the outer orbits. So characteristic spectra result from transitions to a 'hole' in an inner energy level.

Q.14: Explain clearly why x-ray emission lines in the range of 0.1nm are not observed from an x-ray tube when a low atomic number metal is used as the target in the tube?
Ans: For the production of most energetic x-rays, the electrons must be raised from deploying energy levels of the target atoms and certain electrons of innermost shell must be knocked out. The target metal with low atomic number will have x-rays of larger wavelengths. Hence, emission lines of x-rays in the range of 0.001 - 1 nm are not observed.

Q.15: Why do the frequencies of characteristic x-rays depend on the type of the material used for the target?
Ans: The transitions for the emission of characteristic x-rays depend upon the nature of the target material atoms, because frequency of x-rays (v) depends upon the atomic number (z) of the target material [v proportional z-according to Moseley’s law: v: (2.48 x 10^15 Hz) (Z - 1)^2]. Due to Moseley’s work, the characteristic x-ray spectrum became the universally accepted signature of an element.

Q.16: Does the maximum frequency in the ‘bremsstrahlung process’ depend on the nature of the target material?
Ans: No. the maximum frequency and minimum wavelength in the ‘bremsstrahlung process’ do not depend on the material.

Q.17: In laser operation, what process is required to be produced before ‘stimulated emission’?
Ans: Laser operation requires the creation of a non-equilibrium condition, called “population inversion” in which the number of atoms in a high energy state is greater than the number in a lower energy state.

Q.18: Why does laser usually emit only one particular color of light rather than several colors?
Ans: A laser beam is highly coherent and monochromatic, i.e., the emitted photons have the same frequency and wavelength. As each and every color has its own wavelength, so a laser, being monochromatic, emits only one particular color of light.

Advent of Modern Physics - Question Answers - Physics XII

ADVENT OF MODERN PHYSICS

Chapter - 17

Q.1: What do you understand by a frame of reference? What is the difference between inertial frame and non-inertial frame?
Ans: The position and motion of a body can be located with reference to some coordinate system, called the frame of reference.

The frame of reference that is either at rest or moves with uniform velocity is called an inertial frame. It has zero linear or rotational acceleration. Newton’s laws hold well in such a frame. All inertial frames of references are equivalent.

The frame of reference which possesses acceleration is known as a non-inertial frame. Laws of motion do not remain valid in such a frame.

Q.2: Explain why the Compton Effect is not observable with visible light?
Ans: In Compton’s experiment of x-rays of wavelength - 1 Å, equivalent to energy - 140 eV, were directed on a graphite block, where binding energies of bounded electrons were 102 eV. If visible light is used, it possesses low frequency, and these photons have energies - 0.1 eV. This energy is too small to be given to loosely bound electrons to get them scattered.

Q.3: What phenomena require wave description of light? What phenomena required particle picture of light? How are the two aspects related putatively?
Ans: The convincing evidences that light are a wave phenomena are:

Interference of light
Diffraction of light
Polarization of light
Production of electromagnetic waves
The optical Doppler Effect.

The idea of quantum nature of light, i.e., photon (which has a particle nature), was introduced due to the following evidences:

Black body radiation
The photoelectric emission
Compton scattering
X-ray production

The wave and particle aspects are related in de Broglie equation as: $λ = \frac{h}{m v}$ . Thus a particle of non-zero rest mass moves as if it were guided by an associated matter wave. Nevertheless, the ‘particle waves’ are waves of probability. Confirmation of de Broglie wavelength came in 1927 by C.J. Division and L.H. German and, independently, G.P. Thomson. It is astounding to note that Thomson (J.J.), the father, was awarded the Nobel prize in 1906 for having shown that the electron is a particle; and 31 years later Thomson (J.P.), the son, for having shown that the electron is a wave.

Q.4: In what way do the particles of light (photons) differ from the particles of matter, such as electrons and protons?
Ans:
Particles of matter (e.g., electron, proton, etc.) possess certain characteristics:

Non-zero rest mass.
They possess inertia and contain no energy 'packets.'
Their speed is always less than $c$ (speed of light).
Their energy is proportional to the square of the speed ( $E = \frac{1}{2} m v^{2}$ ).
They may be charged or uncharged.
When in motion, they are guided by matter waves.

Particles of light (photons) possess the following distinct characteristics:

Zero rest mass.
They consist of waves in packets of discrete amounts (called 'energy packets' or quanta).
They travel with speed equal to that of light.
Their energy is proportional to frequency ( $E = h ν$ ).
They are always electrically neutral.
They are not guided by matter waves.

Q.5: In the photoelectric effect, the energy of a photoelectron is less than that of an incident photon. Explain?
Ans: When radiation (a photon) strikes a metal surface, it deposits its entire energy on some electron in the absorbing surface. If the energy of the photon (by $h ν$ ) exceeds the energy required by the electron in work against the force binding it to the surface ( $ϕ_{0}$ ), it will be emitted with some energy. As $K . E . = h ν - ϕ_{0}$ , hence $k . e . < h ν$ .

Q.6: How did de Broglie hypothesis help to explain the stability of the atom?
Ans: According to de Broglie's hypothesis, an electron moving around the nucleus is pictured as a kind of wave packet (standing wave). An electron can circle a nucleus indefinitely without radiating energy provided that its orbit contains an integral number of de Broglie wavelengths.

Q.7: What is rest mass of a body?
Ans: Rest mass ( $m_{0}$ ) is the mass of a body when it is at rest with respect to an observer. The relativistic mass of a body moving with certain velocity $v$ is given by

$m = \frac{m_{0}}{\sqrt{1 - \frac{v^{2}}{c^{2}}}}$

Q.8: If we keep applying a force on a material object, can the object gain the speed of light?
Ans: If we keep applying a force which can produce a velocity equal to the velocity of light ( $v = c$ ), then the mass of the material body would become infinite. This is not possible.

Q.9: A block of polished metal having a black spot in the middle is heated above 3000 K and then placed in a dark room. Write your observations?
Ans: If a metal block is heated to incandescence at about 1000K, the metal has a dull red glow. A further high temperature changes into orange, then yellow and finally to white (3000 K) the black spot behaves as a black body. It absorbs maximum energy and appears as black. When seen in a dark room, the black spot radiates more energy (since a good emitter is a good emitter) than the rest of the block. The black spot appears brighter than the rest of the surface.

Q.10: Does the fact that an atom can emit a photon violate the law of conservation of energy? Explain?
Ans: No. An atom in an excited state can emit a photon. The energy received in jumping up is released in the emission.

Q.11: Can matter (e.g., electron) be created or destroyed?
Ans: Matter can be created from energy (photon) in pair production; and can be destroyed as photons in annihilation of matter process.

Q.12: Can pair production take place in vacuum? Explain
Ans: No, because this process requires a heavy nucleus to conserve momentum and energy of the system. The heavy nucleus takes the recoil after stopping the photon.

Q.13: Can an intense beam of television waves focused on a metal cause photoemission?
Ans: No, because TV waves are of low frequency, while a metal requires high threshold frequency for photoemission.

Q.14: Both photoelectric emission and Compton scattering are processes that involve interaction of radiation with matter. How do they differ?
Ans: In photoelectric effect, a low energy photon (e.g., ultraviolet light) can lose all its energy on striking an electron, and the photon vanishes. But in Compton Effect, a high energy photon (e.g., X-ray) loses part of energy and a photon is scattered with the remaining energy (and hence frequency decreases).

Q.15: The speed limit of our highways is 65 km/h. If the speed of light were the same, would you be able to drive at the speed limit?
Ans: No, because mass would become infinite.

Q.16: A ball is dropped from a helicopter flying at constant speed horizontally. Describe its motion relative to the pilot and an observer on earth’s surface.
Ans: According to the observer on the earth, it will fall forward towards the ground following a projectile path.

According to a pilot, it will to a point on the earth vertically.

Q.17: Why does the casing of a large electric transformer have metal blades fastened to it perpendicular to the surface and painted black?
Ans: The blades transfer heat by radiation to the atmosphere by increasing the surface area. They are painted black because black body radiates energy at a faster rate.

Electromagnetic Waves and Electronics - Question Answers - Physics XII

Chapter - 16

Q.1: Under what circumstances does a charge radiate electromagnetic waves?
Ans: A charge radiates electromagnetic (e.m.) waves when it is accelerated.

Q.2: In an electromagnetic wave, what is the relationship, if any, between the variation in the magnetic and electric fields?
Ans: In an electromagnetic wave, the transverse sinusoidal oscillating electric field and magnetic field are propagated at right angles to each other and to the direction of motion.

Q.3: A radio transmitter has a vertical antenna. Does it matter whether the receiving antenna is vertical or horizontal?
Ans: A receiving antenna should be vertical just like a transmitting antenna. A horizontal receiving antenna will intercept much less radio frequency signals.

Q.4: Explain why are light waves able to travel through a vacuum, whereas sound waves cannot?
Ans: Light waves are electromagnetic waves (of wavelength 400 nm to 760 nm). Sound waves are produced due to the vibration of the molecules of a medium. Hence, sound waves require a material medium, whereas light waves do not require a medium for their propagation.

Q.5: Explain the condition under which radiation of electromagnetic waves takes place from a certain source?
Ans: When a transmitting antenna is coupled with an alternating source of potential (known as oscillator), charges (electrons) are accelerated up and down the antenna. This creates a fluctuating electric flux, which generates a magnetic flux. Hence the waves propagated from an antenna are electromagnetic waves.

Q.6: Can a diode be used for amplifying a weak signal?
Ans: Normally, a diode cannot be used for amplifying a weak signal. But specially constructed diode (e.g., tunnel diode) can be used as an amplifier and oscillator for microwave frequencies.

Q.7: Are radio waves form of light?
Ans: Since both radio waves and visible light are electromagnetic in nature, hence we can say that radio waves are a form of light (of frequency 4 x $10^{14}$ Hz), having frequencies much lower (30 kHz to 300 MHz) than light.

Q.8: Can e.m. waves be propagated through a piped vacuum?
Ans: Yes.

Q.9: Why does a semiconductor act as an insulator at 0K and why does its conductivity increase with an increase in temperature? OR - Discuss the effect of temperature on semiconductors?
Ans: In a semiconductor, at 0K, the valence band is completely filled and the conduction band is totally empty. The semiconductor, therefore, behaves like a perfect insulator. At room temperature, some of the electrons in the valence band gain energy from thermal agitation of the lattice atoms and move up into the conduction band, leaving holes in the valence band. If the temperature is increased, due to further thermal agitation, more electrons occupy the conduction band. Thus, the conductivity of the semiconductor increases with an increase in temperature.

Q.10: Explain the role of forbidden band in solids?
Ans:

In conductors, the conduction band and valence bands are overlapping and hence no forbidden band exists.
In insulators, the conduction and valence bands are separated by a large forbidden band.
In semiconductors, the conduction and valence bands are separated by a small forbidden energy gap.

Q.11: Why is light not seen in an ordinary diode but an LED emits light?
Ans: Silicon is opaque to light. So, an ordinary silicon diode does not emit light, but an LED is a junction diode made from gallium arsenide phosphate crystal. When it is forward biased, electron-hole recombination takes place, which results in the release of light.

ELECTRICAL MEASURING INSTRUMENTS

Chapter - 15

Q.1: What is the function of the concave pole pieces and the coaxial soft iron cylinder in the moving coil galvanometer?
Ans: The concave magnetic poles and the cylindrical core make the magnetic field radial and stronger (so the current becomes directly proportional to the deflection).

Q.2: Why is it necessary to have some form of controlling couple in the moving coil galvanometer?
Ans: Controlling couple is necessary to control the motion of the coil, it is proportional to the current to be measured. It is produced by using a spring control method, which consists of two hair springs attached to a spindle wound in the opposite directions. As the coil rotates the spring winds up and produces a counter torque. The coil comes to rest (the final deflection of the pointer is given) when the deflecting torque (or magnetic torque) is counterbalanced by the controlling torque (or restoring torque).

Q.3: What is meant by the sensitivity of a galvanometer? On what a factor does it depend? How can we have large sensitivity of a moving coil galvanometer?
Ans: A galvanometer is sensitive if it gives large deflection for a very small current. The sensitivity of a galvanometer is the current in microamperes required to cause a deflection of 1mm or 1 division.

S = \frac{I (\text{in } \mu A)}{\theta (\text{in div})}

Since $I = K \cdot \theta$ , hence the galvanometer is sensitive if $K (= C/BAN)$ is small. Sensitivity depends on $C$ (couple per unit twist), $N$ (number of turns), $A$ (area of coil) and $B$ (strength of magnetic field).

For large sensitivity a soft iron core (sphere or cylinder) is placed inside the coil and the poles are made circular or cylindrical. This makes $4B$ stronger and radial.

Q.4: Which galvanometer usually has greater sensitivity, aluminum pointer or lamp and scale type? Why?
Ans: Lamp and scale type galvanometer has greater sensitivity ( $10^{-15}$ A/div), because it gives large deflection for a very small current.

Q.5: We want to convert a galvanometer into (a) an ammeter (b) a voltmeter. What do we need to do in each case?
Ans:

To convert a galvanometer into an ammeter, we connect a low resistance in parallel (called shunt).
To convert a galvanometer into a voltmeter, we connect a high resistance in a series (called multiplier).

Q.6: Why is it necessary for an ammeter to have zero or negligibly small resistance?
Ans: An ammeter must have negligibly small resistance so that it may not alter the current being measured.

Q.7: What necessary condition must a voltage measuring device satisfy?
Ans: A voltage measuring device must contain a very high (in fact, infinite) resistance, so that it will, practically, draw no current from the circuit across which it is connected.

Q.8: Why must an ammeter be connected to a circuit in series and a voltmeter in parallel?
Ans: An ammeter must be connected in series to a circuit because its resistance is very small as compared to the total resistance of the circuit. Hence it does not alter the current being measured. But a voltmeter has very high resistance, so it must be connected in parallel to a circuit.

Q.9: An ammeter and voltmeter of suitable ranges are to be used in a circuit. What might happen if by their mistake positions are interchanged?
Ans:

If, by a mistake, an ammeter is connected in parallel to a circuit, its coil will be burnt out to heavy current (because of its extremely low resistance).
When a voltmeter, by mistake, is connected in series to a circuit it will give reading but will not record correct p.d. because it will decrease the current (due to its very high resistance). It will not cause damage.

Q.10: The terminals of ammeters are usually made of thick and bare metal while those of voltmeters are quite thin and well insulated. Explain why?
Ans:

An ammeter must have very low resistance. So its terminals should have almost zero resistance. Hence terminals must be made of thick, bare metal.
A voltmeter must have very high resistance. So its terminals should be thin and well insulated to avoid sparking between the terminals.

Q.11: Why is a potentiometer considered one of the most accurate voltage measuring devices?
Ans: The principle of a potentiometer is that the potential drop across any length of wire of uniform cross-section is directly proportional to the length of the wire. At the balance point, the two terminals of the galvanometer are at the same potential, and no current will flow through it. Hence, a potentiometer is an instrument that can be used to measure the emf of a source and compare potentials without drawing any current from the source. Essentially, it balances an unknown p.d. against an adjustable, measurable p.d.

Q.12: How is a Wheatstone bridge used for measuring an unknown resistance?
Ans: If we connect three resistances $R_1$ , $R_2$ , and $R_3$ of precisely known adjustable values and a fourth unknown resistance $R_4$ , and these are so adjusted that the galvanometer shows no deflection; then in this balanced condition, $R$ square root $R_2$ is equal to $R_3$ square root $R_4$ . Hence $R_4$ can be calculated.

MAGNETISM & ELECTROMAGNETISM

Chapter – 14

Q.1: What is flux density and how is it related to the number of lines of induction expressed in Webers?
Ans: Magnetic flux density $B$ is the magnetic flux per unit area $(B = \Phi / A)$ . The unit of flux density is Weber per $m^2$ (or tesla, $T$ ). Magnetic flux is the total number of magnetic lines of induction passing perpendicularly through an area $(\Phi = B \times A)$ . Its SI unit is ‘Weber’ $(1 Wb = 1 T \cdot m^2)$ . Hence magnetic flux density refers to the number of lines of induction (in Webers) per square meter.

Q.2: Charged particles fired in a vacuum tube hit a fluorescent screen. Will it be possible to know whether they are positive or negative?
Ans: Yes, the charge on particles in motion can be found by applying a magnetic field perpendicular to the motion of the charges and by observing the deflection. A positive charge in an inward perpendicular magnetic field is deflected upward. In an electric field, a positive charge will be deflected towards the negative side (plate).

Q.3: Beams of electrons and protons are made to move with the same velocity at right angles to a uniform magnetic field of induction. Which of them will suffer a greater deflection? What will be the effect on the beam of electrons if their velocity is doubled?
Ans: The radius of the circular path of a particle moving in a magnetic field is $r = \frac{mv}{Bq}$ . Thus $r$ is proportional to $m$ , but deflection is proportional to $1/m$ . Thus, the electron, being lighter, will be deflected more than the proton.

Since $r \propto v$ , if velocity is doubled, the radius will also be doubled; but deflection is halved.

Q.4: A circular loop of wire hangs by a thread in a vertical plane. An electric current is maintained in the loop anti-clockwise when looking at the front face. To what direction will the front face or the coil turn in the earth’s magnetic field?
Ans: Toward the geographic north pole.

Q.5: Imagine that the room in which you are seated is filled with a uniform magnetic field pointing vertically upward. A loop of wire, which is free to rotate about the horizontal axis is placed through its center parallel to its length, has its plane horizontal. For what direction of current in the loop, as viewed from above, will the loop be in a stable equilibrium with respect to forces and torque of magnetic origin?
Ans: Anti-clockwise.

Q.6: Two identical loops, one of copper and the other of aluminum, are similarly rotated in a magnetic field of induction. Explain the reason for their different behavior. Is an electric generator a ‘generator of electricity’? Where is the electricity before it is generated? What do such machines generate?
Ans: Since the conductivities of copper and aluminum are different, they show different behavior with the induced e.m.f. As the conductivity of copper is higher than that of aluminum, a copper loop will have a greater induced current than an identical aluminum loop moving with the same speed in the same magnetic field. An electric generator is not a generator of electricity (i.e., quantity of charge). Electricity is present in the conducting coil of the generator before it was driven in an electrical circuit. A generator provides e.m.f. to drift the haphazardly moving electrons in the conducting coil. In fact, a generator converts mechanical energy into energy of moving charges.

Q.7: A loosely wound helical spring of a stiff wire is mounted vertically with the lower end just touching mercury in a dish. When a current is started in the spring, it excretes a vibratory motion with its lower end jumping out and into mercury. Explain the reason for this behavior?
Ans: When a current is passed through a helical spring, whose one end is just above a mercury pool, a magnetic field is produced. The current through all the loops is in the same direction. This produces attraction between them, so its length decreases. The dipping and moving out of the mercury causes the circuit to break. Due to elasticity, the helix regains its original length. The electrical contact is established again. The process is repeated, so the helix vibrates up and down.

Q.8: What is the mechanism of transfer of energy between the primary and secondary windings of a transformer? A certain amount of power is to be transferred over a long distance. If the voltage is stepped up 10 times, how is the transmission line loss reduced?
Ans: Electromagnetic induction is the phenomenon responsible for the transfer of energy between the primary windings (one circuit) to the secondary windings by means of a changing magnetic field which links the two coils. The mutual induction transforms the voltage or e.m.f. of large or similar value due to a different number of turns in the primary and secondary coils.

Suppose a power line has input power $P$ . The same power can be carried at low current if the voltage is made high. Input current $I_1 \propto \frac{P}{V_1}$ . If voltage is stepped up 10 times:

V_2 = 10V_1, \quad I_2 = \frac{P}{V_2} = \frac{P}{10V_1}, \quad \text{Thus } \frac{I_2}{I_1} = \frac{P / 10V_1}{P / V_1} = \frac{1}{10}

When the current is 10 times smaller, the power loss as heat in the wires $(I^2R)$ is $(10)^2$ , i.e., 100 times smaller.

Q.9: What is the difference between magnetic field a.c. generators? What is meant by the frequency of alternating current?
Ans: An alternating current generator that uses a permanent magnet to provide the magnetic flux rather than an electromagnet is called ‘magnetic’. It is used in systems like petrol engines, motorbikes, and motorboats. The a.c. generator that employs electromagnets is called “alternate.” It has a rotating field magnet (called rotor) and a stationary armature (called stator) or vice versa.

Alternating current (a.c.) is produced by a voltage source whose terminal polarity reverses with time. The number of cycles per second made by an a.c. is called its frequency $f$ . Its unit is hertz (Hz). We have $f = 1/T$ , and an a.c. reverses its polarity $2f$ times per second. An a.c. with a frequency of 50 Hz has a time period of $\frac{1}{50} = 0.02$ seconds. This a.c. reaches zero every 0.01 seconds.

Q.10: In what direction are the magnetic field lines surrounding a straight wire carrying current that is flowing directly towards you?
Ans: Anti-clockwise (using the right-hand rule).

Q.11: What kind of field or fields does or do surround a moving electric charge?
Ans: When an electric charge is in motion, it is surrounded by an electrostatic field as well as a magnetic field.

Q.12: Can an electron at rest be set in motion with a magnet?
Ans: No. When an electron is at rest, it has no magnetic field $(F = qvB = 0 \text{ if } v = 0)$ . So, in the absence of any magnetic field of its own, it cannot interact with a magnet.

Q.13: A beam of electron is directed towards a horizontal wire in which the current flows from left or right. In what direction is the beam deflected?
Ans: If the beam is parallel to the wire, it will follow a spiral path; and if it is perpendicular to the wire, it will adopt a circular path.

Q.14: A charged particle is moving in a circle under the influence of a uniform magnetic field. If an electric field is turned on at that point along the same direction as the magnetic field, what path will the charged particle take?
Ans: When a charged particle is moving in a circle under the influence of a uniform magnetic field; and if an electric field is applied along the same direction, it will exert lateral force on the charged particle. Consequently, the charged particle will move in a cyclic path in the form of a spiral (called helix).

Q.15: A loop of wire is suspended between the poles of a magnet with its plane parallel to the pole faces. What happens if direct current is passed through the coil? What will happen if an alternating current is passed instead?
Ans: When d.c. passes through the loop such that its magnetic field is: (i) opposite the direction of the field of the magnet, the coil will turn round through 180° and then will stay in equilibrium. (ii) Along the field of the magnet, the coil will stay in equilibrium. However, when an a.c. is passed through the loop, it will remain in its initial position (with slight vibration).

Q.16: A current-carrying wire is placed in a magnetic field. How must it be oriented so that the force acting on it is zero or is maximum?
Ans:

Force will be zero if theta is equal to zero (parallel to B);
Force will be maximum if theta is equal to 90° (perpendicular to B).

Q.17: Why is the magnetic field strength greater inside a current-carrying loop of wire?
Ans: In a loop of wire, the direction of current in the opposite sides of the loop is opposite to each other. This is analogous to two parallel conductors carrying current in opposite directions. The directions of both the magnetic fields are along the same direction in the loop. This increases the strength of the field.

Q.18: What exactly does a transformer transform?
Ans: A transformer transforms the magnitude of alternating voltage and current.

Q.19: Can an efficient transformer step up energy? Explain.
Ans: Transformers cannot charge energy. In an ideal transformer, the power remains constant, i.e., power input equals power output $(V_p I_p = V_s I_s)$ . Thus it cannot step up energy.

Q.20: In what three ways can a voltage be induced in a wire?
Ans:

By moving a wire in a magnetic field.
By moving a magnet near it.
By changing current through a circuit near it.

Q.21: Does the voltage output of a generator change if its speed of rotation is increased?
Ans: Yes, because induced e.m.f. = $BNA \, \text{omega} \sin \, (\text{omega} \, \text{time})$ . Thus, an e.m.f. increases if the speed of rotation “Omega” is increased.

Q.22: When a beam of electrons is shot into a certain region of space, the electrons travel a straight line through the region. Can we conclude that in the region there is no electric field? No magnetic field?
Ans: There are two possibilities:

No electric or magnetic field is present.
The electric and magnetic fields are at right angles to each other, and their strengths are exerting equal but opposite forces on the electron beam.

Q.23: A copper ring is placed above a solenoid with an iron core to increase its field. When the current is turned on in the solenoid, the copper ring moves upward. Why?
Ans: When current in a solenoid (with an iron core) increases, an induced current is produced in a copper ring (held above it) in the opposite direction. This is analogous to opposite currents in two parallel wires. Thus they develop similar poles and repel each other. Consequently, the ring moves up.

Q.24: A very long copper pipe is held vertically. Describe the motion of a bar magnet dropped lengthwise down the pipe?
Ans: Suppose a bar magnet falls through a very big copper pipe (under gravity). When the magnet is well inside the pipe, the configuration of the magnetic field remains the same. So it will fall freely with acceleration of gravity only.

Q.25: A solenoid is viewed in such a way that the solenoid current appears clockwise to the viewer. What is the direction of the field within the solenoid?
Ans: The end viewed will develop south polarity. So the direction of the magnetic field will be away from the viewer inside the solenoid.

Q.26: A hollow copper tube carries. Why is $B = 0$ inside the tube? Is $B$ non-zero outside the tube?
Ans: The charges always reside or move on the outer of a conductor. Since inside the tube, current is zero, hence $B = 0$ (according to Ampere’s law). The outer surface of the tube behaves like a set of parallel wires carrying current down their length. The magnetic field outside the tube exists, and its direction is given by the right-hand grip rule.

Q.27: Can a current-carrying coil be used as a compass?
Ans: A current-carrying coil behaves like a bar magnet (magnet dipole). Thus when it is suspended freely, it can be used as a compass.

Q.28: When a charged particle enters a magnetic field, it is deflected by the magnetic force? Can the magnetic force do work on the moving charged particle?
Ans: No, magnetic force can do no work on a moving charged particle because it is always perpendicular to the velocity of the particle.

Q.29: If both electric field (E) and magnetic field (B) act on a charged particle, what is the total force on it?
Ans: The total force is $F = qE + q (v \times B)$ . This force is called the Lorenz force.

Q.30: Can an isolated magnetic pole (monopole) exist? What is the source of the magnetic fields?
Ans: No, magnetic monopoles cannot exist. The only known source of magnetic fields are magnetic dipoles (current loops), even in magnetic materials.

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