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PHYSICS (THEORY)

CLASS XII

Time : 3 hours M.M.: 70

General Instructions:

(i) All questions are compulsory

(ii) There are 30 questions in total. Questions 1 to 18 are very short answer type

questions and carry one mark each.

(iii) Questions 9 to 18 carry two marks each, questions 19 to 27 carry three marks

each and questions 28 to 30 carry five marks each.

(iv)There is no overall choice. However, an internal choice has been provided in onequestion of two marks one question of three marks and all three question of five

marks each. You have to attempt only one of the given choice in such questions.

(v) Use of calculators is not permitted However, you may use log tables if necessary.

(vi) 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

7 14 10 Tm Ao2

1 Nm99 10

24 o c

=

=

Q1. Two identical loops, one of copper and another of constantan are removed from a

magnetic field within same interval of time. In which loop the induced current will begreater?

Q2. Two electrically charged particles, having charges of different magnitude when

placed at a distance d from each other experience a force of attraction F. These

two particles are put in contact and again placed at the same distance from eachother. What is the nature of force between them now?

Q3. A ray of light incident on an equilateral glass prism ( )3glass = moves parallel to thebase of the prism inside it. What is the angle of incidence for this ray?

Q4. A capacitor of capacitance C is being charged up by connecting it across a dc voltage

source of voltage V. How do the conduction and displacement currents in this set up

compare with each other:

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(a) During charging up process.

(b) After the capacitor gets fully charged?

Q5. An - particle and a proton are moving in a plane of paper in a region where there

is uniform magnetic field ( )B

directed normal to the plane of the paper. If the two

particles have equal linear momentum, what will be the ratio of the radii of their

trajectories in the field?

Q6. How does the magnetic moment of an electron in a circular orbit of radius r andmoving with a speed change, when the frequency of revolution is doubled?

Q7. The mean life of a radioactive sample is Tm. What is the time in which 50% of this

sample would get decayed?

Q8. A proton and an alpha particle, both initially at rest, are (suitably) accelerated so asto have the same kinetic energy. What is the ratio of their de-Broglie wavelengths?

Q9. Find the amount of work done in rotating an electric dipole, of dipole moment 3 10-8 Cm. From its position of stable equilibrium to the position of unstable

equilibrium, in a uniform electric field of intensity 104 N/C.

Q10. A wire loop formed by joining two semicircular wires of radii R1 and R2 carries a

current I as shown in the following figure. Find the magnetic field at the centre O.

Q11. The intensity, at the central maxima (O) in a Youngs double slit set up is lo. If the

distance OP equals one third of the fringe width of the pattern, show that the

intensity at point P, would equal lo/4.

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Q12. An X ray tube operates at 10kV. What is the ratio of x ray wavelength to that of

deBroglie wavelength?

Q13. An electric heater is connected turn by turn to a d.c and a.c sources of equal

voltages. Will the rate of heat production be same in the two cases? Explain.

Q14. Two students A and B prepare the following table about the electromagnetic waves.

Rewrite this table in its corrected form.

Direction of Peak Value of

Student Electric field Magnetic field Propagation Electric field Magnetic field

A Along X-axis Along X-axis Along Y-axis E B = cE

B Along Y-axis Along Z-axis Along X-axis E = cB B

Q15. Which type of diode shows this type of I-V characteristic curve? State one use of it

with circuit diagram.

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Q16. A modulating signal is a square wave as shown.

The carrier wave given by c(C) = 4 sin (4t) volts.

(i) Sketch the amplitude modulated waveform.

(ii) What is the modulation index?

Q17. A radioactive nucleus A undergoes a series of decay according to the followingscheme:

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The mass number and atomic number of A are 180 and 72 respectively. What are

these numbers for A4 ?

OR

Write the equations for the two types of -decay. Why is it very difficult to detect

the neutrino?

Q18. Draw the output wave form at X, using the given inputs A,B for the logic circuitshown below. Also identify the (equivalent) gate.

Q19. A resistor of resistance 400 , and a capacitor of reactance 200 , are connected ina series to a 220 V, 50 Hz. a.c. source. If the current in the circuit is 0.49 amperefind the (i) voltage across the resistor and capacitor (ii) value of inductance required

so that voltage and current are in phase.

Q20. The energy levels of an element are as shown below. Which of the shown transitions

will result in the emission of photon of wavelength 482 nm?

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Q21. An equiconvex lens, with radii of curvature of magnitude R each is put over a liquid

layer poured on top of a plane mirror. A small needle, with its tip on the principal

axis of the lens, is moved along the axis until its inverted real image coincides withthe needle itself. The distance of the needle from the lens is measured to be a. On

removing the liquid layer and repeating the experiment the distance is found to beb.

Given that two values of distance measured represent the focal length values in the

two cases, obtain a formula for the refractive index of the liquid.

Q22. In the meter bridge experiment, a student observed a balance point at the point J.

Draw the equivalent wheat stone Bridge circuit diagram for this set up. Find thevalue of unknown resistance X.

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The values of R and X are both doubled and then interchanged. What would be thenew position of the balanced point? If, in this set up, the galvanometer and battery

are interchanged at the balance point position, how will the balance point getaffected?

Q23. In the following diagrams, write which one of the diodes is forward biased and which

is reverse biased?

(c) An ac signal is fed into the circuit X. The corresponding output waveform isshown below. Identify X. Draw its detail circuit diagram.

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OR

The transfer characteristic of base biased transistor in CE configuration is shown.

Name the region corresponding to the values (i) O to V1 (ii) V1 to V2 (iii) greater thanV2 of the input voltage applied to transistor.

Identify the voltage range that should not be used if the transistor has to work as aswitch. What is the practical use of transistor, when it is operated in this voltage

range? Name the source that results in a higher energy of the output of a transistor

operated in this range?

Q24. A metallic square loop ABCD of length L resistance R is moving in its own plane withvelocity in a uniform magnetic field B perpendicular to its plane as shown in the

figure 1. What will be the direction of induced current in the loop? What will be the

induced emf in the loop when half of the loop is outside the field. ?

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Q25. Two convex lenses, of equal focal length, but of aperture A1 and A2 (A2 < A1), are

used as the objective lenses in two astronomical telescopes having identical

eyepieces.

Compare the ratio of their (i)resolving power (ii)(normal) magnifying power and

(iii)intensity of images formed by them. Which of the two telescopes should bepreferred? Why?

Q26. Give reasons for the following:-

(i) For ground wave transmission, size of antenna ( )l should be comparable to

wavelength of signal i.e. / 4

(ii) Audio Signal converted into an em wave, are not directly transmitted as such.

(iii) The amplitude of modulating signal is kept less than the amplitude of carrierwave .

Q27. Find the potential difference between point A and B and the rate of energydissipation in R1 . Assume that both the batteries have zero internal resistance.

Q28. Introduce a small sphere A of radius r, carrying some charge q into a large sphere B

of radius R carrying charge Q .Place it at the centre of the large one.

(i) What will be the potential difference between theses two spheres.

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(ii) What will happen if we connect A and B sphere with a conducting wire?

(iii) Name and state the principle of a machine which works on the above concept.

OR

If two parallel similar plates each of area A having surface charge densities + and are separated by a distance d in air, write the expression for (i) the electric field

at points between the two plates (ii) potential difference between the plates (iii) Thecapacitance of the capacitor so formed.

Q29. (a) A plane wave front approaches a plane surface separating two media. If medium

one is (optically) denser and medium two is (optically) rarer, construct the refractedwave front using Huygenss principle.

Hence prove Snells law.

(b) Draw the shape of the refracted/reflected wave front when a plane wave front is

incident of (i) prism and (ii) convex mirror. Give a brief explanation for the

construction.

OR

(a) State the essential condition for the diffraction of light to take place.

A parallel beam of monochromatic light falls normally on a narrow slit and lightcoming out of the slit is obtained on the screen. Derive an expression for the angularwidth of the central bright maxima obtained on the screen.

(b) State two differences between diffraction and interference.

Q30. (a) Draw the labeled diagram of moving coil galvanometer. Prove that in a radial

magnetic field, the deflection of the coil is directly proportional to the current flowingin the coil.

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(b) A galvanometer can be converted into a voltmeter to measure up to

(i) V volts by connecting a resistance R1

in series with its coil.

(ii)V

2volts by connecting a resistance R2 in series will its coil.

Find the resistance R in terms of R1 and R2 required to convert it in to a voltmeter

that can read up to 2V volts.

OR

(a) Draw diagrams of depict the behavior of magnetic field lines near a bar of:

(i) Copper

(ii) Aluminum

(iii) Mercury, cooled to a very low temperature (4.2 K).

(b) The vertical component of the earths magnetic field at a given place is 3 times

its horizontal component. If total intensity of earths magnetic field at the place is 0.4

G, find the value of :

(i) angle of dip

(ii) the horizontal component of earths magnetic field.