Kathmandu BernHardt HS School
Balkhu, Kathmandu
First Term Examination-2070
Set: A
Grade: XII
Time: 3 hrs.
Group A
(Answer of the numerical problems should be expressed in S.I. System)
Faculty: Science
Subject: Physics
FM: 75
PM: 27
1. Answer, in brief, Any Four questions:
[4×2=8]
a) A large number of free electrons are present in metals, why is there no current in
the absence of electric field across it?
b) What is the resistance of human body and what voltage is dangerous for human
body?
c) What is meant by position of Gauss? What do you understand by TanA and TanB
position?
d) What is the workdone by the magnetic field on a moving charge?
e) An electron beam moving with a uniform velocity is gradually diverging. When it is
accelerated to a high velocity, it starts converging, why? Explain.
2. Attempt Any Four questions:
[4×2=8]
a) Why is clock oil used in Millikan's experiment? Can we use water in place of the
clock oil?
b) What do you mean by zener breakdown?
c) An electron and a proton have same de-Broglies wavelength. Which has the
greater energy?
d) Can electromagnetic wave theory explain the photoelectric effect? Justify your
answer.
e) What are the effects of ozone depletion?
3. Answer, in brief, Any One question:
[1×2=2]
a) What is natural frequency of a body? What are the factors on which it depends?
b) What kinds of energy are associated with waves on a stretched string? Explain.
4. Answer, in brief, Any One question:
[1×2=2]
a) What do you understand by optical path?
b) Why is interference pattern not detected when the two coherent sources are far
apart?
Group B
5. Attempt Any Three questions:
[3×4=12]
a) Describe the principle construction and working of moving coil galvanometer.
b) State Ampere's circuital law. Use it to find the magnetic field due to a long solenoid
carrying current.
c) State and prove tangent law in magnetism.
d) Describe, with a necessary theory, the method of conversion of a galvanometer
into ammeter and voltmeter.
6. Attempt Any Three questions:
[3×4=12]
a) Write Einstein's photoelectric equation. Describe Millikan's experiment to verify it.
b) What is zener diode? Describe how in a zener diode used as a voltage regulator.
c) An electron moving horizontally enters to the vertical uniform electric field. Obtain
the expression for the gain in kinetic energy of the electron.
d) What are the importance of physics for the development of society?
7. Attempt Any One question:
[1×4=4]
a) What is stationary wave? Show that the distances between two consecutive nodes
and antinodes are equal.
b) Prove with necessary diagram that only odd harmonics are present in a closed
organ pipe.
8. Attempt Any One question:
[1×4=4]
a) State Huygen's principle. Use it to prove the laws of refraction of light.
b) What are Newton's rings? Show that the radius of nth dark ring is √𝑛𝑅 in reflected system.
Where  is wavelength of light used and R is radius of curvature of plane convex lens.
Group C
9. Solve Any Two questions:
[2×4=8]
a) A copper wire has 1.0×1029 free electrons per cubic meter a cross sectional area of
2.0 mm² and carries a current of 5.0A. Calculate the force acting on each electron if
the wire is now placed in a magnetic field of flux density 0.15T which is
perpendicular to the wire.
b) The resistance of two conductors in series is 10Ω and their resistance becomes
7.5Ω when connected in parallel. Find the resistances of individual conductor.
c) Two similar magnetic poles have pole strength in the ratio 9:1. If the distance
separating them is 4m at which point between the two, does the intensity of the
magnetic field vanish?
10. Solve Any Two questions:
[2×4=8]
a) The ground state of electron in the hydrogen atom may be represented by the
energy -13.6Ve and the first two excited state by -3.4eV and -1.5eV respectively.
An electron which is completely free of the atom is at zero energy. Calculate
ioinisation potential and excitation potentials.
b) Find the energy of the neutron in electron volts. Whose de-Broglie's wavelength is
1Å? (mn=1.674×10–27kg)
c) An oil drop of mass 3.25×10–15kg falls vertically with uniform velocity, through air
between vertical parallel plates which are 2cm apart. When a p.d. of 1000V is
applied to the plates the drop moves to the negatively charged plate, its path
being inclined at 45° to the vertical. Calculate the charge on oil drop.
11. If the velocity of sound in air a t 30°C is 350m/sec calculate the ratio of the molar
heat capacities of air. (Density of air at 0°C is 1.29 kgm–3)
(4)
12. The radius of curvature of the curved mirror is 20m and the plane mirror is rotated
at 20 revsec–1. Calculate the angle between a ray incident on the plane mirror and
then reflected from it after the light has travelled to the curved mirror and back to
the plane mirror.
(3)
Best of Luck