CLASS 12 - PHYSICS 7.

A: Since matter cannot be concentrated at a point,

ASSERTION AND REASON TYPE therefore point charge is not possible.
QUESTIONS WITH ANSWERS R: An electron is a point charge.
Answer: (3)
Answer: (1)
Both are correct and reason is correct explanation of 8. A: A finite size charged body may behave like a
assertion. point charge if it produces an inverse square electric
Answer: (2) field.
Both are correct but reason is not the correct R: Two charged bodies may be considered as point
explanation of assertion. charges if their distance of separation is very large
Answer: (3) compared to their dimensions.
Reason is wrong. Answer: (2)
Answer: (4)
Both are wrong. 9. A: The path traced by a positive charge is a field
line.
ELECTROSTATICS R: A field line can intersect itself.
Answer: (4)
1. A: A negatively charged body means that the body
has gained electrons while a positively charged body 10. A: If electric flux over a closed surface is
means the body has lost some of its electrons. negative then the surface encloses net negative
R: Charging process involves transfer of electrons. charge.
Answer: (1) R: Electric flux is independent of the charge
distribution inside the surface.
2. A: Particles such as photon or neutrino which Answer: (2)
have no rest mass are uncharged.
R: Charge cannot exist without mass. 11. A: We may have a Gaussian surface in which
Answer: (1) less number of field lines enter and more field lines
come out.
3. A: When a body is charged, its mass changes. R: The electric field E in the Gauss’s law is only due
R: Charge is quantized. to the enclosed charges.
Answer: (2) Answer: (3)

4. A: Though quark particles have fractional 12. A: The equilibrium of a charged particle under
electronic charges, the quantum of charge is still e. the action of electrostatic force alone can never be
R: Quark particles do not exist in Free State. stable.
Answer: (1) R: Coulomb force is an action-reaction pair.
Answer: (2)
5. A: An electron has negative charge by definition.
R: Charge of a body depends on its velocity. 13. A: The field in a cavity inside a conductor is zero
Answer: (3) which causes electrostatic shielding.
R: Dielectric constant of conductors in electrostatics
6. A: A point charge cannot exert force on itself. is infinite.
R: Coulomb force is a central force. Answer: (2)
Answer: (2)
14. A: If dipole moment of water molecules were R: Potential is a scalar quantity.
zero, then microwave cooking would not be possible. Answer: (2)
R: In a microwave oven the water molecules vibrate Both are correct but reason is not the correct
due to oscillating electric field in microwave and explanation of assertion.
heat the food.
Answer: (1) 21. A: Absolute value of potential is not defined.
R: Two equipotential lines cannot intersect each
15. A: Electric field lines are continuous curves in other.
free space. Answer: (2)
R: Electric field lines start from negative charge and Both are correct but reason is not the correct
terminate at positive charge. explanation of assertion.
Answer: (3)
22. A: Two large conducting spheres carrying
16. A: When an electric dipole is placed in uniform charges Q1 and Q2 are brought close to each other.
electric field, net force on it will be zero. The magnitude of electrostatic force between them is
R: Force on the constituent charges of the dipole will exactly given by Q1Q2/4πε0r2, where r is the distance
be equal and opposite when it is in uniform electric between their centers.
field. R: Here charges Q1 and Q2 can be assumed to be
Answer: (1) concentrated at the centers of their respective
spheres.
17. A: Gauss’ theorem is applicable on any closed Answer: (4) Both are wrong.
surface.
R: In order to find the value of electric field due to a 23. A: Work done by the field of a nucleus in a
charge distribution, Gauss’ theorem should be complete orbit of the electron is zero even if the orbit
applied on a symmetrical closed surface. is elliptical.
Answer: (2) R: Electrostatic force is conservative in nature.
Answer: (1)
18. A: For a given potential function, electric Both are correct and reason is correct explanation of
intensity function can be uniquely derived. assertion.
R: For a given electric intensity function, electric
potential function can be uniquely derived. 24. A: Electric field is discontinuous across the
Answer: (3) surface of a charged conductor.
Reason is wrong. Electric potential difference can be R: Electric potential is discontinuous across the
derived but not electric potential function. surface of a charged conductor.
Answer: (3)
19. A: An equipotential surface is normal to electric Reason is wrong. Potential is continuous across the
field lines. surface of charged conductor.
R: Potential increases in the direction of electric
field. 25. A: Water has a much greater dielectric constant
Answer: (3) than any other ordinary substance.
Reason is wrong. Electric potential decreases in the R: Water has permanent dipole moment.
direction of electric field. Answer: (1)
Both are correct and reason is the correct explanation
20. A: One may have zero potential but non-zero of assertion.
electric field at a point in space.
26. A: Electric potential of a positively charged body CURRENT ELECTRICITY
may be negative.
R: The potential of a conductor does not depend on 1. A: For a given conductor, electric current does not
the charge of the conductor. vary even if it’s cross sectional area varies.
Answer: (3) R: A conductor remains uncharged when current
Reason is wrong. Potential of a conductor depends flows through it.
on the charge of the conductor. Answer: (2)

27. A: The potential difference between two 2. A: When a steady current flows through a
concentric spherical shells depends only on the conductor of non-uniform cross-section, the current
charge of inner shell. density, electric field and drift velocity do not remain
R: The electric field in the region in between two constant.
shells depends on the charge of inner shell and R: For a constant current the current density, electric
electric field is the negative of potential gradient. field and drift velocity are inversely proportional to
Answer: (1) cross sectional area.
Both are correct and reason is the correct explanation Answer: (1)
of assertion.
3. A: To a metal wire of diameter d and length L
28. A: If E be electric field at a point, in free space when the applied voltage is doubled, drift velocity
then energy density at that point will be ε0E2/2. gets doubled.
R: Electrostatic field is a conservative field. R: For a constant voltage when the length is doubled,
Answer: (2) drift velocity will be halved but drift velocity is
Both are correct but reason is not the correct independent of diameter.
explanation of assertion. Answer: (2)

29. A: A capacitor is a device which stores electric 4. A: Kirchhoff's Current law is applicable at any
energy in the form of electric field. junction or node in the circuit.
R: Net charge on the capacitor is always zero. R: Kirchhoff's laws are general in nature.
Answer: (2) Answer: (2)
Both are correct but reason is not the correct
explanation of assertion. 5. A: Voltage across a resistor decreases in the
direction of current and increases opposite to the
30. A: When two conductors charged to different direction of current.
potentials are connected to each other, the negative R: Voltage drop or gain across a capacitor depends
charge always flows from lower potential to higher on the direction of current.
potential. Answer: (3)
R: In the charging process, there is always a flow of
electrons only. 6. A: The voltage across a battery may be less, equal
Answer: (2) or more than the emf of the battery.
Both are correct but reason is not the correct R: Voltage across a battery also depends on the
explanation of assertion. magnitude and direction of current.
Answer: (1)

7. A: Practically a voltmeter will measure the voltage

across the battery but not it’s EMF.
R: EMF of a cell is measured with the help of a R: Effective internal resistance of the parallel
potentiometer. combination of two identical cells will be half of the
Answer: (2) internal resistance of each cell.
Answer: (2)
8. A: A potentiometer can act as an ideal voltmeter.
R: An ideal voltmeter has infinite resistance. 15. A: The drift velocity of electrons in a conductor
Answer: (2) is very small still current in a conductor is
established almost instantaneously on closing the
9. A: Ohm's law is universally applicable for all switch.
conducting elements. R: Electric field in the conductor sets up with speed
R: All conducting elements show straight line of light.
graphic variation on (I–V) plot. Answer: (1)
Answer: (4)
16. A: When temperature of a metallic wire is
10. A: A low voltage supply, from which high increased, its resistance increases.
currents are to be withdrawn, must have very low R: As the temperature is increased, average
internal resistance. relaxation time increases.
R: Maximum current drawn from a source is Answer: (3)
inversely proportional to internal resistance.
Answer: (1) 17. A: The potentiometer wire should have uniform
cross sectional area.
11. A: High voltage (high tension) supply must have R: On the potentiometer wire the jockey is gently
very large internal resistance touched, not pressed hard.
R: If the circuit is accidentally shorted, then the Answer: (2)
current drawn will not exceed safety limits if internal
resistance is high. ELECTROMAGNETIC INDUCTION
Answer: (1)
1. A: Total induced emf in a loop is not confined to
12. A: Alloys of metals usually have greater any particular point but it is distributed around the
resistivity than that of their constituent metals. loop in direct proportion to the resistances of its
R: Alloys usually have much lower thermal parts.
coefficient of resistance than pure metals. R: In general when there is no change in magnetic
Answer: (2) flux, no induced emf is produced.
Answer: (2)
13. A: Current density is a vector quantity.
R: Electric current, passing through a given area is 2. A: The induced current flows so as to oppose the
the flux of current density through that area. cause producing it.
Answer: (2) R: Lenz's law is based on energy conservation.
Answer: (1)
14. A: When two cells of equal EMF and equal
internal resistances are connected in parallel with 3. A: Faraday's law is an experimental law.
positive plate of one to the positive plate of the other R: Time varying magnetic field cannot generate
then, the net EMF of the combination will be equal induced emf.
to the EMF of each cell. Answer: (3)
4. A: Unlike electrostatic field the lines of induced 11. A: When an electric motor is started, a variable
field form closed loop. resistance (that decreases with time) is used in series.
R: Electrostatic field is conservative unlike induced This resistance is known as motor starter.
fields. R: The back-emf in the beginning, when motor
Answer: (1) starts, is very small.
Answer: (1)
5. A: The mutual induction between the two coils
infinitely apart is zero. 12. A: When a bar magnet is dropped into a vertical
R: If the mutual induction between the two coils is long hollow metallic tube, the magnet ultimately
zero, it means that their self inductances are also moves with zero acceleration.
zero. R: The magnet falling into metallic tube causes the
Answer: (3) eddy currents in the metal tube, so the motion of the
magnet is damped.
6. A: An inductor is called the inertia of an electric Answer: (1)
circuit.
R: An inductor tends to keep the flux constant. 13. A: The power output of a practical transformer is
Answer: (1) always smaller than the power input.
R: A transformer works on the principle of mutual
7. A: At any instant, if the current through an induction.
inductor is zero, then the induced emf will also be Answer: (2)
zero.
R: In one time constant, the current flows to 37 % of 14. A: Electrical power through transmission lines is
its maximum value in a series LR circuit. transmitted at high voltage.
Answer: (4) R: At high voltage theft of power is checked.
Answer: (3)
8. A: There may be an induced emf in a loop without
induced current. 15. A: The electric field induced due to changing
R: Induced current depends on the resistance of the magnetic field is non-conservative.
loop as well. R: The line integral of the electric field induced due
Answer: (2) to changing magnetic field along a closed loop is
always zero.
9. A: When the magnetic flux through a loop is Answer: (3)
maximum, induced emf is maximum.
R: When the magnetic flux through a loop is
minimum, induced emf is minimum. ALTERNATING CURRENT
Answer: (4)
1. A: Direct current is more dangerous than
10. A: When a conducting loop is kept stationary in a Alternating current of same value.
non-uniform magnetic field an emf is induced. R: An electrocuted person sticks to direct current
R: As per Faraday's law, whenever flux changes, an line. While alternating current repels the person from
emf is induced. the line.
Answer: (4) Answer: (4)

2. A: AC can be transmitted over long distances at

high voltage without much power loss.
R: The average value of AC is defined over any half R: The scale of a dc ammeter is uniformly divided.
cycle. Answer: (2)
Answer: (3)

3. A: An inductor and a capacitor are called low pass ELECTROMAGNETIC WAVES

filter and high pass filter respectively.
R: Reactance of an inductor is low for low frequency 1. A: Different electromagnetic waves differ
signals and that of a capacitor is high for high considerably in their mode of interaction with
frequency signals. matter.
Answer: (3) R: Different electromagnetic waves have different
wavelength or frequency.
4. A: The chief characteristic of series resonant Answer: (2)
circuit is voltage magnification.
R: At resonance the voltage drop across L (or C) is Q 2. A: All electromagnetic waves travel through
times the applied voltage. vacuum with same speed but they have different
Answer: (1) wavelength or frequency.
R: The wavelength of the electromagnetic waves is
5. A: Wires of the transmission lines carrying A.C. often correlated with the characteristic size of the
are made of multiple strands. system that produces and radiates them.
R: A.C. flows on surface of the conductor. Answer: (1)
Answer: (1) 3. A: High frequency electromagnetic waves are
detected by some means based on the physical
6. A: The ammeters and voltmeters used for effects they produce on interacting with matter.
measuring alternating current and voltages have non- R: The oscillating fields of an electromagnetic wave
uniform divisions on their scales. can accelerate charges and can produce oscillating
R: The instruments used for measuring alternating currents therefore, an apparatus designed to detect
current and voltage are based on heating effect of EM waves is based on this fact.
current. Answer: (1)
Answer: (1)
4. A: Infrared waves are often called heat waves.
7. A: A series resonant circuit is also known as an R: Infrared waves vibrate not only the electrons, but
acceptor circuit. entire atoms or molecules of a substance which
R: For large value of Ohmic resistance, the quality increases the internal energy and temperature of the
factor of a series resonant circuit is high. substance.
Answer: (3) Answer: (1)

8. A: For a practical choke coil the power factor is 5. A: Long distance radio broadcasts use short-wave
very small. bands.
R: In a practical choke coil the power dissipation R: Ionosphere reflects waves in these bands.
reduces if frequency of the ac is increased. Answer: (1)
Answer: (2)
6. A: It is necessary to use satellites for long distance
9. A: If a current has both ac and dc components, TV transmission.
then a dc ammeter used to measure this current will
measure the average value of the total current.
R: Television signals are not properly reflected by R: Increase in ozone decreases the amount of UV
the ionosphere therefore, reflection is affected by radiation to earth.
satellites. Answer: (2)
Answer: (1)
14. A: The electrical conductivity of the earth's
7. A: Optical and radio telescopes are built on the atmosphere does not change with altitude.
ground but X-ray astronomy is possible only from R: Cosmic rays from outer space entering the earth's
satellites orbiting the earth. atmosphere do not affect it.
R: Atmosphere absorbs X-rays, while visible and Answer: (4)
radio waves can penetrate it.
Answer: (1) 15. A: Light waves can be polarized.
R: All electromagnetic waves move with same speed
8. A: If the earth did not have an atmosphere, its in vacuum.
average surface temperature would have been lower. Answer: (1)
R: In the absence of atmosphere, the green house
effect will be absent. 16. A: In an electromagnetic wave the energy density
Answer: (1) in electric field is equal to energy density in
magnetic field.
9. A: In an EM wave the magnitude of the electric R: Electromagnetic waves are transverse in nature.
field vector is more than the magnitude of the Answer: (2)
magnetic field vector.
R: Energy of the EM wave is shared equally between 17. A: In a material medium the speed of a particle
the electric and magnetic fields. can be more than the speed of light in that medium.
Answer: (2) R: In the phenomenon of green house effect, low
wavelength radiation is allowed to pass but high
10. A: When cooking in microwave ovens, metal wavelength radiation is not allowed to pass.
containers are used. Answer: (2)
R: Energy of the microwaves can be easily
transferred to the food through metal. WAVE OPTICS
Answer: (4) 1. A: The speed of light in vacuum doesn't depend on
nature of the source, direction of propagation,
11. A: Food is cooked faster by microwaves than by motion of the source or observer wavelength and
conventional gas burner. intensity of the wave.
R: Microwaves have more energy than heat waves. R: The speed of light in vacuum is a universal
Answer: (3) constant independent of all the factors listed and
anything else.
12. A: Microwaves are commonly used in radar to Answer: (1)
locate flying objects.
R: Microwaves have smaller wavelength than radio 2. A: The speed of light, sound waves, water waves
waves. in a medium is independent of the nature of the
Answer: (1) source or intensity (so long it is low).
R: Speed of the waves in a medium depends on
13. A: Environmental damage has depleted the wavelength.
ozone layer in the atmosphere. Answer: (2)
3. A: Speed of light in a medium is independent of Answer: (1)
the motion of the source relative to the medium.
R: Speed of light in a medium depends on the 9. A: The phase difference between any two points
motion of the observer relative to the medium. on a wave front is zero.
Answer: (2) R: Corresponding to a beam of parallel rays of light,
the wave fronts are planes parallel to one another.
4. A: When monochromatic light is incident on a Answer: (2)
surface separating two media, the reflected and
refracted light both have the same frequency as the 10. A: Light waves can be polarized.
incident frequency. R: Light waves are transverse in nature.
R: At any interface between the two media, the Answer: (1)
electric (and magnetic) fields must satisfy certain
boundary conditions for all times and frequency 11. A: The law of conservation of energy is violated
determines the time dependence of fields. during interference.
Answer: (1) R: For sustained interference the phase difference
between the two waves must change with time.
5. A: When light travels from a rarer to a denser Answer: (4)
medium, it loses some speed but it doesn't imply a
reduction in the energy carried by the light wave. 12. A: When the apparatus of YDSE is brought in a
R: Energy carried by a wave depends on the liquid from air, the fringe width decreases.
amplitude of the wave and not on the speed of wave R: The wavelength of light decreases in the liquid.
propagation. Answer: (1)
Answer: (1)
DUAL NATURE OF RADIATION AND
6. A: When a narrow pulse of light is sent through a MATTER
medium, it doesn't retain its shape as it travels
through the medium. 1. A: Every metal has a definite work function. Still
R: Since the speed of propagation in a medium all photoelectrons do not come out with the same
depends on wavelength, different wavelength energy if incident radiation is monochromatic.
components of the pulse travel with different speeds. R: Work function is the minimum energy required
Answer: (1) for the electron in the highest level of the conduction
band to get out of the metal. Not all electrons in the
7. A: In the wave picture of light, intensity of light is metal belong to this level rather they occupy a
determined by the square of the amplitude of the continuous band of levels.
wave. Answer: (1)
R: In the photon picture of light, for a given
frequency, intensity of light is determined by the 2. A: Work function of aluminum is 4.2 eV.
number of photons per unit area. Emission of electrons will be possible by two
Answer: (2) photons, each of 2.5eV energy, striking the electron
of aluminum.
8. A: The speed of light in still water is not same as R: Energy of a photon can be less than the work
that in flowing water. function of the metal, for photoelectron emission.
R: The speed of light in water is not independent of Answer: (4)
the relative motion between the observer and the Both the assertion and reason are incorrect. The
medium. quantized nature of light allows emission only when
the photon striking the electron has enough energy to R: Work function is the minimum energy required to
eject the electron. There will be no emission due to liberate an electron out of a metal. So some electrons
photons with energy less than work function. may require more energy for their liberation.
Answer: (1)
3. A: On increasing the intensity of light, the number
of photoelectrons emitted is more. Also the kinetic 8. A: The photoelectric effect is a proof of the
energy of each photon increases but the photoelectric quantized nature of the light.
current is constant. R: Each photon in a light beam has same amount of
R: Photoelectric current is independent of intensity energy.
but increases with increasing frequency of incident Answer: (3)
radiation. The assertion is correct but reason is completely
Answer: (4) false. It can only be true for completely
On increasing the intensity of light only number of monochromatic laser.
photoelectrons increase and not the KE of electrons
or photons. 9. A: A photon cannot transfer all of its energy to an
isolated electron.
4. A: The process of photoelectron emission and R: When energy of a photon is more than 1.02 MeV,
thermionic emission of electrons is different. it can materialize into two particles called electron
R: Photoelectric emission does not depend upon and positron.
temperature, whereas thermionic emission is Answer: (2)
temperature dependent.
Answer: (1) 10. A: There is almost no time-lag between the
The process of photoelectric emission depends of incidence of light and the emission of
frequency of wave and thermionic emission occurs photoelectrons.
when metal is heated. R: A photon transfers almost all its energy to a single
electron in a metal.
5. A: Wave nature of particles is not visible in daily Answer: (1)
life.
R: In daily life, mass of particles is very high so their ATOMS
de Broglie wavelength is very small.
Answer: (1) 1. A: Both the Thomson's as well as the Rutherford's
The assertion is correct and reason is correct models constitute an unstable system.
explanation λ = h/mv and it is very low for masses in R: Thomson's model is unstable electro-statically
everyday life as his very small. while Rutherford's model is unstable because of
electromagnetic radiation of orbiting electrons.
6. A: If a stationary nucleus emits α particle, the de Answer: (1)
Broglie wavelengths of the daughter nucleus and α
particle are equal. 2. A: Bohr's orbits are regions where the electron
R: The magnitudes of the linear momenta of the may be found with large probability.
daughter nucleus and α particle are the same. R: The orbital picture in Bohr's model of the
Answer: (1) hydrogen atom was inconsistent with the uncertainty
principle.
7. A: When a photon of energy hν is incident on an Answer: (1)
electron in a metal of work function φ (<hν), the
electron will not necessarily come out of the metal.
3. A: Bohr's model with its planet-like electron is not 9. A: The nature of the characteristic X-rays does not
applicable to many electron atoms. depend on accelerating potential.
R: Unlike the situation in the solar system, where R: X-rays are electromagnetic radiation.
planet-planet gravitational forces are very small as Answer: (2)
compared to the gravitational force of the sun on The nature of the characteristic X-rays does not
each planet, the electron-electron electric force depend on characteristic accelerates potential but on
interaction is comparable in magnitude to the the material. Both statements are true, independent
electron nucleus electric force. facts.
Answer: (1)
10. A: If vacuum is not created inside an X-ray tube,
4. A: In Bohr model, the frequency of revolution of X-rays will not be produced.
an electron in its orbit is not connected to the R: Without vacuum inside the X-ray tube the
frequency of spectral line for smaller principal electrons are not emitted by the filament.
quantum number n. Answer: (3)
R: For transitions between large quantum number the If vacuum is created inside the tube the emitted
frequency of revolution of an electron in its orbit is electrons will not hit the molecules of air to produce
connected to the frequency of spectral line, as per X-rays.
Bohr's Correspondence principle.
Answer: (2) 11. A: Gases are insulators at ordinary pressure but
they start conducting at very low pressure.
6. A: If the accelerating potential in an X-ray R: At low pressures, ions have a chance to reach
machine is decreased, the minimum value of the their respective electrodes and constitute a current
wavelength of the emitted X-rays gets increased. but at ordinary pressures; ions undergo collision with
R: The minimum value of the wavelength of the gas molecules and recombination.
emitted X-rays is inversely proportional to the Answer: (1)
accelerating potential.
Answer: (1) 12. A: The oil-drops of Millikan’s experiment should
be of microscopic size.
7. A: According to Bohr’s atomic model the ratio of R: For larger drops the electric fields needed in the
angular momenta of an electron in first excited state experiment will be impractically high.
and in ground state is 2:1. Answer: (1)
R: In a Bohr’s atom the angular momentum of the
electron is directly proportional to the principal 13. A: Stoke’s formula for viscous drag is not really
quantum number. valid for oil-drops of extremely minute sizes.
Answer: (1) R: Stoke’s formula is valid for motion through a
homogeneous continuous medium and the size of the
8. A: If a beam of photons of energy 10.0 eV each, is drop should be much larger than the intermolecular
incident on a sample of hydrogen gas containing all separation in the medium for this assumption to be
atoms in the ground state, then the beam of the valid.
photons is completely transmitted through the gas Answer: (1)
without absorption.
R: The minimum energy required by an electron to
make a transition to an excited state is 10.2 eV.
Answer: (1)
NUCLEAR PHYSICS
6. A: Nuclear density is almost same for all nuclei.
1. A: Uncertainty principle demands that an electron R: The radius (r) of a nucleus depends only on the
confined to a nucleus must have very high energy so mass number (A) as r ∝ A1/3.
that the electron cannot reside in a nucleus. Answer: (1)
R: The electrostatic attraction between electron and
proton is large at such a small distance but is not 7. A: During radioactive disintegration an α-particle
enough to bind such a high-energy electron. and a β-particle do not emit simultaneously from any
Answer: (1) nucleus.
R: An α-particle emits from a nucleus when the N/Z
2. A: A free proton is stable but inside a nucleus, a ratio is less than the stability range (where N =
proton gets converted into a neutron, a positron and a number of neutrons and Z = number of protons in a
neutrino (p → n + e+ + ν). nucleus).
R: Inside a nucleus, neutron decay (n → p + e–+ ν–) Answer: (2)
as well as proton decay are possible, since other The two common modes of radioactive decay are
nucleons can share energy and momentum to alpha and beta types. In both these decays the other
conserve energy as well as momentum and both the particle is not ejected.
decays are in dynamic equilibrium.
Answer: (1) 8. A: In β-decay an electron is emitted by the
nucleus.
3. A: Exothermic reactions are possible when two R: Electrons are not present inside the nucleus.
light nuclei fuse or when a heavy nucleus undergoes Answer: (2)
fission into intermediate mass nuclei.
R: The nature of nuclear binding energy curve is 9. A: A radioactive substance has half life of 1 hour.
such that it rises for lighter nuclei and slightly Therefore, if two nuclei of the substance are present
decreasing for heavier nuclei. initially, after 1 hour only one will remain
Answer: (1) undissociated.
R: When a nucleus makes a transition from excited
4. A: For fusion, the light nuclei must have sufficient state to ground state, it emits a beta particle.
initial energy to cross the Coulomb barrier. Hence, Answer: (4)
fusion requires high temperature; however, the actual Both the statements are wrong. Nuclear reaction
temperature required is somewhat less than expected being spontaneous may occur at any time.
classically.
R: It is due to quantum mechanical tunneling of the 10. A: Fast moving neutrons do not cause fission of a
potential barrier. uranium nucleus.
Answer: (1) R: A fast moving neutron spends very little time
inside the nucleus.
5. A: Only in low or medium energy nuclear Answer: (1)
reactions, the number of protons and number of
neutrons are separately conserved.
R: In high energy reactions, protons and neutrons
can be converted into other particles and a new
quantum number, the Baryon number is however,
always conserved.
Answer: (2)
SEMICONDUCTOR ELECTRONICS 7. A: The energy gap between the valence band and
conduction band is greater in silicon than in
1. A: The conductivity of an intrinsic semiconductor germanium.
at 0K is zero. R: Thermal energy produces fewer minority carriers
R: The bond strength of the semiconductor at 0K is in silicon than in germanium.
much higher as compared to the bond strength at Answer: (2)
room temperature.
Answer: (3) 8. A: Light emitting diode (LED) emits spontaneous
radiation.
2. A: When base region has larger width, the R: LEDs are forward biased p-n junctions.
collector current is small. Answer: (1)
R: At larger width of the base region the rate of The flow of current through forward wired PN
electron-hole recombination is more which results in junction career radial.
larger value of base current.
Answer: (1) 9. A: NAND or NOR gates are called digital
building blocks.
3. A: The conductivity of a pure semiconductor R: The repeated use of NAND (or NOR) gates can
increases on doping. produce all the basic or complicated gates.
R: Doping causes the reduction in bond strength. Answer: (1)
Answer: (3)
10. A: In NPN transistors, electrons are current
4. A: Semiconductors do not obey Ohm’s law. carriers inside as well as outside the transistor
R: In semiconductors the rate of flow of charge not circuits.
only depends on the applied electric field but also on R: In PNP transistors, holes are responsible for
the availability of charge carriers. current inside the transistors but outside the
Answer: (1) transistor, electrons are the current carriers.
Answer: (2)
5. A: When a pure semiconductor is doped with a
pentavalent impurity, the number of conduction
electrons is increased while the number of holes is
decreased.
R: Some of the holes get recombined with the
conduction electrons as the concentration of the
conduction electrons is increased.
Answer: (1)

6. A: In transistor common emitter mode as an

amplifier is preferred over common base mode.
R: In common emitter mode, the input signal is
connect in series with the voltage applied to the base
emitter junction.
Answer: (2)
COMMUNICATION SYSTEMS R: Microwaves have frequencies 100 to 300 GHz
which have very good directional properties.
1. A: Modulator is an essential component of a Answer: (1)
transmitter.
R: Modulator superimposes a low frequency 8. A: In satellite communication, generally we keep
message signal on a high frequency carrier wave. different uplink and downlink frequencies.
Answer: (2) R: In case of failures, the detection of faulty link
becomes easier if frequencies are kept different.
2. A: Short wave bands are used for transmission of Answer: (1)
radio waves to a large distance.
R: Short waves are reflected by ionosphere. 9. A: Remote sensing satellites can send continuous
Answer: (1) pictures of the earth even when clouds are present or
it is dark night.
3. A: If the amplitude of a message signal is half of R: These satellites use infrared as well as
the amplitude of the carrier wave, then the index of microwaves apart from visible light photography.
modulation is 50%. Answer: (1)
R: In amplitude modulation the amplitude of the
carrier wave varies in accordance with the signal 10. A: Optical communication system is more
voltage of the message signal. economical than other systems of communications.
Answer: (2) R: The information carrying capacity of a
µ = Am/Ac. Hence assertion is true. communication system is directly proportional to its
band width.
4. A: The bandwidth of a modulated signal is 2fm in Answer: (1)
amplitude modulation, where fm is the frequency of
the modulating signal.
R: In case of amplitude modulation the frequency of
a modulated signal is equal to the frequency of the
carrier wave.
Answer: (2)

5. A: The process of demodulation is carried out to

retrieve the message signal.
R: The range of the line-of-sight propagation is
limited mainly due to earth’s curvature.
Answer: (2)

6. A: Transducer in communication system converts

electrical signal into a physical quantity.
R: For information signal to be transmitted directly
to long distances, modulation is not a necessary
process.
Answer: (4)

7. A: Microwave propagation is better than the sky

wave propagation.