KENDRIYA VIDYALAYA NTPC SHAKTINAGAR
IstPre Board Exam 2015-16
Subject – Physics
Class – XIIth
Time Duration: 3.00 Hrs M. Marks: 70
General Instructions:-a. All questions are compulsory
b. Internal choices have been provided in some questions. You have to attend only one of the choices in such question.
c. Question numbers 1 to 8 are very short answer questions carrying one mark each.
d. Question numbers 9 to 18 are short answer questions carrying two marks each.
e. Question numbers 19 to 27 are also very short answers questions carrying three mark each.
f. Question numbers 28 to 30 are long answer type question carrying 5 marks each
g. Use log tables if necessary.
1. Name the electromagnetic wave used for the treatment of cancer?
2. A magnet is moving towards a coil with a uniform speed v as shown in the
figure. State the direction of the induced current in the resistor R.
3. Define resistivity of a metal .State it’s SI unit.
4. Two nuclei have mass numbers in the ratio 1:8 , what is the ratio of their nuclei radii?
5. What is meant by electrostatic shielding?
6. Write Einstein’s photoelectric equation explaining different terms .
7. What is the magnetic moment of an electron orbiting in a circular orbit of radius ‘r’ with a speed ‘V’?
8. At what temperature the intrinsic semiconductor behaves as an insulator.
9. Write the relation between half life and mean life of a radioactive element.
10. Two bulbs are marked 220V-100W and 220V-50W. They are connected in series to 220V mains. Find the ratio of heat generated in them.
11. Define the term angle of dip. What is the value of the angle of dip at the magnetic equator ?
12. Find the ratio of intensities of two points on a screen in young’s double slit experiment , when waves from the two slits have a path difference of (i) 0 and (II) ג /4.
13. A charge Q is placed at a distance a/2 above the centre of a horizontal square surface of edge a. Find the flux of electric field through each square surface.
14. From the following , identify the E. M. Waves having the (i) maximum(ii) minimum frequency and give one use of both waves. (a)Radio Waves (b) Gamma rays (c) Visible light (d) Microwaves (e) Ultraviolet rays and (f) Infrared rays
15. The work function for a certain metal is 4.2 eV . Will this metal give photoelectric emission for incident radiation of wavelength is 330 nm ?
16. What is nuclear fission ?Give one representative equation.
OR
How does the size of a nucleus depend on its mass number ? Hence, explain why the density of nuclear matter is independent of the size of nucleus?
17. With the help of a circuit diagram ,explain how a p-n junction diode can be used as a half wave rectifier?.
18. Explain space wave propagation?Give two examples of communication system which use space wave mode.
19. A 200 V variable frequency AC source is connected to a series combination of L = 5 H, C = 80µF and R = 40Ω. Calculate
(a) Angular frequency of source to get the maximum current
(b) Current amplitude at resonance and
(c) Power dissipation in the circuit.
20. Write the truth table for the circuits given in Fig. consisting of NOR gates only. Identify the logic operations (OR, AND, NOT) performed by the two circuits.
21. Derive a mathematical expression for the width of interference fringes obtained in Young’s double slit experiment with the help of a suitable diagram.
22. Figure shows a 2.0 V potentiometer used for the determination of internal resistance of 1.5 v cell. The balance point of the cell in open circuit is 76.3 cm. When a resistor of 9.5 is used in the external circuit of the cell, the balance point shifts to 64.8 cm length of the potentiometer wire. Determine the internal resistance of the cell.
2.0 V
1.5 V G
G
9.5 Ω
23. What do you mean by self induction. Define self inductance. Derive expression for self inductance of a long air – core solenoid of length l , cross – sectional area A and having number of turns N.
24. Draw a labelled ray diagram for the formation of image by a compound microscope . Acompound microscope uses two lenses of power 10D and 1 D.
(i) State with reason, which lens is preferred of objective and eye-piece.
(ii) Calculate the magnifying power of the compound microscope , if the final image is formed at the least distance of distinct vision.
25. Draw the circuit diagram to study the characteristics of npn transistor in common emitter(CE)configuration.Sketch typical (i) input characteristics (ii) out put characteristics for such a configuration.Explain how the current gain of a transistor is calculated from output characteristics.
OR
(a) Define modulation index.
(b) Give its physical significance .
(c) For an amplitude modulated wave , maximum amplitude is found to be 10 V while the minimum amplitude is 2V . Determine the modulation index.
26. State three postulates of Bohr‘s theory of H – atom.
27. In a winter night, sonu was sitting with his grandmother, she told him “when I was of your age, paisa coins were used like one paisa, two paisa etc., and those paisa coins were having great value at that time.”
Sonu asked his grandmother, “ I don’t know about it, as I am not educated and I even never tried to know about that”.
But sonu’s elder brother monu was standing behind and listening to the conversation between them, he told sonu that those were made up of aluminium magnesium alloy. Grandmother felt happy with her grandsons.
(a) Write about monu’sintellegency.
(b) A paisa coin weighs 0.75 g and is square shaped with diagonal 17 mm.
Suppose it is electrically neutral. If the coin is only made up of aluminium. Find the magnitude of equal number of + ve and – ve charges.
28. (a) Define plane polarised lights. (b) State and prove Brewster law of polarisation. (c) If the angle between the pass axis of polarizer and the analyser is 450 , write the ratio of the intensities of original light and the transmitted light after passing through the analyser.
OR
Derive the expression for the refractive index of the material of the prism in terms of the angle of the prism and angle of minimum deviation.Also prove that for a small angled prism δ=A(µ -1), where the symbols have their usual meanings.
29. Define electric flux .What is it’s SI unit?What is Gauss theorem in electrostatics? Prove that the electric field at a point due to a uniformly charged infinite plane sheet is independent of the distance from it.
OR
Two capacitors with capacitances C1 and C2 are charged to potentials V1and V2 respectively and than connected in parallel. Calculate the common potential across the combination, the charge on each capacitor , the electrostatic energy stored in the system and the charge in electrostatic energy from its initial value.
30. (a) What do you mean by current sensitivity of a moving coil galvanometer. On what factor does it depend ?
(b)State two reasons why a galvanometer cannot be used as such to
measure current in a given circuit ?
(c) With the help of a circuit , show how a moving coil galvanometer can be converted into an ammeter of an given range.
OR
(a) Write any three characteristics, a ferromagnetic substance should possess if it is to be used to make a permanent magnet . Give one example of such a material.
(b) The susceptibility of a magnetic material is – 0.085. Identify the magnetic type of the material.
(c) Draw the field lines due to an external magnetic field near a (i) diamagnetic (ii) paramagnetic substance.
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