Exam-Pre Board-1st
Class-XII (Physics)
Time Allowed: 3hrs
Maximum Marks: 70
General Instructions
(i)All the question are compulsory
(ii) Question No 1-5 are very short answer questions and carry 1 mark each.
(ii) Question No 6-10 are short answer questions and carry 2 marks each.
(ii) Question No 11-22 are also short answer questions and carry 3 marks each.
(ii) Question No 23 may be value based carrying 4 marks.
(ii) Question No 24-26 are long answer questions and carry 5 marks each.
1. The instantaneous current and voltage of an a.c. circuit are given by i=Sin300t ampere and
V=200Sin300t volt. What is the power dissipation in the circuit?
2. Which frequency of microwaves will be most suitable to warm food?
3. Two lines A and B, in the plot given below show the variation of de-Broglie wavelength,λ
versus 1/√V Where V is the accelerating potential difference, for two particles carrying the
same charge. Which one of the two represents a particle of smaller mass?
B
A
4. State Lenz’s law.
5. A rectangular loop PQRS is moved slowly in uniform magnetic field as shown. What is the
direction of instantaneous induced current?
×
×
×
×
×
×
×
P ×
× S ×
×
×
×
×
×
×
×
Q ×
× R
×
×
×
×
×
×
6. An electric dipole free to move is placed in a uniform electric field. Explain with diagram its
motion when it is placed.
(i) parallel to the field
(ii) Perpendicular to the field.
7. Which one of the two; an α particle or a β particle will (i) experience larger force , (ii)
describe smaller circle when projected with the same velocity V perpendicular to a uniform
magnetic field: Give reasons.
OR
A proton and an α particle having same velocity enter a region of uniform magnetic field ,
acting perpendicular to their direction of motion. Deduce the ratio of the radii of their circular
path.
8. When can a charge act as a source of electromagnetic waves? How are the direction of
electric and magnetic field related to each other and to the direction of propagation of the
wave? Represent the same graphically.
Which quantity if any has the same value in vacuum for waves belonging to the different part of
the electromagnetic spectrum?
9. What is a dipole antenna? What role it play in the communication system? How does the
length of the antenna vary with frequency?
10. What is the difference between analog and digital signals? Draw a diagram to represent
voltage v/s time variation of two types of signals.
11. Calculate the electrostatic force between two electrons placed a distance of 1cm apart in
vacuum. What will the force if
(i) distance between the electrons is doubled
(ii) the electrons are replaced by protons
(iii) one of the electrons are replaced by a deuteron.
(iv) the electrons are replaced by alpha particles.
12. Obtain the relation between the electrostatic field intensity and the electrostatic potential
at a point in an electric field due to a point charge.
13. Write expression for joule heating produced in a resistance when current is passed through
it. Show that the rise in temperature of a wire due to the passage of current in it is independent
of its length.
14. If the frequency of the incident radiation of the cathode of a photocell is doubled, how will
the following change?
(i) Kinetic energy of the electron
(ii) Photoelectric current.
(iii) Stopping potential.
Justify your answer in each case.
15. A circular coil of 200 turns and radius 10 cm is placed in a uniform magnetic field of 0.5 T,
normal to the plane of the coil. If the current in the coil is 3.0 A, calculate the
(i) total torque on the coil.
(ii) total force on the coil.
(iii) average force on each electron in the coil, due to the magnetic field.
Assume the area of cross-section of the wire to be 10-15 m2 and the free electron density is
1029/m3.
16. State Biot-Savart law. Using the law, obtain an expression for the magnetic field intensity
at the centre of a circular coil carrying current. Represent graphically the variation of the
magnetic field intensity with number of turns in the coil.
17. In an a.c. generator, coil of N turns and area A is rotated at v revolutions per second in a
uniform magnetic field B. Write the expression for the emf produced.
A 100 turn coil of area 0.1 m2 rotates at half a revolution per second. It is placed in a magnetic
field of intensity of 0.1. T perpendicular to the axis of rotation of the coil. Calculate the
maximum voltage generated in the coil.
18. (i) Define the term self-induction.
(ii) The current through a coil of self-inductance 0.25 H changes as I= 5 Sin 50t ampere.
What is the peak emf generated in the coil.
19. An object is placed at a distance ‘a’ from a convex lens of focal length f. Its diminished
image is obtained on a screen held at a distance b from the lens. Draw a diagram for the above
and obtain a relation between ‘a’, ‘b’, and ‘f ‘.
20. Give reason for each of the following:
(i) Sunrise and sunset is abrupt on the surface of the moon.
(ii) sky appears to be dark on moon.
(iii) No rainbow is formed in on moon.
21. Draw the path of a ray of light passing through a glass slab. Mark lateral shift in the ray.
Obtain an expression for the same in term of µ; I and t; the refractive index, angle of incidence
and the thickness of the slab.
22. Explain, why increasingly high frequency carrier waves are being used for effective
transmission network of signals.
A massage signal of 12 KHz and peak voltage 20 V used to modulate a carrier wave of frequency
12 MHz and peak voltage 30 V. Calculate the (i) modulation index (ii) side-band frequencies.
OR
Define the term work function of a metal. The threshold frequency for a metal is f0. When the
light of frequency 2f0 is incident on the metal plate, the maximum velocity of electrons emitted
is v1. Let v2 and v3 respectively denote the maximum velocities of the photo-electrons emitted
when the incident light is replaced by
(i) light of same frequency but twice the intensity
(ii) light of frequency 5f0.
Find the ratio v2 to v1 and v3 to v1. Justify your answer.
23. In order to tap natural water resource in an area and to provide electric supply to a remote
village in a hilly area, a power plant proposed to be erected. The villagers protested against it
thinking that the water from the source will become useless after production of electricity. A
meeting of the grampanchayat was called to discuss the issue. Hemant, a teacher in the village
talked to the members of the panchayat and explained to them the advantage of the power
supply which the village will get. He also explained to them that the water will still remain as
good for agriculture after power production as otherwise. He also apprised them of the
possible improvement in the quality of life in the village. The village agreed.
(i) List two qualities shown by Hemant.
(ii) What is the principle on which the generator produces electricity?
(iii) If 5000 kg water falls per sec from 40 m height, calculate the power likely to be available
assuming 70% efficiency.
24. What is a polarization of light?
Draw diagram to represent both polarized light and unpolarised light.
State and use Brewster law to obtain an expression for angle of polarization when some part of
incident light is reflected and refracted from the surface of an medium of refractive index µ.
The critical angle for a given transparent medium is ic .A ray of light enters from air to this
medium at an angle ip which is also the angle of polarization.Deduce a relation between ip and ic
OR
(a) What are coherent sources of light? Two slits in Young’s double slit experiment are
illuminated by two different sodium lamps emitting light of the same wavelength . Why is no
interference pattern observed?
(b) Obtain the condition for getting dark and bright fringes in Young’s experiments and write
the expression for the fringe width.
(c) If s is the size of the source and D it’s distance from the plane of the two slits, what should
be the criterion for the interference fringe to be seen?
25. With reference to nuclear reactor explain the process of chain reaction. Draw a neat and
labeled diagram of nuclear reactor. What are the main difficulties in starting chain reaction in
natural uranium?
OR
How is energy being obtained in sun with fusion process? Why is this process not possible at
ordinary temperatures? Using necessary equations, explain two types of possible fusion
reactions on the sun. Why is fusion considered cleaner than fusion?
26. (a) Draw a system of (i) NOR gates to produce AND gate (ii) NAND gates to produce OR gate.
(b) Draw symbol of a photodiode and explain its working.
(c) A photodiode is fabricated from a semiconductor with a band gap of 2.8 eV. Can it detect a
wavelength of 600 nm?
OR
Draw a circuit diagram to study the output characteristic of a typical PNP transistor in CE mode.
From a typical characteristic , explain the meaning of saturation and cutoff. What bias
conditions exist at a transistor junctions in the saturation and cutoff state?