Thursday, 21 Feb 2019

CBSE exams 2019: Class 12 Physics sample paper

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CBSE exams 2019: Check the sample paper of Class 12 Physics

– Written by Ambarish Kumar Dixit

CBSE exams 2019: CBSE has released the date sheet of class 12 exams, according to which, exams will begin from February 15. The board has scheduled the Physics paper on March 5. Here is the sample paper for students:

General Instructions:

(i) All questions are compulsory.

(ii) There are 27 questions in all.

(iii) This question paper has four sections: Section A, Section B, Section C and Section D.

(iv) Section A contains five questions of one mark each, Section B contains seven questions of two
marks each, Section C contains twelve questions of three marks each, and Section D contains three
questions of five marks each.

(v) There is no overall choice. However, internal choices have been provided in two questions of one
mark, two questions of two marks, four questions of three marks and three questions of five marks
weightage. You have to attempt only one of the choices in such questions.

(vi) You may use the following values of physical constants wherever necessary

SECTION A

1. How does electric potential change in the direction of electric field?

2. Define mobility. Write its SI unit.

3. By what percentage will the transmission range of a T.V. tower be affected when the height of the
tower is increased by 21 %?

4. If total energy transferred to a surface by an electromagnetic wave in time t is U, what would be the
magnitude of the total momentum delivered to this surface?

Or

Name the part of the electromagnetic spectrum of wavelength 210 m and mention its one application.

5. A plot of magnetic flux versus current (I) is shown in the figure for two inductors A and B.
Which of the two has larger value of self-inductance?

Or

Define self-inductance of a coil. Write its SI units.

SECTION B

6. The oscillating magnetic field in a plane electromagnetic wave is given by

(i) Calculate the wavelength of the electromagnetic wave.

(ii) Write down the expression for the oscillating electric field.

7. Derive an expression for the de-Broglie wavelength associated with an electron accelerated through
a potential V. Draw a schematic diagram of a localised-wave describing the wave nature of the
moving electron.

8. A current is induced in coil C 1 due to the motion of current carrying coil C 2 . (a) Write any two ways
by which a large deflection can be obtained in the galvanometer G. (b) Suggest an alternative device
to demonstrate the induced current in place of a galvanometer.

9. What sky wave communication? Why this mode of propagation is restricted to the frequencies only
up to few MHz?

Or

For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum
amplitude is 2.V. Calculate the modulation index. Why modulation index is generally kept less than
one?

10. Draw a plot of potential energy of a pair of nucleons as a function of their separation. Write two
important conclusions which you can draw regarding the nature of nuclear forces.

11. Define the resolving power of a microscope. How is this affected when
(i) the wavelength of illuminating radiations is decreased, and
(ii) the diameter of the objective lens is decreased?

Justify your answer.

Or

Draw a labelled ray diagram of a reflecting telescope. Mention its two advantages over the
refracting telescope.

12. In the given circuit, assuming point A to be at zero potential, use Kirchhoff’s rules to determine the
potential at point B.

SECTION C

13. State Gauss’s law in electrostatic. A cube with each side ‘a’ is kept in an electric field given by
ECi
(as is shown in the figure) where C is a positive dimensional constant. Find out

(i) the electric flux through the cube, and

(ii) the net charge inside the cube.

14. A capacitor of unknown capacitance C is connected across a battery of V volts. The charge stored in
it is 360 C . When potential across the capacitor is reduced by 120 V, the charge stored in it
becomes 120 C . Calculate:

(i) The potential V and the unknown capacitance C.

(ii) What will be the charge stored in the capacitor, if the voltage applied had increased by 120 V?

15. Calculate the value of the resistance R in the circuit shown in the figure so that the current in the
circuit is 0.2 A. What would be the potential difference between points B and E?

16. (i) How does angle of dip change as one goes from magnetic pole to magnetic equator of the Earth?

(ii) A uniform magnetic field gets modified as shown below when two specimens X and Y are
placed in it. Identify whether specimens X and Y are diamagnetic, paramagnetic or ferromagnetic..

(iii) How is the magnetic permeability of specimen X different from that of specimen Y?

Or

Depict the field-line pattern due to a current carrying solenoid of finite length.

(i) In what way do these lines differ from those due to an electric dipole?

(ii) Why can’t two magnetic field lines intersect each other?

17. Figure shows a rectangular loop conducting PQRS in which the arm PQ is free to move. A uniform
magnetic field acts in the direction perpendicular to the plane of the loop. Arm PQ is moved with a
velocity v towards the arm RS. Assuming that the arms QR, RS and SP have negligible resistances
and the moving arm PQ has the resistance r, obtain the expression for (i) the current in the loop (ii)
the force and (iii) the power required to move the arm PQ.

18. A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 120 rev/min in a plane
normal to the horizontal component of the Earth’s magnetic field. The Earth’s magnetic field at the
plane is 0.4 G and the angle of dip is 60°. Calculate the emf induced between the axle and the rim of
the wheel. How will the value of emf be affected if the number of spokes were increased?

Or

An inductor 200 mH, capacitor 500 F , resistor 10 are connected in series with a 100 V, variable
frequency ac source. Calculate the

(i) frequency at which the power factor of the circuit is unity.

(ii) current amplitude at this frequency.

(iii) Q-factor.

19. In Young’s double slit experiment, monochromatic light of wavelength 630 nm illuminates the pair
of slits and produces an interference pattern in which two consecutive bright fringes are separated
by 8.1 mm. Another source of monochromatic light produces the interference pattern in which the
two consecutive bright fringes are separated by 7.2 mm. Find the wavelength of light from the
second source.

What is the effect on the interference fringes if the monochromatic source is replaced by a source of
white light?

Or

In Young’s double slit experiment, the two slits 0.15 mm apart are illuminated by monochromatic
light of wavelength 450 nm. The screen is 1.0 m away from the slits.

(a) Find the distance of the second (i) bright fringe, (ii) dark fringe from the central maximum.

(b) How will the fringe pattern change if the screen is moved away from the slits?

20. A jar of height h is filled with a transparent liquid of refractive index n. There is a small bulb at the
centre of the bottom of the jar. Find the minimum diameter of a disc, such that when it is placed on
the top surface symmetrically about the centre, the bulb is invisible. (Consider the bulb to be a point
source.)

21. Define the terms ‘threshold frequency’ and ‘stopping potential’ in the study of photoelectric
emission. Explain briefly the reasons why wave theory of light is not able to explain the observed
features in photoelectric effect.

22. The energy levels of a hypothetical atom are shown below. Which of the shown transitions will
result in the emission of a photon of wavelength 275 nm?
Which of these transitions correspond to emission of radiation of (i) maximum and (ii) minimum
wavelength?

23. (i) What characteristic property of nuclear force explains the constancy of binding energy per
nucleon (BE/A) in the range of mass number ‘A’ lying 30 < A < 170?
(ii) Show that the density of nucleus over a wide range of nuclei is constant independent of mass
number A.

Or

In a Geiger–Marsden experiment, calculate the distance of closest approach to the nucleus of Z=80,
-particle of 8 MeV energy impinges on it before it comes momentarily to rest and
reverses its direction. How will the distance of closest approach be affected when the kinetic energy
of the a-particle is doubled?

24. Name the type of waves which are used for line of sight (LOS) communication. What is the range
of their frequencies?

A transmitting antenna at the top of a tower has a height of 20 m and the height of the receiving
antenna is 45 m. Calculate the maximum distance between them for satisfactory communication in

LOS mode. (Radius of the Earth = 6.4 × 10 6 m)

SECTION D

25. A voltage osin VVt is applied to a series LCR circuit drives a current osinii in the circuit.

Deduce the expression for the average power dissipated in the circuit. For circuits used for
transporting electric power, a low power factor implies large power loss in the transmission.
Explain. Also, define the term “Wattless current”.

Or

(a) Show that in an ac circuit containing a pure inductor, the voltage is ahead of current by /2 in
phase.

(b) A horizontal straight wire of length L extending from east to west is falling with speed v at right
angles to the horizontal component of Earth’s magnetic field B.

(i) Write the expression for the instantaneous value of the e.m.f. induced in the wire.

(ii) What is the direction of the e.m.f.?

(iii) Which end of the wire is at the higher potential?

26. Define magnifying power of a telescope. Write its expression.
A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length
5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final
image when it is formed 25 cm away from the ey epiece.

Or

How is the working of a telescope different from that of a microscope?
The focal lengths of the objective and eyepiece of a microscope are 1.25 cm and 5 cm respectively.
Find the position of the object relative to the objective in order to obtain an angular magnification of
30 in normal adjustment.

27. (a) Why is a Zener diode considered as a special purpose semiconductor diode?

(b) Draw the I-V characteristics of Zener diode and explain briefly, how reverse current suddenly
increase at the breakdown voltage?

(c) Describe briefly with the help of a circuit diagram, how Zener diode works as voltage regulator?

OR

(a) Draw the circuit arrangement for studying the V-I characteristics of a p-n junction diode in (i)
forward and (ii) reverse bias. Briefly explain how the typical V-I characteristics of a diode are obtained and draw these characteristics.

(b) With the help of the necessary circuit diagram, explain the working of a photodiode used for
detecting optical signal.