MBOSE Class 12 Question Paper 2020 for Physics Old Course

Total No. of Printed Pages—11
HS/XII/Sc/Ph/OC/20

2020

PHYSICS

( Theory )

( Old Course )

Full Marks : 70

Time : 3 hours

The figures in the margin indicate full marks for the questions

General Instructions :

(i) All questions are compulsory.

(ii) All the answers are to be written in the Answer Script.

(iii) There is no overall choice. However, internal choices
have been provided in two questions of 2 marks,
two questions of 3 marks and all questions of 5 marks.

(iv) Use of non-programmable ordinary scientific calculator
and/or logarithmic table is allowed.

(v) Use of Mobile Phones, Pagers and such other
electronic gadgets is not allowed in the Examination
Hall.

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(vi) Use the following values of physical constants,
wherever necessary :
Speed of light in vacuum, c  3  108 m s 1

Planck’s constant, h  6  63  1034 J s

Permittivity of free space,
0  8  86  1012 C2 N1 m2

Permeability of free space, 0  4   10 7 T m A 1

Mass of electron, me  9  11  10 31 kg

Mass of proton, m p  1  67  1027 kg

Electronic charge, e  1  6  1019 C

GROUP—A
( Multiple choice type questions )
Choose and write the correct option for the following : ½×8=4

1. An electric dipole consisting of charge  q and  q is

placed in a uniform electric field E such that its dipole
 
moment p makes an angle  with the direction of E .

The net force F and torque  acting on the dipole are

(A) F  0, 0

(B) F  0, 0

(C) F  0, 0

(D) F  0, 0

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2. When air between the plates of a capacitor is replaced
by mica of dielectric constant k = 6, then the capacitance
of the parallel-plate capacitor

(A) remains unaffected

(B) reduces to 1/6 times

(C) becomes 6 times

(D) None of the above

3. The total resistance of two resistors when connected
in series is 9  and when connected in parallel is 2 .
The values of two resistances are

(A) 4  and 5 

(B) 2  and 7 

(C) 3  and 6 

(D) 1  and 8 

4. Kirchhoff’s second law in electricity is based on the law
of conservation of

(A) sum of mass and energy

(B) momentum

(C) energy

(D) charge

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5. To convert a given galvanometer into an ammeter of
desired range

(A) low resistance is connected in series

(B) low resistance is connected in parallel

(C) high resistance is connected in series

(D) high resistance is connected in parallel


6. A charge q moves
 with velocity v in a region where
electric field E and magnetic field B both exist. Then
the force acting on the charge q is
  
(A) qE  q (v  B )
 
(B) q (v  B )
  
(C) qE  q (B  v )
  
(D) qB  q (E  v )

7. If a convex lens of focal length 80 cm and a concave lens
of focal length 50 cm are combined by placing them
together in contact, then their resulting power will be

(A) + 6·5 D

(B) – 6·5 D

(C) + 7·5 D

(D) – 7·5 D

HS/XII/Sc/Ph/OC/20/37 [ Contd.

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8. For relaxed eye, the magnifying power of a microscope
is

v0 D
(A) 
u0 f e

v0 f e
(B) 
u0 D

u0 D
(C) 
v0 f e

u0  D 
(D)  
v 0  f e 

GROUP—B
( Very short answer type questions )
Answer the following questions in one sentence/step each :
1×8=8

9. Two equal negative point charges – 2 C each are held
1 m apart. Calculate their potential energy. 1

10. On which factors does the capacitance of a capacitor
depend? 1

11. Define resistivity of the material of a wire. State its
SI unit. 1

12. A heating element is marked 210 V, 630 W. What is the
value of current drawn by the element when connected
to a 210 V d.c. source? 1

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13. When a direct current of 2·5 A is passed through a coil of
500 turns, the magnetic flux produced is 1·4×10–4 Wb.
What is the inductance of the coil? 1

14. Which of the following belong to the electromagnetic
spectrum?
-rays, -rays, -rays, Cathode rays, X-rays,
UV rays, Microwaves, Ultrasonic waves, Radio waves,
Infrared rays
Arrange them in order of increasing frequency. 1

15. In Young’s double-slit experiment, the separation of the
slits is doubled and the distance between the slits and
the screen is halved. What will be the effect on the fringe
width? 1

16. The stopping potential in an experiment on a
photoelectric effect is 1·5 V. What is the maximum
kinetic energy of the photoelectron emitted? 1

GROUP—C
( Short answer type–I questions )
Answer the following questions within 30 words each,
wherever applicable : 2×8=16

17. Prove that the energy stored in a parallel-plate capacitor
is given by U  1 CV 2 , where the symbols have their
2
usual meanings. 2

HS/XII/Sc/Ph/OC/20/37 [ Contd.

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18. Write down a simple relation to show the variation of
resistance with temperature, and hence define the
temperature coefficient of resistance. 2

19. Either
In a hydrogen atom, an electron of charge –e revolves in
a circular path of radius r with a speed v. Deduce an
expression for the magnetic dipole moment of the
revolving electron. 2
Or
A moving-coil galvanometer consists of a rectangular coil
of n turns, each of area A, suspended in a radial
magnetic field of flux density B. At equilibrium position,
show that the current passing through the coil is
directly proportional to the angle of deflection of the
coil. 2

20. Either
How is Eddy current produced? Give two applications of
Eddy current. 1+1
Or
Define the term ‘mutual inductance’ and give its SI unit.
State two factors on which the coefficient of mutual
inductance between a pair of coils depends. 1+1

21. What do you mean by power factor of an a.c. circuit?
On what factors does it depend? 1+1

22. What are microwaves? State any two uses of
microwaves. 1+1

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23. The binding energies of 8 O16 and 17 Cl35 are 127·35 MeV
and 289·3 MeV respectively. (a) Calculate the binding
energy per nucleon of 8 O16 and 17 Cl35 , and (b) state
which of the two nuclei is more stable. 1+1

24. What are the basic elements of a communication
system? Draw the block diagram of a basic
communication system. 1+1

GROUP—D
( Short answer type–II questions )
Answer the following questions within 30 to 40 words each,
wherever applicable : 3×9=27

25. Obtain an expression for the electric potential energy of
an electric dipole placed in a uniform electric field. State
when it will be (a) minimum and (b) maximum. 2+½+½

26. Either
In the network shown below

find the currents I1 , I 2 and I 3 . 3

HS/XII/Sc/Ph/OC/20/37 [ Contd.

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Or
Find the value of the unknown resistance X in the
circuit shown in the figure below if no current flows
through the section AO. Also calculate the current
drawn by the circuit from the battery of e.m.f. 6 V and
negligible resistance. 3

27. State the principle of working of a transformer. Explain
how it works and mention two of its applications. 1+2

28. Either
State Brewster’s law. Show that the reflected ray and
refracted ray are perpendicular to each other when the
angle of incidence is equal to polarizing angle. 1+2
Or
State Huygens’ wave principles. Using the principles,
verify the law of reflection of light. 1+2

29. (a) What is the frequency of a photon whose energy is
75 eV? 2
(b) Derive Einstein’s photoelectric relation. 1

30. Using Bohr’s postulates, obtain an expression for the
total energy of an electron in the stationary orbit. 3

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31. What is rectification? With the help of a labelled circuit
diagram, explain full-wave rectification using
semiconductor diode. 1+2

32. Define the terms ‘mass defect’ and ‘binding energy’.
How are they related? 1+1+1

33. Calculate the modulation factor when signal amplitude
is—
(a) zero;
(b) equal to carrier amplitude;
(c) half the carrier amplitude. 1+1+1

GROUP—E
( Long answer type questions )
Answer the following questions in 70 to 80 words each,
wherever applicable : 5×3=15

34. Either
(a) Show that the path of a  charged particle moving
inside the electric field E is parabolic in nature. 3

(b) A cathode-ray tube is operated at 2500 V. What is
the speed of electrons emitted? 2

Or

(a) Draw a labelled diagram of a cyclotron. Explain its
working principle. Show that the frequency is
independent of speed and radius of the orbit. 1+1+1

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(b) A cyclotron is to accelerate protons to a kinetic
energy of 5·0 MeV. If the strength of the magnetic
field in the cyclotron is 2·0 T, find—
(i) the frequency needed for the applied
alternating voltage;
(ii) the radius of the cyclotron. 1+1

35. Either
Establish a relation for refraction at convex spherical
refracting surface of a denser medium when—
(a) the image formed is real;
(b) the image formed is virtual. 2½+2½
Or
What is meant by diffraction of light? Deduce the
condition for the position of secondary minima of
diffraction pattern in term of path difference.
Also show graphically the relative intensity distribution
for a single-slit diffraction pattern as a function of sin .
1+3+1
36. Either
Draw a circuit diagram to study the input and output
characteristics of an n-p-n transistor in a common-
emitter configuration. Show these characteristics
graphically. Explain how current amplification factor of
the transistor is calculated using output characteristics.
1+1+2+1
Or
Explain why photodiode is preferably operated under
reverse bias? Give two simple applications of a
photodiode with justification. 3+2

  

HS/XII/Sc/Ph/OC/20/37 20K