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KENDRIYA VIDYALAYA SANGATHAN (MUMBAI REGION)
FIRST PREBOARD 2016
PHYSICS (THEORY)QP CODE : 1304
CLASS –XII SCI
TIME ALLOWED: 3 hours Maximum Marks: 70
General Instructions:
(i)
(ii)
(iii)
(iv)
(v)
All questions are compulsory. There are 26 questions in all .
This question paper has five sections : Section A, Section B, Section C, Section D and
Section E.
Section A contains five questions of one mark each, Section B contains five questions of
two marks each, Section C contains twelve questions of three marks each, Section D
contains one value based question of four marks and Section E contains three questions
of five marks each.
There is no overall choice. However , an internal choice has been provided in one question
of two marks , one question of three marks and all the three questions of five marks
weightage. You have to attempt only one of the choices in such questions.
You may use the following values of physical constants wherever necessary :
c = 3 x 10 8 m/s
h = 6.63 x 10 -34js
e = 1.6 x 10 -19 C
µo = 4π x 10 -7 T m A -1
εo = 8.854 x 10 -12 C2N -1 m -2
1/4πεo = 9 x10 9 N m 2 C-2
me = 9-1 x 10 -31 kg
m n = 1-675 x 10 -27 Kg
m p = 1-673 x 10 -27 Kg
Avogadro’s number = 6.023 x 10 23 per gram mole
Boltzmann Constant = 1.38 x 10 -23 J K -1
SECTION A
1)Explain why two field lines never cross each other at any point ?
2)Identify the electromagnetic radiation as per the wavelengthgiven (a) 1mm and write one
application.
3) A beam of α- particles projected along + x-axis, experiences a force due to a magnetic
field along the + z- axis. What is the direction of the magnetic field?
4) State Lenz’s law.
5) Two conducting wires X and Y of same diameter but different materials are joined in series across a
battery. If the no. of density of electrons in X is twice that in Y, find the ratio of drift velocity of
electrons in the two wires.
SECTION B
6) (a) Draw equipotential surfaces due to a point Q> 0 .
(b)Are these surfaces equidistant from each other ? If not , explain why.
OR
A test charge q is moved without acceleration from A to C along the path from A to B and then from B
to C in electric field where the co-ordinates of points A , B and C are (2, -1) (-2 , 2) , (-2, -1 )
respectively . calculate the potential difference between A and C .
7 )(a)On what principle does a meter bridge work ? Draw a circuit diagram for its determination of an
unknown resistance.
(b) Why is it generally preferred to obtain the balance point in the middle of the meter bridge wire?
8) An a.c voltage given by V = 70 sin100πt is connected across a pure resistor of 25Ω . Find the
frequency of the source and the rms current through the resistor .
9) Three identical resistors, each resistance R , when connected in series with d.c source, dissipate
power X . If the resistors are connected in parallel to the same d.c source, how much power will be
dissipated ?
10) Deduce the expression for the electrostatics energy stored in a capacitor C and having charge Q .
SECTION C
11) Apply the Kirchhoff’s rules to the loops PRSP and PRQP to find the currents I1, I2 and I 3 in the given
circuit. Figure
12)Draw magnetic field lines when a diamagnetic and paramagnetic substance is placed in an external
magnetic field and write one difference between its in terms of magnetic susceptibility.
13) Two long straight parallel conductors X and Y , carrying steady currents I1 and I2 in
the same direction are separated by a distance d . Obtain the expression for the force per unit
lengthbetween two conductors X and Y with the help of suitable diagram.
14 Astep down transformer operates on 2.5 KV line . It supplies a load with 20 A . The ratio of the
primary winding to the secondary is 10:1 . If the transformer is 90% efficient , Calculate the output
power , voltage and current in the secondary .
15 (a) How are electromagnetic waves produced?
(b) Draw a sketch of a plane em wave propagating along the z-direction . Depict clearly the directions
of electric and magnetic fields varying sinusoidally with z .
(c) Define displacement current.
16 )Define the resolving power of a microscope . How is this affected when
(a ) the diameter of the objective lens is decreased , and
(b ) the wavelength of illuminating radiations is decreased
OR
What are the optical fibres?
(a) On which principle do they work? and
(b) Write two conditions of its .
17)Write three differences between the interference and diffraction observed patter.
18 (i) Define stopping potential and threshold frequency in the phenomenon of photoelectric effect.
(ii)Write Einstein’s photoelectric equation. State any two characteristics properties of photons on
which this equation is based .
19) The ground state energy of hydrogen atom is – 13.6 eV. If an electron makes a transition
from an energy level -0.85 eV to -3.4 eV, calculate the wavelength of the spectral line emitted.
To which series of hydrogen spectrum does this wavelength belong?
20) A radioactive nucleus A undergoes a series of decays according to the following scheme A
αAβ
1
A2 αA3ƴA4.The mass no. and atomic no. of A are 180 and 72 respectively . What are
these nos. for A4
b) Plot a graph showing the variation of potential energy of pair of nucleons as a function of
their separation.
21)(i) Draw the block diagram of a communication systems.
(ii)What is the function of atransducer in communication system.
22) i) Write any two factor which justify need of modulating signal .
(i ) A transmitting antenna at the top of a tower has a height of 20 m and the height of the
receiving antenna is 45 m . Calculate the maximum distance between them for communication
in LOS mode . R of earth= 6400 km
SECTION D
23) Rohit was watching a program on the topic MOON on the Discovery channel. He came to know
from the observations recorded from the surface of MOON that the sky appears dark from there .
He was surprised and wanted to know the reason behind it . He discussed it with his friends , and
they had the reasons as 1) phenomenon of refraction of light 2) phenomenon of scattering of light
Answer the following questions i) What value was displaced by Rohit ?
ii) What values were displayed by his friends ?
iii) Write some examples where you can seethese phenomena
SECTION E
24) (i)With the help of a labeled diagram, explain the principle and working of a moving coil
galvanometer.
(ii) What is the function of a radial magnetic field ?
(iii) Define current sensitivity and Voltage sensitivity of a Galvanometer.
OR
(I ) Write the expression for the force F , acting on a charged particle of charge q moving with a
velocity V in the presence of both electric field E and magnetic field B . Obtain the condition
under which the particle moves undeflected through the fields .
(ii ) A rectangular loop of size l x b carrying a steady current I is placed in a uniform magnetic field B
. Prove that the torque Ʈ acting on the loop is given by Ʈ = m x B , where m is the magnetic moment
of the loop .
25(a)Draw a circuit diagram to study the input and output characteristics of an NPN transistor in
common emitter configuration . Draw the graphs showing the nature of input and output curves .
(b) Identify the logic operations carried out by the circuit given below .write its truth table.
OR
(a) Draw the circuit diagrams of P-N junction diode in (I ) forward bias (ii ) reverse bias . Draw the graph
showing V- I characteristics . Name the semiconductor device which operates under the reverse bias in
the breakdown region.
(b) Identify the logic operations carried out by the circuit given below .write its truth table.
26(I ) Derive the lens formula 1/f = 1/v – 1/u for thin concave lens, Using the necessary ray diagram.
(ii)Two thin convex lenses of focal lengths f1 and f2respectively , are placed coaxially in contact . Derive
the expression for the focal length of the combined system .
OR
(i )Derive the expression for the fringe width in young’s double slit experiment.
(ii) The ratio of the intensities at minima to maxima in the Young’s double slit experiment is 9 : 25 .
Find the ratio of the widths of the two slits .