ISC Class 12 Question Paper 2020 for Physics

PHYSICS
PAPER – 1
(THEORY)
(Maximum Marks: 70)
(Time allowed: Three hours)
(Candidates are allowed additional 15 minutes for only reading the paper.
They must NOT start writing during this time.)
------------------------------------------------------------------------------------------------------------
All questions are compulsory.
This question paper is divided into 4 Sections, A, B, C and D as follows:
Section A
Question number 1 is of twelve marks. All parts of this question are compulsory.
Section B
Question numbers 2 to 12 carry 2 marks each with two questions having internal choice.
Section C
Question numbers 13 to 19 carry 3 marks each with two questions having internal choice.
Section D
Question numbers 20 to 22 are long-answer type questions and carry 5 marks each.
Each question has an internal choice.
The intended marks for questions are given in brackets [ ].
All working, including rough work, should be done on the same sheet as and
adjacent to the rest of the answer.
Answers to sub parts of the same question must be given in one place only. A list of
useful physical constants is given at the end of this paper.
A simple scientific calculator without a programmable memory may be used for
calculations.
Section A
Question 1
(A) Choose the correct alternative (a), (b), (c) or (d) for each of the questions given [5×1]
below:
(i) A point charge ‘q’ is kept at each of the vertices of an equilateral triangle
having each side ‘a’. Total electrostatic potential energy of the system
is:

(a)
 1  3q 2 (b)
 1  3q
  2  
 4 o  a  4 o  a

(c)
 1  3q 2 (d)
 1  3q
    2
 4 o  a  4 o  a
This Paper consists of 8 printed pages.
1220-861A Turn over
© Copyright reserved.

(ii) Curie temperature is the temperature above which:
(a) a ferromagnetic substance behaves like a paramagnetic substance.
(b) a paramagnetic substance behaves like a diamagnetic substance.
(c) a ferromagnetic substance behaves like a diamagnetic substance.
(d) a paramagnetic substance behaves like a ferromagnetic substance.
(iii) In an astronomical telescope of refracting type:
(a) Objective should have small focal length.
(b) Objective should have large focal length.
(c) Eyepiece should have large focal length.
(d) Both objective and eyepiece should have large focal length.
(iv) In photoelectric effect experiment, the slope of the graph of the
stopping potential versus frequency gives the value of:

(a)
h (b) h
e
(c)
e (d)
hc
h e
(v) In a nuclear reactor, cadmium rods are used as:
(a) Control rods
(b) Fuel rods
(c) Coolant
(d) Moderator
(B) Answer the following questions briefly and to the point: [7×1]
(i) State Gauss’ theorem.
(ii) A metallic wire having a resistance of 20Ω is bent in order to form a
complete circle. Calculate the resistance between any two diametrically
opposite points on the circle.
(iii) How can a moving coil galvanometer be converted into a voltmeter?
(iv) Write Biot-Savart’s law in vector form.
(v) What is the phase difference between any two points lying on the
same wavefront?
(vi) Name the physical principle on the basis of which optical fibres work.

(vii) What is Pair production?

2

Section B
Question 2 [2]
(a) A uniform copper wire having a cross sectional area of 1mm2 carries a current
of 5A. Calculate the drift speed of free electrons in it.
(Free electron number density of copper = 2×1028/m3.)
OR
(b) An electric bulb is rated as 250V, 750W. Calculate the:
(i) Electric current flowing through it, when it is operated on a 250V
supply.
(ii) Resistance of its filament.

Question 3 [2]
Write an expression for force per unit length between two long current carrying
wires, kept parallel to each other, in vacuum and hence define an ampere, the SI unit
of current.

Question 4 [2]
(i) Define angle of dip.
(ii) State the relation between magnetic susceptibility (  ) and
relative permeability ( r ) of a magnetic substance.

Question 5 [2]
(a) Figure 1 below shows a metallic rod MN of length l = 80cm, kept in a uniform
magnetic field of flux density B = 0·5T, on two parallel metallic rails P and
Q. Calculate the emf that will be induced between its two ends, when it is
moved towards right with a constant velocity v of 36 km/hr.
M P

x x x x x

l = 80cm
x x x x x

N Q
Figure 1

3
1220-861A Turn over

OR
(b) When current flowing through one coil changes from 0 Amp to 15 Amp in
0·2 s, an emf of 750V is induced in an adjacent coil. Calculate the coefficient
of mutual inductance of the two coils.

Question 6 [2]
(i) State any one use of infrared radiations.
(ii) State any one source of ultraviolet radiations.

Question 7 [2]
Where will you keep an object in front of a:
(i) Convex lens in order to get a virtual and magnified image?
(ii) Concave mirror to get a real and diminished image?

Question 8 [2]
Draw a labelled graph of angle of deviation (𝛿) versus angle of incidence (i) for a
prism.

Question 9 [2]
(i) State de Broglie hypothesis.
(ii) What conclusion can be drawn from Davisson and Germer’s experiment?

Question 10 [2]

Calculate binding energy of oxygen nucleus ( O) from the data given below:
16
8

Mass of a proton = 1·007825u
Mass of a neutron = 1·008665u
Mass of ( O)
16
8
= 15·994915u

Question 11 [2]
For a radioactive substance, write the relation between:
(i) Half life (T) and disintegration constant (λ).
(ii) Mean life ( ) and disintegration constant (λ).

Question 12 [2]
With reference to communication systems, what is meant by:
(i) modulation?
4

(ii) demodulation?
Section C

Question 13 [3]
Show that intensity of electric field E at a point in broadside on position is given by:

 1  p
E=  2 2 32 ,
 4 o  ( r + l )
where the terms have their usual meaning.

Question 14 [3]
A parallel plate capacitor is charged by a battery, which is then disconnected.
A dielectric slab having dielectric constant (relative permittivity) K, is now
introduced between its two plates in order to occupy the space completely.
State, in terms of K, its effect on the following:
(i) The capacitance of the capacitor.
(ii) The potential difference between its plates.
(iii) The energy stored in the capacitor.

Question 15 [3]
(a) E1 and E2 are two batteries having emfs of 3V and 4V and internal resistances
of 2Ω and 1Ω respectively. They are connected as shown in Figure 2 below.
Using Kirchhoff’s Laws of electrical circuits, calculate the currents I1 and I2.

A I1 B I2 C
• • •

3V, 2Ω E1 E2 4V, 1Ω
R3 8Ω
R1=4Ω R2=7Ω

• • •
F E D
Figure 2

OR

5
1220-861A Turn over

(b) A potentiometer circuit is shown in Figure 3 below. AB is a uniform metallic
wire having length of 2m and resistance of 8Ω. The batteries E1 and E2 have
emfs of 4V and 1·5V and their internal resistances are 1Ω and 2Ω respectively.

E1 R
•
4V, 1Ω 7Ω
2m, 8Ω
A• C B
•
J
E2
G
1·5V, 2Ω
Figure 3
(i) When the jockey J does not touch the wire AB, calculate:
(a) the current flowing through the potentiometer wire AB.
(b) the potential gradient across the wire AB.
(ii) Now the jockey J is made to touch the wire AB at a point C such that
the galvanometer (G) shows no deflection. Calculate the length AC.

Question 16 [3]
For two thin lenses kept in contact with each other, show that:
1 1 1
= +
F f1 f 2
where the terms have their usual meaning.

Question 17 [3]
(a) A compound microscope consists of two convex lenses having focal length
of 1·5cm and 5cm. When an object is kept at a distance of 1·6cm from the
objective, the final image is virtual and lies at a distance of 25cm from the
eyepiece. Calculate magnifying power of the compound microscope in this
set-up.
OR
(b) In Young’s double slit experiment, the screen is kept at a distance of 1·2m
from the plane of the slits. The two slits are separated by 5mm and illuminated
with monochromatic light having wavelength 600nm. Calculate:
(i) Fringe width i.e. fringe separation of the interference pattern.
(ii) Distance of 10th bright fringe from the centre of the pattern.

6

Question 18 [3]
Draw the energy level diagram of hydrogen atom and show the transitions
responsible for:
(i) absorption lines of Lyman series.
(ii) emission lines of Balmer series.

Question 19 [3]
(i) State any one difference between energy band diagram of conductors and that
of insulators.
(ii) Give a relation between 𝛼 and 𝛽 for a transistor.
(Derivation is not required.)
(iii) What is the advantage of an LED bulb over the filament electric bulb?

Section D
Question 20 [5]
(a) (i) A 400Ω resistor, a 3H inductor and a 5µF capacitor are connected in
series to a 220V, 50Hz ac source. Calculate the:
(1) Impedance of the circuit.
(2) Current flowing through the circuit.
(ii) Draw a labelled graph showing the variation of impedance (Z) of a
series LCR circuit versus frequency ( f ) of the ac supply.
OR
(b) (i) When an alternating emf e = 310 sin (100𝜋t)V is applied to a series LCR
circuit, current flowing through it is i = 5 sin(100𝜋t + 𝜋/3)A.
(1) What is the phase difference between the current and the emf?
(2) Calculate the average power consumed by the circuit.
(ii) Obtain an expression for the resonant frequency (fo) of a series LCR
circuit.

Question 21 [5]
(a) (i) Derive an expression for refraction at a single (convex) spherical
surface, i.e. a relation between u, v, R, n1 (rarer medium) and
n2 (denser medium), where the terms have their usual meaning.
(ii) Name the phenomenon due to which the sun appears reddish at sunset.
OR
7
1220-861A Turn over

(b) (i) Draw a labelled graph of intensity of diffracted light (I) versus
angle (𝜃) in the Fraunhofer diffraction experiment for a single slit
diffraction.
(ii) State the law of Malus.
(iii) How will you distinguish experimentally between ordinary light and
plane polarized light?

Question 22 [5]
(a) (i) In a semiconductor diode, what is meant by potential barrier?
(ii) Draw a labelled circuit diagram of a Zener diode as a voltage
regulator.
(iii) Show with the help of a diagram, how you will obtain an AND gate
using only NAND gates. (Truth table is not required.)
OR
(b) (i) Draw a labelled circuit diagram of a transistor acting as a
common emitter amplifier. What is meant by phase reversal?
(ii) Draw the symbol of a NAND gate and write its truth table.

Useful Constant and Relation:

1u = 931MeV

8