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Physics-XII2

CBSE Solved Paper

5. The motion of copper plate is damped, when it is allowed to oscillate between thetwo poles of a magnet. What is the cause of this damping?

6. Define the activity of a given radioactivesubstance. Write its SI unit.

7. Welders wear special goggles or face masks with glass windows to protect their eyesfrom electromagnetic radiation. Name theradiations and write the range of their frequency.

8. Write the expression for the de-Broglie wavelength associated with a chargedparticle having charge q and mass m , whenit is accelerated by a potential V .

9. Draw typical output characteristics of an n-p-n transistor in CE configuration. Showhow these characteristics can be used todetermine output resistance?

10. A parallel beam of light of 500 nm falls on anarrow slit and the resulting diffractionpattern is observed on a screen 1 m away. Itis observed that the first minimum is at adistance of 2.5 mm from the centre of thescreen. Calculate the width of the slit.

11. A slab of material of dielectric constant K has the same area as that of the plates of aparallel plate capacitor but has thethickness d /2, where d is the separationbetween the plates. Find out the expressionfor its capacitance, when the slab isinserted between the plates of thecapacitor.

12. A capacitor made of two parallel plateseach of the plate A and separation d , isbeing charged by an external AC source.Show that the displacement current insidethe capacitor is the same as the currentcharging the capacitor.

13. Explain the term drift velocity of electronsin conductor. Hence, obtain the expressionfor the current through a conductor interms of drift velocity.

Or

Describe briefly with the help of a circuitdiagram, how a potentiometer is used todetermine the internal resistance of a cell.

14. A convex lens of focal length f 1 is kept incontact with a concave lens of focal length f 2 .Find the focal length of the combination.

15. In the block diagram of a simple modulator for obtaining an AM signal, shown in the figure,identify the boxes A and B. Write their function.

16. In the circuit shown in the figure, identify theequivalent gate of the circuit and make itstruth table.

17. ( i) For a given AC, i i t m= sin , show that theaverage power dissipated in a resistor Rover a complete cycle is 1

22 i R m .

( ii) A lightbulbis rated at100 W for a 220 VACsupply. Calculate the resistance of thebulb.

18. A rectangularconductor LMNO is placedin auniform magnetic field of 0.5 T. The field isdirected perpendicular to the plane of theconductor. When the arm MN of length20 cm is moved towards left with a velocity of 10 1ms , calculate the emf induced in thearm. Given, the resistance of the arm to be5 (assuming that other arms are of negligible resistance), find the value of thecurrent in the arm.

Or

A wheel with 8 metallic spokes each50 cm long is rotated with a speed of 120 rev/min in a plane normal to thehorizontal component of the Earths magneticfield. The Earths magnetic field at the place is0.4 G andthe angle of dip is 60 . Calculate theemf induced between the axle and the rim of

wheel. How will the value of emf be affected,if the number of spokes were increased?

A B AM wave y t ( ) x t ( )

Modulatingsignal

Carrier wave

A

B

A

BY

l

L

O

N

M


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19. Define the current sensitivity of agalvanometer. Write its SI unit.Figure shows two circuits each having agalvanometer and battery of 3 V.

When the galvanometer in eacharrangement do not show any deflection,obtain the ratio R R1 2 / .

20. A wire AB is carrying a steady current of 12 A and is lying on the table. Another wireCD carrying 5 A is held directly above AB ata height of 1 mm. Find the mass per unitlength of the wire CD , so that it remainssuspended at its position when left free.Give the direction of the current flowing inCD with respect to that in AB .[Take the value of g =10 ms 2 ]

21. Draw V-I characteristics of a p-n junctiondiode. Answer the following questions,giving reasons.( i) Why is the current under reverse bias

almost independent of the appliedpotential upto a critical voltage?

( ii) Why does the reverse current show asudden increase at the critical voltage?

Name any semiconductor device whichoperates under the reverse bias in the

breakdown region.22. Draw a labelled ray diagram of a refracting

telescope. Define its magnifying power and write the expression for it. Write two important limitations of arefracting telescope over a reflecting typetelescope.

23. Write Einsteins photoelectric equation andpoint out any two characteristic properties of photons on which this equation is based.

Briefly explain the three observed features which can be explained by this equation.

24. Name the type of waves which are used for Line of Sight (LOS) communication. What isthe range of their frequencies?

A transmitting antenna at the top of a tower has a height of 20 m and the height of thereceiving antenna is 45 m. Calculate themaximum distance between them for satisfactory communication in Line of Sightmode. (Radius of the Earth = 6 4 10 6. km)

25. ( i) What is linearly polarised light? Describebriefly using a diagram, how sunlight ispolarised.

( ii) Unpolarised light is incident on apolaroid. How would the intensity of transmitted light change when thepolaroid is rotated?

26. One day Chetans mother developed asevere stomach ache all of a sudden. She

was rushed to the doctor who suggested for an immediate endoscopy test and gave anestimate of expenditure for the same.Chetan immediately contacted his classteacherand shared the information withher.The class teacher arranged the money andrushed to the hospital. On realising thatChetan belonged to a belowaverage incomegroup family, even the doctor offeredconcession for the test fee. The test wasconducted successfully.

Answer the following questions based onthe above information( i) Which principle in optics is made use of in

endoscopy?( ii) Briefly explain the values reflected in the

action taken by the teacher.( iii ) In what way do you appreciate the

response of the doctor on the givensituations?

27. ( i) Using Biot-Savarts law, derive theexpression for the magnetic field in thevector form at a point on the axis of acircular current loop.

( ii) What does a toroid consist of ? Find out theexpression for the magnetic field inside atoroid for N turns of the coil having theaverage radius r and carrying a current I .Show that the magnetic field in the openspace inside and exterior to the toroid is

zero.

Physics-XII3

CBSE Solved Paper

3.0 V

R1

G 60

9 6

4

R2

3.0 V

8

6 12

G1.20


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Physics-XII4

CBSE Solved Paper

Or ( i) Draw a schematic sketch of a cyclotron.

Explain clearly the role of crossed electric

and magnetic field in accelerating thecharge. Hence derive the expression for thekinetic energy acquired by the particles.

( ii) An -particle and a proton are released fromthe centre of the cyclotron and made toaccelerate.(a) Can both be accelerated at the same

cyclotron frequency? Give reason to justifyyour answer.

(b) When they are accelerated in turn, which of the two will have higher velocity at the exitslit of the dees?

28. ( i) Define electric dipole moment. Is it a scalar or a vector? Derive the expression for theelectric field of a dipole at a point on theequatorial plane of the dipole.

( ii) Draw the equipotential surfaces due to anelectric dipole. Locate the points where thepotential due to the dipole is zero.

Or

Using Gauss law, deduce the expression for the electric field due to a uniformly chargedspherical conducting shell of radius R at apoint( i) outside and ( ii) inside the shell.

Plot a graph showing variation of electricfield as a function of r R> and r R > , then E pr

=

14 0 3

(ii) Equipotential surface due to electric dipole

The potential due to the dipole is zero at the linebisecting the dipole length.

Or Electric field due to a uniformly charged thinspherical shell

E 1 1 s n

E1 cos

E2 cos

E 2

E2 sin

( +

)

r

a

2

2

qB A

O + q a a

( + ) r a2 2

P

r

q + qp

2l

1

1

1

1

1/2

112

v =

2

I I

P

Q B

O

Charged sphericalshell

Gaussian surface

R

dsP

E

r

1


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Physics-XII13

CBSE Solved Paper

(i) When point P lies outside the spherical shellSuppose that we have to calculate electric field atthe point P at a distance r r R( )> from its centre.

Draw the Gaussian surface through point P , so asto enclose the charged spherical shell. TheGaussian surface is a spherical shell of radius r and centre O .Let E be the electric field at point P . Then, theelectric flux through area element d S is given by

d d = E SSince, d S is also along normal to the surface d E ds =Total electric flux through the Gaussian surface

is given by

= = E d E d S S

S S

Now, d r S = 4 2 = E r 4 2 (i)Since, the charge enclosed by the Gaussiansurface is q, according to Gauss theorem.

=q

0(ii)

From Eqs. (i) and (ii), we obtain

E r q =

4 2

0

E qr

=

14 0

2 [For r R> ]

(ii) When point P lies inside the spherical shellIn such a case, the Gaussian surface enclosed nocharge.According to Gauss law,

E r =4 02i e. ., E = 0 [ ]For r R

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