KENDRIYA VIDYALAYA SANGATHAN
PRE- BOARD EXAM 2014-15
Class XII
MAX. MARKS: - 70
SUBJECT: - PHYSICS
MAX. TIME: - 3:00 HOURS
1st
General Instructions:
1. All questions are compulsory.
2. There is no overall choice. However, internal choice has been provided in some questions. You are to attempt
only one option in such questions.
3. Questions 1 to 5 carry 1 mark each.
4. Questions 6 to 10 carry 2 marks each
5. Questions 11 to 22 carry 3 marks each
6. Question 23 carries 4 marks.
7. Questions 24 to 26 carry 5 marks each.
1. A point charge q is placed at O as in figure : O•------------------•P-----------•Q . Is VP – VQ is +ve or -ve .
When (i) q < 0 (ii) q > 0. Justify.
2. What is the phase difference between the voltage across an inductor and a capacitor in an a.c. circuit?
3. Red light, however bright, cannot cause emission of electrons from a clean Zinc surface. But even weak ultra
violet radiation can do so. Why?
4. How does magnetic moment of an electron change, when frequency of revolution is doubled?
5. Explain why the I-V characteristics of a resistor are obtained to deviate from a straight line from higher values
of current.
6. Two identical metallic spheres having unequal opposite charges at distance 0.90 m in air. After bringing in
contact, they are placed at same distance. Now force of repulsion is 0.025 N. Calculate the final charge on each
of them.
7. What is difference between electron & β-particle?
8. Write the truth table for the following combination of gates:
9. Which one of the following describe smallest circle in the
magnetic field (i) α – particle (ii) β – particle, if magnetic field and speed of charged is same?
10. Why ground wave transmission is restricted to 1500 kHz?
11. A capacitor of 100µF is charged to 200V. After disconnecting from battery, distance between the plates is
doubled. Find (i) potential difference between the plates (ii) energy stored?
12. A series battery of 6 lead accumulators of e.m.f. 2V and internal resistance of 0.5 Ω is charged by a 100V d.c.
supply. What series resistance should be used in charging circuit in order to limit the current to 8A? Obtain
(i) power supplied by d.c. source (ii) power dissipated as heat.
13. State Ampere’s circuital law. Using Ampere’s circuital law derive the expression for magnetic field along the
axis of (i) an air cored solenoid
14. A galvanometer of 12Ω gives full deflection for 2.5mA. Calculate the value of resistance to convert it into (i) an
ammeter of range 0 to 7.5A (ii) a voltmeter of range 0 to 10 V.
15. In LCR circuit, L=5H,C=80µF and R = 40Ω are connected to a variable frequency 230 V source .
(i)Determine resonance frequency, (ii) impedance of circuit (iii) amplitude of current at resonance
(iv) rms potential drop across each component.
16. Derive an expression for range up to which signals transmitted by a T.V. tower can be received?
17. Derive prism formula. Draw graph for δ& i. Also give the conditions for minimum deviation.
18. A convex lens & concave lens of n2 is held in a medium of refractive index n1 , separately . Trace the path of
parallel beam of light passing through each lens, when (i) n1> n2 (ii) n1< n2 (iii) n1 = n2 .
19. Two lenses of +15D and -15D are in contact with each other forming a combination lens.
(i)
What is the focal length of this combination?
(ii)
An object of size 3 cm is placed at 30cm from this combination of lens. Calculate the position and size of
the image formed.
20. Explain the term: (i) Stopping potential (ii) Threshold Frequency (iii) Work Function (iv) Saturation Current
with reference to Photo Electric Effect.
21. Draw the graph to show variation of Binding Energy per Nucleon with mass number of different nuclei .
Explain the process of nuclear fussion and fission on this basis .
22. Define the term ‘critical frequency. On a particular day the max. Frequency reflected by ionosphere is 10MHz.
on another day it was found to decrease to 8MHz. calculate the ratio of the max. electron densities of
ionosphere on two days. ?
23. Isabella used an induction stove and explains to her neighbor that due to shortage of LPG one must utilize
other sources that are available to produce heat energy; being a Physics teacher Isabella, explains that the Oil
companies are trying their best to meet the demands for LPG and that as a good citizen one must use other
sources wherever feasible. Isabella uses an induction stove having a value of 7H inductor and the flow of
current is from 10Ato 7A in 9 x 10-2 seconds.
a) In the above, what is the quality you find in the Physics teacher?
b) Calculate the emf generated in the above.
24. For a spherical surface separating two media of n2& n1 , ( n2 > n1 ) . Derive the relation
𝑛2
𝑣
-
𝑛1
𝑢
=
𝑛2−𝑛1
𝑅
. Write the assumptions and sign conventions used.
Or
24. (i) Explain the phenomenon of total internal reflection. Deduce the
relation
for critical angle and refractive index of the medium
(ii) The refractive indices of material of prism for red, green and blue wave
lengths are 1.39, 1.44 and 1.47 respectively. Which of three will come out
of face AC.?
25. (i) Explain the formation of depletion layer and barrier potential in a p-n junction.
(ii). How the thickness of depletion layer in a p-n junction diode changes in (a) forward biased (b) reverse
biased. (iii) With diagram explain the use of a p-n diode as a full wave rectifier. Draw the input &output wave
forms.
Or
25. (i) Draw the circuit diagram to study the input and output characteristics of npn transistor in CE
configuration.
(ii)Explain how these graphs are used to calculate, input resistance, output resistance and current
amplification factor.
(iii)
Explain how output voltage is out of phase by 180˚ with input voltage.
26. State Gauss Theorem in Electrostatics. Use it to obtain Electric Field Intensity due to hollow charged
conducting sphere. Draw the graph between electric field intensity and distance from the centre to far off
point.
Or
26. Give principle, working and construction of Van de Graff generator. How leakage can be minimized?
S.
No.
1.
2.
3.
Unit
Electrostatics
Current Electricity
Magnetic Effects of Current
&
Magnetism
4.
Electromagnetic Induction
and
Alternating Current
5.
Electromagnetic waves
6.
Optics
7.
Dual Nature of Matter
8.
Atom and Nuclei
9.
Electronic Devices
10.
Communication System
Total
BLUE PRINT
V.S.A.
S.A.1
(1mark) (2 mark)
1
1
1
1
S.A.2
(3 mark)
2
2
1
1
1
1
1
3
1
1
5(5)
1
1
1
10(5)
VBQ
L.A.
(4marks (5mark)
1
8 (3)
7(3)
8(3)
1
8(3)
1
1
1
36(12)
Total
4 (1)
15(3)
3(1)
14(4)
4(2)
6(3)
7(2)
5(2)
70(26)