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P.M. CLASSES
PRELIMS
CLASS 12 - PHYSICS

Time Allowed: 2 hours Maximum Marks : 50

Section A 9) A photon of energy 7 eV is incident on a metal sur-

face having the work function of 3.75 eV. The stopping
1) Three small charged spheres X, Y and Z carrying charges
potential is: [1]
+ q, q and + q respectively are placed equidistant from
a) 1.75 eV b) 3.25 eV
each other, as shown in the figure. The spheres Y and
c) 2.45 eV d) 3.75 eV
Z are held in place. Initially X is also held in place, but
is otherwise free to move. When X is released, the path 10) The radius of27
13 X nucleus is R. The radius of 53 Y
125
nucleus
followed by it will be: will be [1]
a) 5
R
(3 13 )1/3
b) 53 R
( )1/3
c) 53 R
( )1/3
d) 13 R
[1]
53

a) C b) A Section B
c) D d) B
2) The capacitance of a parallel plate capacitor having a 11) Define electric field intensity and derive an expression
medium of dielectric constant K = 4 in between the for it at a point on the axial line of a dipole. Also,
plates is C. If this medium is removed, then the capaci- determine its direction. [2]
tance of the capacitor becomes [1] 12) i. Draw the graphs showing the variation of the fol-
a) C b) 2C lowing with the frequency of ac source in a circuit:
c) 4C d) C4 i. Resistance
ii. Capacitive reactance
3) The potential difference across a cell in an open circuit iii. Inductive reactance
is 8 V. It falls to 4 V when a current of 4 A is drawn ii. Can the voltage drop across the inductor or the
from it. The internal resistance of the cell is: [1] capacitor in a series LCR circuit be greater than
a) 4Ω b) 1Ω the applied voltage of the ac source? Justify your
c) 2Ω d) 3Ω answer.
4) A circular coil of radius 10 cm is placed in a magnetic [2]
fieldB
⃗ = (1.0î+0.5ĵ)mT such that the outward unit vector 13) In a single slit diffraction experiment, the width of the slit
normal to the surface of the coil is (0 · 6î + 0 · 8ĵ) . The is made double the original width. How does this affect
magnetic flux linked with the coil is: [1] the size and intensity of the central diffraction band. [2]
o
a) 0.314µ Wb 14) The threshold wavelength for a metal is 3315A . What
b) 3 · 14µ Wb should be the wavelength of light incident on the metal
c) 31 · 4µ Wb surface so that the maximum kinetic energy of the emitted
d) 1.256µ Wb photoelectrons be 1.25 eV? [2]
15) Explain, with the help of a circuit diagram, the working
5) The magnetic moment has dimensions of [1] of a p - n junction diode as a half - wave rectifier. [2]
a) [L2 A] b) [L2 T - 1 A]
c) [LT A]
- 1
d) [LA] Section C
6) The emf induced in a coil rotating in a magnetic field
doesnot depend upon the following: [1] 16) Find the expression for the capacitance of a parallel plate
capacitor of area A and plate separation d if (i) a di-
a) Number of turns in the coil electric slab of thickness f and (ii) a metallic slab of
b) Area of the coil thickness f, where (t < d) by one between the plates of
c) Angular speed of rotation of the coil the capacitor. In which case would the capacitance be
d) Resistance of the coil more and why? [3]
7) The current flowing in a step down transformer 220 V 17) i. Write an expression of magnetic moment associated
to 22 V having impedance220Ω is [1] with a current (I) carrying circular coil of radius r
a) 0.1 A b) 0.1 mA having N turns.
c) 1 A d) 1 mA ii. Consider the above mentioned coil placed in YZ
plane with its centre at the origin. Derive expres-
8) Welders wear special glass goggles or face masks with
sion for the value of magnetic field due to it at
glass windows to protect their eyes from radiations pro-
point (x, 0, 0).
duced by welding arcs. These radiations are: [1]
[3]
a) X - rays b) Gamma rays
18) What do you mean by diamagnetism, paramagnetism and
c) Infrared waves d) Ultraviolet rays
ferromagnetism? [3]
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19) A series LCR ac circuit has L = 2.0 H, C = 32µ F and

R = 10Ω .
i. At what angular frequency of ac will it resonate?
ii. Calculate the Q value of the circuit.
[3]
20) i. State Bohr’s quantization condition for defining sta-
tionary orbits. [5]
ii. Use the energy level diagram shown below to obtain 22) i. A particle of massm and charge q is moving with
the relation between three wavelengthsλ1 , λ2 and λ3 a velocity ⃗v in a magnetic field B
⃗ as shown in the
emitted due to the transition of electron from the figure. Show that it follows a helical path. Hence,
energy states C and B. obtain its frequency of revolution.

[3]

Section D ii. In a hydrogen atom, the electron moves in an orbit

o
21) i. Derive an expression for the electric field at a point of radius2A making 8 × 1014 revolutions per second.
on the equatorial plane of an electric dipole consist- Find the magnetic moment associated with the orbital
ing of charges q and - q separated by a distance motion of the electron.
2a. [5]
ii. The distance of a far off point on the equatorial 23) i. Draw a labelled ray diagram of a compound micro-
plane of an electric dipole is halved. How will the scope showing image formation at least distance of
electric field be affected for the dipole? distinct vision. Derive an expression for its magni-
iii. Two identical electric dipoles are placed
√ along the fying power.
diagonals of a square ABCD of side 2 m as shown ii. A telescope consists of two lenses of focal length
in the figure. Obtain the magnitude and direction of 100 cm and 5 cm. Find the magnifying power when
the net electric field at the centre (O) of the square. the final image is formed at infinity.
[5]