# CBSE Previous Year Question Papers Class 12 Physics With Solutions

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## Class 12 Subject Physics Paper Set 3 with Solutions

SECTION A

Note : Select the most appropriate option from those given below each question :

Question 1: (Marks 1)

If a positive charge is displaced against the electric field in which it was situated, then

(A) work will be done by the electric field on the charge.

(B) the intensity of the electric field decreases.

(C) energy of the system will decrease.

(D) energy will be provided by external source displacing the charge.

Answer :

(D) energy will be provided by external source displacing the charge

Question 2: (Marks 1) Answer :

(A) Question 3: (Marks 1) Answer :

(A) Question 4: (Marks 1)

The electrical resistance of a conductor

(A) varies directly proportional to its area of cross-section.

(B) decreases with increase in its temperature.

(C) decreases with increase in its conductivity.

(D) is independent of its shape but depends only on its volume.

Answer :

(C) Decreases with increase in its conductivity

Question 5: (Marks 1)

m2V–1s–1 is the SI unit of which of the following ?

(A) Drift velocity

(B) Mobility

(C) Resistivity

(D) Potential gradient

Answer :

(B) Mobility

Question 6: (Marks 1) Answer :

(D) Question 7: (Marks 1) Answer :

(D) Question 8: (Marks 1)

A zener diode has

(A) heavily doped p-side and lightly doped n-side.

(B) heavily doped n-side and lightly doped p-side.

(C) heavily doped n-side as well as p-side.

(D) lightly doped n-side as well as p-side.

Answer :

(C) heavily doped n-side as well as p-side

Question 9: (Marks 1)

A region has a uniform magnetic field in it. A proton enters into the region with velocity making an angle of 45° with the direction of the magnetic field. In this region the proton will move on a path having the shape of a

(A) straight line

(B) circle

(C) spiral

(D) helix

Answer :

(D) Helix

Question 10: (Marks 1) Answer :

(D) -F

Note : Fill in the blanks with appropriate answer :

Question 11: (Marks 1)

The shape of the wavefront originating from a line source is __________ .

Answer :

Cylindrical

Question 12: (Marks 1)

The refractive index of the material of a converging lens is 1·5. If air is replaced by a medium of refractive index 1·6, then the lens will now behave as a __________ lens.

Answer :

Divergent lens/ Concave lens

Question 13: (Marks 1)

In Young’s double slit experiment, the separation between the two slits is halved. The new fringe width will be __________ times its initial value.

Answer :

Two

Question 14: (Marks 1) Answer :

√3

Question 15: (Marks 1)

In photoelectric effect, the number of emitted photoelectrons is proportional to __________ of incident light.

OR

Light of frequency v is incident on a photosensitive surface of threshold frequency v0 (v > v0). The value of kinetic energy of the emitted photoelectrons will be __________ .

Answer :

Intensity

OR

h(v - v0)

Note : Answer the following :

Question 16: (Marks 1)

An ac is passed through a series LCR circuit. What is the impedance of the circuit at resonance ?

Answer :

Z=R Alternatively, Impedance=Resistance

Question 17: (Marks 1)

Two identical coils, one of copper and the other of aluminium are rotated with the same angular speed in an external magnetic field. In which of the two coils will the induced current be more ?

Answer :

Copper

Question 18: (Marks 1) Answer :

Zero Eddy currents are produced in metal block / block gets heated

Question 19: (Marks 1)

Mention the contribution of Indian physicist J.C. Bose in the production of electromagnetic waves.

Answer :

J.C Bose observed / produced electromagnetic waves of short wavelength/ did very significant work in production of e.m waves.

Question 20: (Marks 1)

Write one use of the electromagnetic waves of frequency range from 1016 Hz to 1020 Hz.

Answer :

X rays are used as diagnostic tool in medicine /

Gamma rays are used to destroy cancer cells

SECTION B

Question 21: (Marks 2) Answer : Question 22: (Marks 2)

Two identical bars, one of paramagnetic material and other of diamagnetic material are kept in a uniform external magnetic field parallel to it. Draw diagrammatically the modifications in the magnetic field pattern in each case.

Answer : Question 23: (Marks 2)

Two coplanar and concentric coils 1 and 2 have respectively the number of turns N1 and N2 and radii r1 and r2 (r2 >> r1). Deduce the expression for mutual inductance of this system.

Answer : Question 24: (Marks 2)

How does an oscillating charge radiate an electromagnetic wave ? Give the relation between the frequency of radiated wave and the frequency of oscillating charge.

OR

(a) Explain briefly the fact that electromagnetic waves carry energy.

(b) Why do we not feel the pressure due to sunshine ?

Answer :

An oscillating charge produces an oscillating electric field in space, which produces an oscillating magnetic field, which in turn, is a source of oscillating electric field, and so on. The oscillating electric and magnetic fields thus regenerate each other, as the wave propagates through the space.

The frequency of the electromagnetic wave equals the frequency of oscillation of the charge.

OR

a) Consider a plane perpendicular to the direction of propagation of the electromagnetic wave. If there are, on this plane, electric charges, they will be set and sustained in motion by the electric and magnetic fields of the electromagnetic wave. The charges thus acquire energy and momentum from the waves.

b) When the sun shines on your hand, you feel the energy being absorbed from the electromagnetic waves (your hands get warm). Electromagnetic waves also transfer momentum to your hand but because c is very large, the amount of momentum transferred is extremely small and you do not feel the pressure.

Question 25: (Marks 2)

A converging lens of focal length f1 is placed coaxially in contact with a diverging lens of focal length f2 (f1 > f2). Determine the power and nature of the combination in terms of f1 and f2.

OR

How is the resolving power of a compound microscope affected if (a) wavelength of light used is decreased, and (b) the diameter of its objective lens is increased ? Justify your answers.

Answer : Justification of the following is based on the above formula:

a) If λ decreases, Resolving Power increases.

b) If diameter of objective lens is increased, sinθ increases, Resolving Power increases

Question 26: (Marks 2)

Define the terms (a) threshold frequency, and (b) stopping potential. How were these terms incorporated in Einstein’s photoelectric equation ?

Answer :

(a) Threshold Frequency: The minimum cut off frequency v0 below which no photoelectric emission is possible, even if the intensity is large

(b) Stopping Potential: The minimum negative (retarding) potential v0 given to the plate for which the photocurrent stops or becomes zero is called the cut off or stopping potential. Question 27: (Marks 2)

A hydrogen atom is in its third excited state.

(a) How many spectral lines can be emitted by it before coming to the ground state ? Show these transitions in the energy level diagram.

(b) In which of the above transitions will the spectral line of shortest wavelength be emitted ?

Answer : SECTION C

Question 28: (Marks 3)

(a) Differentiate between the random velocity and the drift velocity of electrons in an electrical conductor. Give their order of magnitudes.

(b) A conductor of uniform cross-sectional area is connected across a dc source of variable voltage. Draw a graph showing variation of drift velocity of electrons (vd) as a function of current density (J) in it.

Answer : Question 29: (Marks 3) Answer : Question 30: (Marks 3)

(a) Using the necessary ray diagram, derive the mirror formula for a concave mirror.

(b) In the magnified image of a measuring scale (with equidistant markings) lying along the principal axis of a concave mirror, the markings are not equidistant. Explain.

Answer : Question 31: (Marks 3)

(a) The density of the nuclear matter is tremendously larger than the physical density of the material. Explain.

(b) The nuclear forces are not coulomb forces between nucleons. Explain.

(c) Draw a plot of the potential energy between a pair of nucleons as a function of distance between them inside a nucleus.

Answer : Question 32: (Marks 3)

What do you mean by wave nature of an electron ? How was quantisation of angular momentum of the orbiting electron in Bohr’s model of hydrogen atom explained by de Broglie hypothesis ?

Answer : Question 33: (Marks 3)

Name the diode which can act as a voltage regulator. Explain its working with the help of its labelled circuit diagram. Draw its V – I characteristic.

Answer : Question 34: (Marks 3)

(a) Why is an intrinsic semiconductor deliberately converted into an extrinsic semiconductor by adding impurity atoms ?

(b) Explain briefly the two processes that occur in p-n junction region to create a potential barrier.

Answer :

a) To increase the electrical conductivity / to increase the number density of charge carriers

b) Diffusion and Drift

Explanation Diffusion: During the formation of p-n junction, due to the concentration gradient across the p and n sides, the motion of majority charge carriers give rise to diffusion current.

Drift: Due to the electric field developed at the junction, the motion of the minority charge carriers due to electric field is called drift.

With the passage of time, diffusion current decreases whereas drift current increases and balance each other. This, creates a potential barrier.

SECTION D

Question 35: (Marks 5) Answer : Question 36: (Marks 5) Answer : Question 37: (Marks 5) Answer : ☞ Click here for privious year Question papers

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