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KENDRIYA VIDYALAYA SANGATHAN (RAIPUR REGION)
FIRST PRE BOARD EXAMINATION 2012-2013
QUESTION PAPER
PHYSICS – XII
SET I
Time – 3 hours
MM – 70
General Instructions:
a) All questions are compulsory.
b) There are30 questions in total. Questions 1 to 8 are very short answer
type questions and carry one mark each.
c) Questions 9 to 18 carry two marks each, questions. 19 to 27 carry three
marks each and questions 28 to 30 carry five marks each.
d) There is no overall choice. However, an internal choice has been
provided in one question of two marks, one question of three marks and
three questions of five marks each. You have to attempt only one of the
given choices in such questions.
e) Use of calculators is not permitted. However, you may use log tables if
necessary.
f) You may use following physical constants whenever necessary.
c= 3x108 m/s
h=6.63x10-34Js
e=1.6x10-19C
μo=4x10-7TmA-1
1 = 9x109 Nm2C-1
4πEo
1.
2.
3.
Two point charges placed at a distance r in air exert a force F on each
other. At what distance will these charges experience the same force F in
a medium of dielectric constant K ?
Explain radial magnetic field.
Fig. 1 shows a horizontal solenoid connected to a battery and a switch. A
copper ring is placed on a frictionless track, the axis of the ring being
along the axis of the solenoid. What happens to the ring, as the switch
isclosed ?
Figure 1
4.
5.
Draw a sketch showing the incident, reflected and transmitted rays
when light is incident at the polarising angle on a glass slab.
What modification was made by Maxwell in Ampere circuital law?
6.
An electron and alpha particle have the same de-Borglie wavelength
associated with them. How are their kinetic energies related to each
other?
7.
The radioactivity of the sample is R1 at timet1 andR2 at time t2. The mean
life of the sample is . What is the number of nuclei that have
disintegrated in the time interval (t1 – t2)?
8.
If the base region of a transistor is made large, as compared to a usual
transistor, how does it affect (i) the collector current and (ii) current gain
of this transistor?
9.
A slab of material of dielectric constant has the same area as the plates
of a parallel plate capacitor but has a thickness 3d / 4, where d is the
separation of the plates. How is the capacitance changed when the slab
is inserted between the plates?
10.
State two properties of the material of wire used for suspension of coil in
a moving coil galvanometer?
11.
Meera lives in a town where the government is providing free electricity
that is why most families in her locality waste electricity. They use
traditional bulbs, use electric heaters for cooking, leave thier fans
running even though no one is using the room. Meera one day attends a
seminar on the conservation of electricity in her school. There she comes
to know that there are many villages in her state without electricity. She
convinces her father to replace traditional bulbs with CFL bulbs and
conserve electricity and requests her father to convince his friends for
the same. Slowly everyone in the locality takes inspiration from Meera’s
family and start conserving electricity so that the surplus electricity could
be used for lighting un electrified rural area.
What according to you, are the values displayed by Meera and her
family, friends and neighbours to help her uncle ?
12.
How does the mutual inductance of a pair of coils change when
1) Distance between the coils is increased.
2) No. of turns of the coil is decreased.
13.
14.
The electric potential as a function of distance x is shown in Fig. 3.
Construct a graph of the electric field strength E.
Fig. 3
An inductor 'L' of reactance XL, is connected in series with a bulb 'B' to an
a.c. source as shown in Fig. 4
Fig. 4
Briefly explain how does the brightness of the bulb change, when (i)
number of turns of the inductor is reduced and (ii) a capacitor of
reactance XC=XL is included in series in the same circuit.
A bulb B and a capacitor C are connected in series to the a.c. mains as
shown in Fig. 5.
15.
16.
17.
18.
19.
20.
21.
Figure 5
The bulb glows with some brightness. How will the glow of the bulb
change when a dielectric slab is introduced between the plates of the
capacitor? Give reasons in support of your answer.
State one use and one method of detecting the rays beyond the visiblered end of the electromagnetic spectrum.
Explain why does a convex lens behave as a converging lens when
immersed in water (µ=1.33) and as a diverging lens, when immersed in
carbon disulphide (µ=1.6).
A radioactive isotope has a half life of T years. After how much time is its
activity reduced to 25% of its original activity?
Why are sky waves not used in transmission of T.V signals?
State the Huygens principle .Prove Snell,s laws of refraction on the basis
of Huygens principle.
A parallel plate capacitor, each with plate area A and separation d, is
charged to a potential difference V. The battery used to charge it is then
disconnected. A dielectric slab of thickness d and dielectric constant  is
now placed between the plates. What change, if any, will take place in :
(i)
charge on plates,
(ii)
electric field intensity between the plates
(iii)
capacitance of the capacitor
Justify your answer in each case.
Or
Define electric potential. Derive an expression for the electric potential at
a distance r from a point charge q.
Two convex lenses A and B of an astronomical telescope having focal
lengths 5 cm and 20 cm respectively, are arranged as shown in the Fig. 6.
(i)
(ii)
Fig.6
Which one of the two lenses you will select to use as the objective
lens and why?
What should be the change in the distance between the lenses to
have the telescope in its normal adjustment position?
(iii)
22.
23.
24.
Calculate the magnifying power of the telescope in the normal
adjustment position.
You are given two circuits as shown in Fig. 7, which consist of NAND
gates. Identify the logic operation carried out by the two circuits.
Fig. 7
Show that the de-Broglie wavelength  of electrons accelerated through
a potential difference of V volts can be expressed as
Derive an expression for the power dissipated in LCR series circuit,hence
define power factor.
25.
Calculate binding energy per nucleon of an alpha particle from the
following data:
Mass of free proton =1.00785 a.m.u.
Mass of free neutron =1.008665 a.m.u.
Mass of helium nucleus =4.002800 a.m.u.
1 a.m.u. =931.5MeV.
26.
27.
28.
A transmitting antenna at the top of a tower has a height 32 m and that
of the receiving antenna is 50 m. What is the maximum distance
between them for satisfactory communication in LOS mode? Given
radius of earth 6.4 x 106 m.
Draw the input and output characteristic curve of a transistor in common
emitter configuration. Explain the meaning of its cut off and saturation
regions. For what practical purpose, do we use the transistor in these
regions?
State Kirchhoff's laws for an electrical network. Using Kirchhoff's laws,
find the relation between the resistances of four arms of a Wheatstone
bridge when the bridge is balanced.
Draw a circuit diagram to determine the unknown resistance of a
metallic conductor using a metre bridge.
Or
29.
Define the term potential gradient. Using this concept, explain the
method for comparison of emfs of two primary cells using a
potentiometer. Establish the relation used. Write two possible causes of
potentiometer giving only one-sided deflection.
Write an expression for the force experienced by a charged particle
moving in a uniform magnetic field B. With the help of a diagram,
explain the principle and working of a cyclotron. Show that cyclotron
frequency does not depend on the speed of the particle.
Or
30.
Derive an expression for the torque acting on a loop of N turns, areaA,
carrying current I, when held in a magnetic field B. With the help of a
circuit diagram, show how a moving coil galvanometer can be converted
into an ammeter of given range. Write the necessary mathematical
formula.
With the help of a ray diagram explain the phenomenon of total internal
reflection. Obtain the relation between critical angle and the refractive
index of the medium. Draw ray diagrams to show how a right angled
isosceles prism can be used to (i) deviate the ray through 180°, (ii) deviate
the ray through 90°, and (iii) invert the ray.
Or
With the help of a ray diagram, show the formation of image of a point
object by refraction of light at a spherical surface separating two media
of refractive indices n1 and n2 (n2>n1) respectively. Using this diagram,
derive the relation.
n2


n1 n2  n1

u
R
Write the sign conventions used. What happens to the focal length of
convex lens when it is immersed in water?
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