Physics Sample Paper For Class 12 With Solution NCERT CBSE Board 2020

Class 12 Subject Physics Paper Set 4 with Solutions

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Question 1: (Marks 1)

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Answer :

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Question 2: (Marks 1)

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Answer :

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Question 3: (Marks 1)

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Answer :

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Question 4: (Marks 1)

A circular loop of radius r, carrying a current I lies in y-z plane with its centre at the origin. The net magnetic flux through the loop is :

(a) directly proportional to r

(b) zero

(c) inversely proportional to r

(d) directly proportional to I

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Answer :

(b) Zero

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Question 5: (Marks 1)

The kinetic energy of a proton and that of an α-particle are 4 eV and 1 eV, respectively. The ratio of the de-Broglie wavelengths associated with them, will be

(a) 2:1

(b) 1:1

(c) 1:2

(d) 4:1

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Answer :

(b) 1:1

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Question 6: (Marks 1)

A photocell connected in an electrical circuit is placed at a distance ‘d’ from a source of light. As a result, current I flows in the circuit. What will be the current in the circuit when the distance is reduced to ‘d/2’ ?

(a) I

(b) 2I

(c) 4I

(d) I/2

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Answer :

(c) 4 I

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Question 7: (Marks 1)

A current of 10A is flowing from east to west in a long straight wire kept on a horizontal table. The magnetic field developed at a distance of 10 cm due north on the table is :

(a) 2 × 10^–5 T, acting downwards

(b) 2 × 10^–5 T, acting upwards

(c) 4 × 10^–5 T, acting downwards

(d) 4 × 10^–5 T, acting upwards

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Answer :

(a) 2 × 10^–5 T, acting downwards

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Question 8: (Marks 1)

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Answer :

(c) π

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Question 9: (Marks 1)

Paschen series of atomic spectrum of hydrogen gas lies in :

(a) Infrared region

(b) Ultraviolet region

(c) Visible region

(d) Partly in ultraviolet and partly in visible region

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Answer :

(a) Infra red region

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Question 10: (Marks 1)

In the α-article scattering experiment, the shape of the trajectory of the scattered α-particles depend upon :

(a) only on impact parameter.

(b) only on the source of α-particles.

(c) both impact parameter and source of α-particles.

(d) impact parameter and the screen material of the detector.

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Answer :

(a) Only on impact parameter

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Question 11: (Marks 1)

Torque acting on an electric dipole placed in an electric field is maximum when the angle between the electric field and the dipole moment is _______.

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Answer :

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Question 12: (Marks 1)

A proton released from rest in an electric field, will start moving towards a region of _______ potential in the field.

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Answer :

Decreasing/Lower

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Question 13: (Marks 1)

To minimize the percentage error in the determination of unknown resistance of a conductor in meter bridge experiment, the balance point is adjusted near _______ of the wire.

OR

In potentiometer, a long uniform wire is used to _______ potential gradient along the wire.

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Answer :

Middle/mid point /center

OR

Decrease

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Question 14: (Marks 1)

Unpolarised light of intensity I_o is incident on two crossed polaroids. The intensity of light transmitted by the combination will be _______.

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Answer :

Zero

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Question 15: (Marks 1)

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Answer :

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Question 16: (Marks 1)

The work done in moving a charge particle between two points in an uniform electric field, does not depend on the path followed by the particle. Why ?

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Answer :

Because the electrostatic force is conservative in nature

Alternatively:-

Electric field is conservative in nature / work done by or against the electric field does not depend upon the path followed.

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Question 17: (Marks 1)

Define ‘magnetic declination’ at a place on earth.

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Answer :

Magnetic declination is the angle between the magnetic meridian and the geographic meridian at a place on the earth.

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Question 18: (Marks 1)

An A.C. source with variable frequency is connected to a parallel plate capacitor. How will the displacement current be affected with the decrease in frequency of the source ?

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Answer :

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Question 19: (Marks 1)

An astronomical telescope may be a refracting type or a reflecting type. Which of the two produces image of better quality ? Justify your answer.

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Answer :

Reflecting type telescope

Reason/Justification :-

Mirror have large aperture/high resolving power/ free from chromatic aberration /free from spherical aberration. ( Any one)

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Question 20: (Marks 1)

Can a slab of p-type semi-conductor be physically joined to another n-type semiconductor slab to form p-n junction ? Justify your answer.

OR

In a p-n junction diode the forward bias resistance is low as compared to the reverse bias resistance. Give reason.

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Answer :

No

As there will be discontinuity for the flow of charge carriers / no contact at atomic level.

(Any One Justification)

OR

The forward current is large due to majority charge carriers which are very large in number. Hence resistance in forward bias is low. Alternatively: Depletion region decreases or barrier potential decreases.

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Question 21: (Marks 2)

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Answer :

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Question 22: (Marks 2)

A wire of length L_o has a resistance R_o. It is gradually stretched till its length becomes 2 L_o.

(a) Plot a graph showing variation of its resistance R with its length l during stretching.

(b) What will be its resistance when its length becomes 2 L_o?

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Answer :

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Question 23: (Marks 2)

A resistor R and an inductor L are connected in series to a source of voltage V = V_o sin ωt. The voltage is found to lead current in phase by π/4. If the inductor is replaced by a capacitor C, the voltage lags behind current in phase by π/4. When L, C and R are connected in series with the same source,

Find the :

(i) average power dissipated and

(ii) instantaneous current in the circuit.

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Answer :

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Question 24: (Marks 2)

Light of same wavelength is incident on three photo-sensitive surfaces A, B and C. The following observations are recorded.

(i) From surface A, photo electrons are not emitted.

(ii) From surface B, photo electrons are just emitted.

(iii) From surface C, photo electrons with some kinetic energy are emitted.

Compare the threshold frequencies of the three surfaces and justify your answer.

OR

If the frequency of light incident on the cathode of a photo-cell is increased, how will the following be affected ? Justify your answer.

(i) Energy of the photo electrons.

(ii) Photo current.

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Answer :

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Question 25: (Marks 2)

Briefly explain how a potential barrier is set up across a p-n junction as a result of diffusion and drift of the charge carriers.

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Answer :

Diffusion current is set up across the junction due to the concentration difference of the majority charge carriers on the two sides of the junction.

This diffusion develops an electric field from n- side to p- side across the junction which creates a drift current in the opposite direction.

When diffusion and drift current become equal in magnitude the potential difference across the junction is the barrier potential.

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Question 26: (Marks 2)

(a) Why is a photo diode operated under reverse bias condition ?

(b) Draw V-I characteristic curves of photo diode for incident light of intensities I₁ and I₂ (I₁ > I₂).

OR

(a) State the level of doping and biasing condition used in light emitting diode (LED).

(b) Write any two advantages of LED over the conventional low power lamps.

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Answer :

(a) Because the fractional change in the minority carriers dominated very weak reverse current is more easily measurable than fractional change in forward biased large current

(b)

I_1I2

OR

(a) It is a heavily doped p-n junction. It operates in forward biasing

(b) Advantages Low operational voltage/less power /fast action / nearly monochromatic / long life ( Any two)

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Question 27: (Marks 2)

(a) Explain the formation of energy bands in crystalline solids.

(b) Draw the energy band diagrams of (i) a metal and (ii) a semiconductor

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Answer :

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Question 28: (Marks 3)

A hollow conducting sphere of inner radius r₁ and outer radius r₂ has a charge Q on its surface. A point charge –q is also placed at the centre of the sphere.

(a) What is the surface charge density on the (i) inner and (ii) outer surface of the sphere ?

(b) Use Gauss’ law of electrostatics to obtain the expression for the electric field at a point lying outside the sphere.

OR

(a) An infinitely long thin straight wire has a uniform linear charge density λ. Obtain the expression for the electric field (E) at a point lying at a distance x from the wire, using Gauss’ law.

(b) Show graphically the variation of this electric field E as a function of distance x from the wire.

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Answer :

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Question 29: (Marks 3)

(a) Explain the principle of working of a potentiometer.

(b) In a potentiometer, a standard source of emf 5V and negligible internal resistance maintains a steady current through the potentiometer wire of length 10m. Two primary cells of emf E₁ and E₂ are joined together in a series with (i) same polarity and (ii) opposite polarity. The combination is connected to the potentiometer circuit in each case. The balancing length of the wire in the two cases are found to be 700 cm and 100 cm, respectively. Find the values of emf of the two cells.

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Answer :

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Question 30: (Marks 3)

(a) Differentiate between self inductance and mutual inductance.

(b) The mutual inductance of two coaxial coils is 2H. The current in one coil is changed uniformly from zero to 0.5A in 100 ms. Find the :

(i) change in magnetic flux through the other coil.

(ii) emf induced in the other coil during the change.

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Answer :

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Question 31: (Marks 3)

Explain with the help of a diagram, the working of a step-down transformer. Why is a laminated iron core used in a transformer ?

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Answer :

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Question 32: (Marks 3)

Name the electro-magnetic waves with their frequency range, produced in

(a) some radioactive decay

(b) sparks during electric welding

(c) TV remote

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Answer :

(a) Gamma rays, frequency range 10¹⁹ to 10²⁴ Hz

(a) UV rays, frequency range 10¹⁵ to 10¹⁷ Hz

(b) Infrared rays, frequency range 10¹² to 10¹⁴ Hz

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Question 33: (Marks 3)

Two coherent light waves of intensity 5 × 10^–2 Wm^–2 each super-impose and produce the interference pattern on a screen. At a point where the path difference between the waves is λ/6, λ being wavelength of the wave, find the

(a) phase difference between the waves.

(b) resultant intensity at the point.

(c) resultant intensity in terms of the intensity at the maximum.

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Answer :

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Question 34: (Marks 3)

Two objects P and Q when placed at different positions in front of a concave mirror of focal length 20 cm, form real images of equal size. Size of object P is three times size of object Q. If the distance of P is 50 cm from the mirror, find the distance of Q from the mirror.

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Answer :

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Question 35: (Marks 5)

(a) Show that a current carrying solenoid behaves like a small bar magnet. Obtain the expression for the magnetic field at an external point lying on its axis.

(b) A steady current of 2A flows through a circular coil having 5 turns of radius 7 cm. The coil lies in X-Y plane with its centre at the origin. Find the magnitude and direction of the magnetic dipole moment of the coil.

OR

(a) Derive the expression for the force acting between two long parallel current carrying conductors. Hence, define 1 A current.

(b) A bar magnet of dipole moment 3 Am² rests with its centre on a frictionless pivot. A force F is applied at right angles to the axis of the magnet, 10 cm from the pivot. It is observed that an external magnetic field of 0.25 T is required to hold the magnet in equilibrium at an angle of 30° with the field.

Calculate the value of F.

How will the equilibrium be effected if F is withdrawn ?

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Answer :

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Question 36: (Marks 5)

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Answer :

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Question 37: (Marks 5)

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Answer :

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