FIRST PREBOARD EXAMINATION 2014-15
KENDRIYA VIDYALYA
I.T.I , MANKAPUR
PHYSICS(XII)
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TIME: 3 HRS
M.M.: 70
General instructions:
1. All questions are compulsory.
2. There are 26 questions in total. Questions 1-5 are very short answer questions and
carry one mark each.
3. Questions 6-10 carry two marks each. Questions 11-22 carry three marks each.
One question is having internal option.
4. Questions no.23 is is a value based question, carry 4 marks.
5. Questions 24 – 26 are long answer questions, carry 5 marks each. Each question
having internal option.
----------------------------------------------------------------------------------------------------------------------Two wires of equal length, a one of copper and other of manganin have same resistance.
Which wire is thicker?
A glass lens of refractive index 1.45 disappears when immersed in liquid. What is the value
of refractive index of liquid?
Which type of wave front emerge from distant source?
The maximum kinetic energy of photoelectron is 3eV. What is its stopping potential?
Write any two characteristics of nuclear force.
Two point charges 3 C and -3 C are placed at points A and B, 5 cm apart.
i)Draw the equipotential surface of the system.
ii)Why do equipotential surface get close to each other near the point charge?
The following figure shows the variation of intensity of magnetisation versus the applied
magnetic field intensity for two magnetic materials A and B.
I
A
H
B
i) Identify the materials A and B.
ii)Draw the variation of susceptibility with temperature for B.
8. The energy of an electron in the ground state of hydrogen atom is -13.6eV
i) What does the negative sign signify?
ii)How much energy is required to take an electron in this atom from ground state
to the first excited state?
OR
Draw binding energy per nucleon curve. Mention its two characteristics.
9. I) Identify the logic gates marked P and Q in the given logic circuit.
ii)Write down the output at X for the inputs, A=0, B=0 and A=1, B=1.
A
B
P
Q
X
10. Write two factors justifying the need of a modulating signal.
11. Draw a block diagram showing the components of transmitter. Write the
functions of these components.
12. i) What is meant by half life of a radioactive element?
ii) The half life of a radioactive substance is 20s. Calculate
a)decay constant
b) time taken for a sample to decay be 7/8 of the initial value?
13. A proton and alpha particle are accelerated through the same potential. Which of two
has:
i) greater value of de-broglie wavelength associated with it.
ii) less kinetic energy?
Justify your answers.
14. Draw a labelled ray diagram of a refracting type telescope. Define its magnifying power
and write expression for it. Write two limitations of a refracting type telescope over a
reflecting type telescope.
15. Identify the following electromagnetic radiations as per wavelength given below. Write
one application of each.
i) 1mm
ii) 10-12m
iii) 10-8m
16. i) Predict the direction of induced current in metal rings 1 and 2 when current I in the
wire is steadily decreasing?
1
I
2
ii) A rod of length l is rotated at a constant angular speed ω in a uniform magnetic field B
from one end. Deduce expression for maximum induced emf in the coil.
17. Derive formula for magnetic field due to a current carrying circular coil at any point on
its axis using bio – savart’s law.
OR
With the help of a diagram explain the principle and working of a moving coil
galvanometer.
18. Using gauss’s law obtain the expression for the electric field due to spherical shell of
radius R at a point outside the shell. Draw graph showing the variation electric field with r,
for r ˃ R and r˂ R.
19. Net capacitance of three identical capacitors in series is 1μF. What will be net
capacitance if connected in parallel? Find the ratio of energy stored in parallel combination
to the energy stored in series combination, if same source is used in both combinations.
20. Calculate the value of resistance R in the circuit shown in figure so that current in the
circuit is 0.2A. What would be the potential difference between B and E? Find the value of R
using kirchoff’s law.
8V 0.2A B
15Ω
3V
5Ω
30Ω
C
10Ω
10Ω
R
E
D
21. Draw a circuit diagram for measuring internal resistance of a cell by using
potentiometer. If balancing length for emf of a cell is 250 cm and for potential difference in
the presence of external resistance of 5 is 200cm. Find internal resistance of cell. What will
be effect on internal resistance of a cell if external resistance is increased?
22. A point charge of 10μC is moving with constant speed of 20î m/s in a uniform magnetic
field of 0.5ĵ T. What should be the magnitude and direction of electric field so that charge
moves undeviated along the same path?
23. Shobha and naina are two friends who are married; Shobha is very poor and childless
widow whereas naina is having a good economic status. Once naina noticed that shobha’s
eyesight has became weak and she was not even able to read big letters of the newspaper.
Naina asked to shobha that why she did not take care of this, shobha told that she did not
have enough amount for its remedy. Listening to this naina took shobha to an eye specialist
and asked to him about the whole expenditure for its remedy. Doctor said that it is thecase
of myopia and can be resolved without any major expenditure. After the treatment,
shobha’s eyesight were now better than before.
a)What are the benefits of true friendship?
b)The far point of shobha’s eye is at a distance of 1.5m. Which type of lens is used for its
remedy ? Calculate the power of lens .
24. a) What are coherent sources of light ? Derive a mathematical expression for width of
bright fringe obtained in young’s double slit experiment with the help of suitable diagram.
b) The light of wavelength 600nm is incident normally on a slit of width 3mm. Calculate
the width of central maxima on a screen kept 3m away from the slit.
OR
a) With the help of ray diagram , show the formation of image of a point by refraction of
light at a spherical lens of refractive index μ2 kept in medium of refractive indexμ2(μ2˃ μ1).
Using this diagram, derive the lens maker’s formula.
b) A convex lens of focal length 20 cm and refractive index 1.5 in air. Calculate the
change in focal length when it is immersed in water of refractive index 4/3.
25. a) An AC source generating a voltage V=V0sinωt is connected to a series L-C-R circuit .
Derive the expression for instantaneous current and its phase relationship to the applied
voltage With the help of phasor diagram.
b) An inductor 200mH, capacitor 100μf and a resistance 10Ω are connected in series to
an AC source 100V, having variable frequency. At what frequency of the AC source, will the
power factor of the circuit be 1?
OR
a) Define self-inductance. Write its S.I. unit.
b)Derive an expression for self-inductance of a long solenoid of length l, cross-sectional
area A having N number of turns.
c) Current in a circuit falls steadily from 2A to 0.2 A in 10 ms. If an average emf
200V is induced, calculate the self-inductance of the circuit.
26. a) Draw the circuit diagram of p-n junction diode in i) forward bias ii) reverse bias. Draw
their typical I-V characteristics. What is the effect on depletion layer in both cases?
b) Name the device which used as a voltage regulator. Draw the necessary circuit
diagram and explain its working.
OR
a)Draw a circuit diagram to study the input and output characteristics of p-n-p transistor
in a common emitter configuration. Show these characteristics graphically. Explain how
current amplification factor of transistor is calculated using output characteristics?
b)Explain the function of base region of a transistor. Why is this region made thin and
lightly doped?