Download NAVODAYA VIDYALAYA SAMITI : NEW DELHI

HSB COACHING CENTRE
SAMPLE PAPER-IV
PHYSICS FOR CLASS XII
General Instructions:
(a) All questions are compulsory
(b) There are 30 questions in total. Qns. 1 to 8 carry 1 mark each. Qns. 9 to 18 carry 2 marks
each, Qns. 19 to 27 carry 3 mark each and Qns. 28 to 30 carry 5 marks each.
(c) There is no overall choice. However, an internal choice has been provided in one question of
2 marks, one question of 3 marks and all three questions of 5 marks each. You have to attempt
only one of the given choices in such questions.
(d) Use of calculators is not permitted.
(e) You may use the following physical constants wherever necessary.
c = 3 x 108 ms-1
h = 6.6 x 10-34 Js
e = 1.6 x 10-19C
µo = 4π x 10-7 TmA-1
1/4πεo = 9 x 109 N m2C-2
Avogadro number
NA = 6.023 x 1023 mol-1
Mass of the neutron = 1.675 x 10-27 kg
Boltzmann constant, k = 1.38 x 1023 J K-1
1
2
3
4
5
6
A choke coil and a bulb are connected in series to a d c source. The bulb shines
brightly. How does its brightness change, when an iron core is inserted inside the
choke coil?
The de Broglie wavelengths, associated with a proton and a neutron, are found to
be equal. Which of the two has a higher value for kinetic energy?
Explain with an example, whether the neutron – proton ratio in a nucleus
increases or decreases due to β – decay.
A proton moves along the axis of a straight solenoid of 1000 turns per metre and
carrying a current of 4 A with a velocity of 4 x 10 4 ms-1. What is the magnitude
of the force acting on the proton?
Sketch a graph to show the variation of charge stored in a capacitor with
potential difference across its plates.
The graph showing the variation of resistivity with temperature for a material is
given below.
Resistivity
ρo
O
temp.
What type of material is it referring to? What does the positive slope of the
graph indicate?
7
For the potentiometer circuit shown in the given figure, points X and Y represent
the two terminals of an unknown e m f E'. A student observed that when the
jockey in moved from the end A to the end B of the potentiometer wire, the
deflection in the galvanometer remains in the same direction and the deflection
at the end B is more than that at the end A. What is the possible cause for this?
E
A
B
E’
X
8
9
10
11
12
13
G
Y
How does the focal length of a convex lens change, if the incident violet light is
replaced by red light?
A charged metallic sphere A is suspended by a nylon thread. Another charged
metallic sphere B is brought close to A such that the distance between their
centres is 10 cm. The resulting repulsion of A is noted. Spheres A and B are
touched by uncharged spheres C and D respectively. C and D are then removed
and B is brought closer to A to a distance of 5.0 cm between their centres. What
is the expected repulsion of A on the basis of Coulomb’s law? Spheres A and C
and spheres B and D have identical sizes. Ignore the sizes of A and B in
comparison to the separation between their centres.
Two charges 2 μC and –2 μC are placed at points A and B 6 cm apart.
(a)
Identify an equipotential surface of the system.
(b)
What is the direction of the electric field at every point on this surface?
A cylindrical metallic wire is stretched to increase its length by 5%. Calculate
the percentage change in its resistance.
Draw the magnetic hysterisis curves for soft iron and steel. Based on it state the
reason for the use of soft iron as core of transformers and chokes.
A short bar magnet placed with its axis at 30o to a uniform magnetic field of
0.2T, experiences a torque of 0.060 Nm.
(i)
Calculate the magnetic moment of the magnet
(ii)
Find out potential energy of the magnet when its orientation leads to its
stable equilibrium in the field.
OR
A charged particle having a charge ‘q’ is moving with a speed ‘v’ along the X –
axis. It enters a region of space where electric field E ( = E ĵ) and the magnetic
field B are both present. The particle on emerging from this region is observed to
be moving along the X – axis only. Obtain an expression for the magnitude of B
in terms of ‘v’ and E. Give the direction of B.
14 Distinguish between the terms 'average value' and 'r m s value' of an alternating
current. The instantaneous current from an a.c. source is I = 5 sin (314 t) ampere.
What are the average and r m s values of the current?
15 A beam of parallel rays of light is incident on a concave lens of refractive index
n1 held in a medium of refractive index n2. Trace the path of the refracted rays
when (i) n2 > n1
(ii) n2 < n1.
16 Two different radioactive elements with half lives 1 hr and 2 hr have equal
number of (un decayed) atoms respectively present at a given instant. Determine
the ratio of their activities after 2 hr.
17 Determine the currents through the resistance R of the circuits 1 and 2 when
similar diodes D1 and D2 are connected as shown here.
D1
D2
D1
20Ω
2V
D2
20Ω
2V
18 What do you mean by detection of an amplitude modulated wave? Explain the
working of a diode detector used in an A M receiver, with the help of a circuit
diagram.
19 A spherical conductor of radius 12 cm has a charge of 1.6 x 10-7 C charge
distributed uniformly over its surface. Calculate the electric field at a point:
(i)
at a distance of 8 cm from the centre
(ii)
outside and close to the surface of the sphere
(iii)
at a distance of 18 cm from the centre.
20 In the potentiometer circuit shown, the balance point is at X. State with reason
where the balance point will be shifted when;
(i)
resistance, R is increased keeping all parameters unchanged.
(ii)
resistance, S is increased keeping R constant
(iii)
cell P is replaced by another cell whose e m f is lower than that of cell Q.
P
R
X
A
B
Q
S
G
OR
A series battery of six cells each of e m f 2 V and internal resistance 0.5 Ω is
charged by a 100 V d c supply. What series resistance should be used in the
charging circuit in order to limit the current to 8 A? Using the required resistor
obtain (i)
the power supplied by the d c source
(ii) terminal voltage across the battery
(iii) the power dissipated as heat.
21 Explain briefly the elements required to specify the earth’s magnetic field at a
given place. How does the orientation of a freely suspended magnetic needle
change, when it is moved from the geo magnetic equator to the geo magnetic
poles?
22 The amplitude of electric field in an electromagnetic wave is Eo = 120 N/C and
its frequency is ν = 50.0 MHz. (a) Determine, Bo,ω, k, and λ. (b) Find
expressions for E and B.
23 What is total internal reflection? Write the conditions under which it takes place.
An object PQ is placed in front of a right angled prism as shown in the figure.
The prism is made of crown glass with critical angle 41o. Trace the path of rays
from P and Q normal to the hypotenuse.
P
Q
24 State the conditions for obtaining sustained interference of light from different
sources. The ratio of intensities of maxima and minima in an interference pattern
is found to be 25:9. Calculate the ratio of intensities of the sources producing
this pattern.
25 Draw a schematic diagram of the experimental arrangement used by Davisson
and Germer to establish the wave nature of electrons. Explain briefly how the
de-Broglie relation was experimentally verified in case of electrons.
26 The spectrum of a star in the visible and the ultraviolet region was observed and
the wavelength of some of the lines that could be identified were found to be :
824Ả, 970Ả, 1120Ả, 2504Ả, 5173Ả, 6100Ả
Which of these lines cannot belong to hydrogen atom spectrum? [Given Rydberg
constant R = 1.03 X 107m-1 and (1/R) = 970 Ả.] Support your answer with
suitable calculations.
27 What do you mean by sky wave propagation? Why is it restricted to the
frequency of 30 M Hz.
A T V tower has a height of 400 m at a given place. Calculate the coverage
range, if the radius of the Earth is 6400km.
28 Distinguish between the terms resistance, reactance and impedance of an a c
circuit. Graphically represent their variation with frequency of the applied a c.
An inductor L, a capacitor 20 µF and a resistor 10 Ω are connected in series
with an a c source of frequency 50 Hz. If the current is in phase with voltage,
calculate the impedance of the inductor.
OR
Explain with the help of a labelled diagram, the principle, construction and
working of an a c generator.
A girl peddles a stationary bicycle the pedals of the bicycle are attached to a 100
turn coil of area 0.10 m2. The coil rotates at half a revolution per second and it is
placed in a uniform magnetic field of 0.01 T perpendicular to the axis of rotation
of the coil. What is the maximum voltage generated in the coil?
29 What is meant by diffraction of light? Draw a graph to show the relative
intensity distribution for a single slit diffraction pattern. Obtain an expression for
the linear width of the central maximum.
Light of wavelength 600 nm is incident on a single slit of width 0.5 mm at
normal incidence. Calculate the separation between two dark bands on either
side of the central maximum, if the diffraction pattern is observed on a screen
placed at 2 m from the slit.
OR
Define magnifying power of a microscope. With the help of a ray diagram,
explain the formation of the image in a compound microscope. Derive an
expression for its magnifying power. How can its magnifying power be
increased?
A compound microscope consists of an objective lens of focal length 2.0cm and
an eyepiece of focal length 6.25cm separated by a distance of 15cm. How far
from the objective should an object be placed in order to obtain the final image
at (a) the least distance of distinct vision (25cm), and (b) at infinity? What is the
magnifying power of the microscope in each case?
30 What is a p – n junction? Explain with the help of a diagram, how depletion
layer is formed near the junction. Explain also what happens to this layer when
the junction is;
(i) forward biased (ii) reverse biased.
Draw the V – I characteristics of a p – n junction diode in forward and reverse
bias.
OR
Draw the symbolic representation of a (i) p-n-p, (ii) n-p-n transistor. Why is the
base region of transistor thin and lightly doped? With proper circuit diagram,
show the biasing of a p-n-p transistor in common emitter configuration. Explain
the movement of charge carriers through different parts of the transistor in such
a configuration and show that IE = IC + IB.
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