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GOA BOARD OF SECONDARY AND HIGHER SECONDARY EDUCATION
MODEL QUESTION PAPER
PHYSICS XII
Time: 3 hrs
Max Marks: 70
General Instructions:
i. All questions are compulsory
ii. Answers to the Multiple Choice Questions should be written by choosing and writing the correct alternative
iii. There is no overall choice. However internal choice has been provided in two questions of four marks each.
iv. Use of calculators is not permitted. However, you may ask for mathematical tables.
v. You may use the following values of physical constants wherever necessary :
1 A
What is the direction of (i) electric intensity and (ii) electric dipole moment along the
line joining two equal and opposite charges as shown below?
1
B Using Gauss’ Law, obtain the expression for the electric field at a point outside an
infinitely long straight uniformly charged wire.
2
C Which among the curves shown below cannot possibly represent electrostatic field lines?
1
(a)
(b)




(c)
Only (a)
Only (b)
(a) and (b)
(b) and (c)
1
D A metre bridge is used to find the unknown resistance P (R is the known resistance). At a 4
certain distance l1 from A, the galvanometer shows null defection. How will you find the
resistance P?
P
R
If another resistance X is connected in parallel with P, which side will the null point
shift?
Why should the null point be obtained near the centre of the wire?
OR
D A carbon resistor has the following colour bands on it
4
What is the value of the resistor? What is its tolerance? If the temperature were to
increase, what effect will it have on its resistance? Why are carbon resistors preferred in
most electronic circuits?
E A metal wire of certain resistance is stretched to 4 times its original length. Will there be
any change in its resistivity? Which wire, shorter or longer will have more resistance?
2
2
2 A Two infinitely long straight conductors A and B carry currents in the same direction..
Will they attract or repel each other? Derive the formula for the force per unit length
between them. Hence define SI unit of current.
OR
A State Biot-Savart’s law. Using this law, derive the expression for the magnetic field
due to a current carrying circular loop of radius ‘R’, at a point which is at a distance
‘x’ from its centre along the axis of the loop. Hence obtain the expression for the
magnetic field at the centre of the loop.
4
4
B An electric dipole consisting of two charges of magnitude q each is enclosed inside a
Gaussian surface What is the total electric flux over the Gaussian surface?
 q
 q2l
 zero
 infinite
1
C Why is the direction of the electric field changed alternately in a cyclotron?
1
D The amplitude of the magnetic field component of a harmonic electromagnetic wave in
vacuum is B0 = 515 nT. What is the amplitude of the electric component of the wave?
2
E The figure given below shows a 2V potentiometer used for the determination of internal
resistance of a 1.5 V cell. The balance point of the cell in open circuit is
78 cm.
When a resistor of 9.5 Ωis used in the external circuit of the cell, the balance point
shifts to 65 cm length of the potentiometer wire. Determine the internal resistance of the
cell.
2
3 A Assuming the law of radioactive decay, obtain an expression for the number of atoms of
a radioactive substance present at any given instant.
2
3
B In the nuclear process
 5B + e
6C
What does X stands for?
   particle
   particle
 neutrino
 anti neutrino
11
11
+
+ X
1
C What happens if a bar magnet is cut into two pieces along its length?
1
D What type of material do the following band diagrams represent?
4
In which case will there be better conduction? Why?
A given p-n junction is biased in two different ways as shown in the figure. Identify the
type of biasing used in each case. What is the effect of these biasings on the barrier
potential across the p-n junction?
4
E The capacity of a parallel plate capacitor is 4  F. It is charged to the potential of 80 V. If
the area of the each plate of the capacitor is 5 cm2 and the distance between the plates is
2mm, calculate the energy density of the capacitor.
4 A What is a logic gate?
2
2
Name the logic gate given in the above diagram and give its truth table.
B The following curve represents the hysteresis curve for a ferromagnetic material .Explain
the various parts of the curve on the basis of the domain theory.
3
C State one reason why induced emf is also known as back emf. The magnetic induction B
in the core of a spark coil changes from 1.5 T to 0.1 T in 0.21 ms. If the cross sectional
area of the core is 450mm2, what is the average emf induced in the coil if it has 6000
turns?
3
D A toroidal solenoid has 1000 turns per meter. If the magnetic field inside the solenoid is
4π x 10-3 T .Find the current through it.
1
E A virtual image larger than the object can be obtained by:
 concave lens
 concave mirror
 convex mirror
 plane mirror
1
5
5 A Two monochromatic, coherent waves arrive at a point. State the conditions for
constructive and destructive interference at the point in terms of path difference
1
B With neat circuit diagram and graphs show the phase relationship between current and 3
applied a.c.voltage in a circuit containing an inductor only. Indicate the phase difference
using a phasor diagram
C A point source is placed at the principal focus of a convex lens. What is the nature of the 1
emerging wavefront on the other side of the lens?
 Convex spherical
 Concave spherical
 Plane
 Cylindrical
D A ray of light is incident on a convex refracting surface separating a denser medium from 3
a rarer medium. Obtain the relation between object and image distances in terms of the
refractive indices of the media and the radius of curvature of the spherical surface.
E A coil having an inductance 0.14H and resistance 14Ω is connected across a 110V, 2
50 Hz line. Find the phase angle between the current and supply voltage.
6 A The minimum orbital angular momentum of the electron in a hydrogen atom is
 h
h

2
h

2
h


B What is the electric potential energy of a system of two charges in an electric field?
Derive an expression for the electric potential energy of an electric dipole placed in an
external electric field.
1
3
C The threshold frequency for a certain metal is 3.3 x 1014 Hz. If light of frequency 8.2 x 2
1014 Hz is incident on the metal, predict the cut-off voltage for the photoelectric emission
D Explain two main drawbacks of Rutherford’s atomic model.
The total energy of an electron in a Bohr atom is –ve. What does this imply?
What would happen if it was +ve?
3
E An audio signal needs to be modulated before transmission. Why?( Give any one point)
1
6
7 A State two points of difference between point to point and broadcast mode of
communication..
2
B The equation E  pc is valid
 for an electron as well as for a photon
 for an electron but not for a photon
 for a photon but not for an electron
 neither for an electron nor for a photon
1
C
1
Name the cutoff region and the saturation region of a transistor in the above transfer
characteristics of transistor in CE mode.
D Two narrow slits which are very close together are illuminated by a single 3
monochromatic source of light. A screen is placed at a distance from the sources. Name
the pattern obtained on the screen.
One of the slits is now completely covered. What is the name of the pattern now obtained
on the screen?
Give two differences between the patterns obtained in the two cases.
E At what angle should a ray of light be incident on the face of a prism of refracting angle 3
60o so that it just suffers total internal reflection at the other face? The refractive index of
the material of the prism is 1.523.
7