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Blue Print of Question Paper

Sl. No.

Unit VSA SA -1 SA-2 VALUE BASED

LA Marks

1 ELECTROSTATICS 1(1) 1 1 152 CURRENT AND ELECTRICITY 1(1) 1 13 MAGNETIC EFFECT OF ELECTRIC

CURRENT AND MAGNETISM 2 16

4 EMI AND AC 1(1) 1 15 ELECTROMAGNETIC WAVES 1 176 OPTICS 1(1) 1 2 17 DUAL NATURE OF MATTER &

RADIATION1 10

8 ATOM & NUCLEI 2 19 SEMI-CONDUCTORS 2 1210 COMMUNICATION SYSTEMS 1(1) 1 1

TOTAL 5(5) 10(5) 36(12) 4(1) 15(3) 70SAMPLE PAPER 6

B SESHA SAI/KV, EOI/KATHMANDU/XI/PHY /2017-18

PHYSICS (THEORY)

TIME : 3 hours CLASS XII Max. Marks: 70

Read all of the following information before starting the exam:

All questions are compulsory. There are 26 questions in total. Questions 1 to 5 carry one mark each, questions 6

to 10 carry two marks each, questions 11 to 22 carry three marks each, question 23 is value based which carry four marks and questions 24 to 26 carry five marks each.

Use of calculators is not permitted. You may use the following values of physical constants wherever necessary.

c = 3 × 108 m/s , h = 6.63 × 10–34 Js , e = 1.6 × 10–19 C , μ0 = 4π × 10–7 T mA–1

1/4πεo = 9 × 109 N m2 C–2 , me = 9.1 × 10–31 kg, εo = 8.85 × 10-12 C2 N-1m-

2

Mass of Neutrons = 1.675 × 10–27 kg , Mass of proton = 1.673 × 10–27 kg

SECTION A1. A charge Q is distributed over a metal sphere of radius R. What is the electric field and electric potential at the centre?2. V – I graph for a metallic wire at two different temperatures T1 and T2 is as shown in the figure. Which of the two temperatures is higher and why?

3. Show graphically the variation of magnetic field due to a straight conductor of uniform cross-section of radius ‘a’ and carrying steady currently as a function of distance r (a > r) from the axis of the conductor.4. When light travels from an optically denser medium to a rarer medium, why does the critical angle of incidence depend on the colour of light?5. What is the function of a band pass filter used in a modulator for obtaining AM signal?

SECTION B6. Draw a graph to show the temperature dependency of resistivity for a typical semiconductor and alloy.7. When the electron orbiting in hydrogen atom in its ground state moves to the third excited state, show how the de Broglie wavelength associated with it would be affected.8. Define a wavefront. Using Huygens’ Principle, draw the shape of a refracted wavefront, when a plane wave is incident on a convex lens.OR(a) When a wave is propagating from a rarer to a denser medium, which characteristic of the wave does not change and why?


(b) What is the ratio of the velocity of the wave in the two media of refractive indices µ1 and µ2?9. Distinguish between point to point and broadcast communication modes. Give one example of each.10. From the relation R = R0 A1/3 where R0 is a constant and A is the mass number of a nucleus, show that nuclear matter density is nearly constant.

SECTION C11. Deduce the expression for the torque τ acting on a planar loop of area A and carrying current I placed in a uniform magnetic field B. If the loop is free to rotate, what would be its orientation in stable equilibrium?12. How are electromagnetic waves produced? What is the source of the energy carried by a propagating electromagnetic wave? Identify the electromagnetic radiations used(i) in remote switches of household electronic devices; and(ii) as diagnostic tool in medicine.13. (a) A ray of light is incident normally on the face AB of a right-angled glass prism of refractive index aµg = 1·5. The prism is partly immersed in a liquid of unknown refractive index. Find the value of refractive index of the liquid so that the ray grazes along the face BC after refraction through the prism.

(b) Trace the path of the rays if it were incident normally on the face AC.

14. The outputs of two NOT gates are fed to a NOR gate. Draw the logic circuit of the combination of gates. Write its truth table. Identify the gate equivalent to this circuit.ORYou are given two circuits (a) and (b) as shown in the figures, which consist of NAND gates. Identify the logic operation carried out by the two. Write the truth tables for each. Identify the gates equivalent to the two circuits.

15. With the help of a circuit diagram, explain the working of a junction diode as a full wave rectifier. Draw its input and output waveforms. Which characteristic property makes the junction diode suitable for rectification?16. A potential difference V is applied across a conductor of length L and diameter D. How is the drift velocity, vd, of charge carriers in the conductor affected when (i) V is halved, (ii) L is doubled and (iii) D is halved? Justify your answer in each case.17. In a chamber of a uniform magnetic field 6.5G is maintained. An electron is shot into the field with a speed of 4.8 x 106 ms–1 normal to the field. Explain why


the path of electron is a circle. (a) Determine the radius of the circular orbit (e = 1.6 x 10–19 C, me = 9.1 x 10–31 kg)(b) Obtain the frequency of revolution of the electron in its circular orbit.18. (a) In what way is diffraction from each slit related to the interference pattern in a double slit experiment.(b) Two wavelengths of sodium light 590 nm and 596 nm are used, in turn to study the diffraction taking place at a single slit of aperture 2 × 10−4 m. The distance between the slit and the screen is 1.5 m. Calculate the separation between the positions of the first maxima of the diffraction pattern obtained in the two cases.19. (a) The figure shows the plot of binding energy (BE) per nucleon as a function of mass number A. The letters A, B, C, D and E represent the positions of typical nuclei on the curve. Point out, giving reasons, the two processes (in terms of A, B, C, D and E), one of which can occur due to nuclear fission and the other due to nuclear fusion.

(b) Identify the nature of the radioactive radiations emitted in each step of the decay process given below.

20. Name the three different modes of propagation in a communication system.State briefly why do the electromagnetic waves with frequency range from a few MHz upto 30 MHz can reflect back to the earth. What happens when the frequency range exceeds this limit ?21. Define the terms ‘stopping potential’ and ‘threshold frequency’ in relation to photoelectric effect. How does one determine these physical quantities using Einstein’s equation?22. A network of four 10 μF capacitors is connected to a 500 V supply, as shown in Fig. Determine (a) the equivalent capacitance of the network and (b) the charge on each capacitor. (Note, the charge on a capacitor is the charge on the plate with higher potential, equal and opposite to the charge on the plate with lower potential.)

23. Sunita and her friends visited an exhibition. The policeman asked them to pass through a metal detector. Sunita’s friends were initially scared of it. Sunita, however, explained to them the purpose and working of the metal detector.


Answer the following questions:(a) On what principle does a metal detector work?(b) Why does the detector emit sound when a person carrying any metallic object walks through it?(c) State any two qualities which Sunita displayed while explaining the purpose of walking through the detector.

SECTION E

24. (a) A point-object is placed on the principal axis of a convex spherical surface of radius of curvature R, which separates the two media o refractive indices n1 and n2 (n2 > n1). Draw the ray diagram and deduce the relation between the distance of the object (u), distance of the image (v) and the radius of curvature (R) for refraction totake place at the convex spherical surface from rarer to denser medium.(b) Use the above relation to obtain the condition on the position of the object and the radius of curvature in terms of n1 and n2 when the real image is formed.OR(a) Draw a labelled ray diagram showing the formation of image by a compound microscope in normal adjustment. Derive the expression for its magnifying power.(b) How does the resolving power of a microscope change when (i) the diameter of the objective lens is decreased, (ii) the wavelength of the incident light is increased ?Justify your answer in each case.

25. (a) State Faraday’s law of electromagnetic induction. (b) Explain, with the help of a suitable example, how we can show that Lenz’s law is a consequence of the principle of conservation of energy.(c) Use the expression for Lorentz force acting on the charge carriers of a conductor to obtain the expression for the induced emf across the conductor of length l moving with velocity v through a magnetic field B acting perpendicular to its length.

OR(a) Using phasor diagram, derive the expression for the current flowing in an ideal inductor connected to an a.c. source of voltage v = vo sin ωt. Hence plot graphs showing variation of (i) applied voltage and (ii) the current as a function of ωt.(b) Derive an expression for the average power dissipated in a series LCR circuit.

26. (a) State Gauss’s law in electrostatics. Show, with the help of a suitable example along with the figure, that the outward flux due to a point charge ‘q’, in vacuum within a closed surface, is independent of its size or shape and is given by q/εo .(b) Two parallel uniformly charged infinite plane sheets, ‘1’ and ‘2’, have charge densities + σ and – 2σ respectively. Give the magnitude and direction of the net electric field at a point(i) in between the two sheets and(ii) outside near the sheet ‘1’.

OR(a) Define electrostatic potential at a point. Write its S.I. unit Three point charges q1, q2 and q3 are kept respectively at points A, B and C as shown in the figure. Derive the expression for the electrostatic potential energy of the system.


(b) Depict the equipotential surfaces due to (i) an electric dipole,(ii) two identical positive charges separated by a distance.

SOLUTION OF SAMPLE PAPER

Q.No.

Answer points Marks

1 Electric field inside conductor E = 0 ½

½

2 T1 > T2 1


Slope of T1 is higher than that of T2.(or Resistance, at T1, is higher than that of T2)

3 1

4 Critical angle depends upon the refractive index (n) of the mediumand refractive index is different for different colours of light.

½½

5 It rejects dc and sinusoids of frequency 𝜔m, 2𝜔m and 2𝜔c and retainfrequencies 𝜔c, 𝜔c ± 𝜔m.(Alternatively: It allows only the desired/ required frequencies to passthrough it)

1

6 For semiconductor

For alloy

1

1


7 1

1

8

1

1

1

1

9 Point to point communication: The communication over a link between a single transmitter and receiver.Example: Telephone Broadcast mode: In such a mode large number of receiver is linked with single transmitter.Example: Radio

½

½½

½


½

½

110 ½

1

½

11 ½

½1

1


12 1

1

½½


13

½

½

½½

1

14

1


Or

1

1

½½

½

½

½½


15 1

1/2

½

½

1/2

16 1

1

1

17

MHz

1½

1½


18 (a) If the width of each slit is comparable to the wavelength of light used, the interference pattern thus obtained in the double-slit experiment is modified by diffraction from each of the two slits.

(b) Given that: Wavelength of the light beam, λ1 = 590 nm = 5.9 ×10-7 mWavelength of another light beam, λ2 = 596 nm = 5.96 ×10-7 mDistance of the slits from the screen = D = 1.5 mDistance between the two slits = a = 2 ×10-4 m

For the first secondary maxima,

1

1

1

19 1

1

120 1

1

1

21 1

1

1


22 1

½

1

1/2

23 1

1

2


24

1

½

½

1

1 ½

½


(Award 1 mark if the student draws the diagram for image at distance ofdistinct vision, deduct ½ mark for not showing the direction of Propogation ofray)Derivation:

(b) The resolving power of microscope(i) Will decrease with decrease of the diameter of objective lens as resolvingpower is directly proportional to the diameter.(ii) Will decrease with increase of the wavelength of the incident light asresolving power is inversely proportional to the wave length

1

1

½

½

1

1


25 2

1 ½

1 ½

1

1,

1

1+1


26 2

1

1

1

1

1

1


(Award these 1 mark if the student directly writes the expressionfor 𝑈)(b) Equipotential surface due to(i) An electric dipole

(ii) Two identical positive changes

1

1


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