PHYSICS (Theory)

�Rias fail,� {"1c0-1Paas)

CBSE QUESTION PAPER

Maximum Marks: 100 � '3icFi: 100

Ti me allowed : 3 hours

f.:mfftrr-'ff1fll:3�

Class-XII

General Instructions :

(i) · All questions are compulsory.

(ii) There are 30 questions in total. Questions 1 to 8 carry one markeach, questions 9 to 18 carry two marks each, questions 19 to 27carry three marks each ands questions 28 to 30 carry ffoe marks each.

(iii) There is no overall choice. However, an internal choice has beenprovided in one question of two marks, one question of three marksand all three questions of five marks each. You have to attemptonly one of the choices in such questions.

(iv) Use of calculators is not permitted.

(v) You may use the following values of physica_l constants wherevernecessary

mirRl �:

c = 3 x 108 ms-1

h = 6-626 X 10- 34 Jg

e = 1-602 x 10-19 C

µ0

= 47t x 10-7 T Ill A-1

-1- = 9 x 109 N�2 c-2 41tE

Mass of electron me = 9· l x 10-31 kg

Mass of neutron mn

= 1-675 x 10-27 kg

Boltzmann's constant k = 1381 x 10-23 J K -1

Avogadro's number NA= �-022 x 1023 moJ-1

Radius of earth = 6400 km

(i) wft JfR � f I

(ii) � JTR-Tff if jiff 30 ffl f I JfR 1 8 8 ff ffl ffl � 3fcn <fir �, ffl

9 # 18 ff ffl ffl � 3icfi cfif �. ffl 19 'ft 27 if ffl 'JfH 'ffR Jtcfi cfif i

n2lT 'JfH 28 # 30 -if ffl fl Tlhf 3fq; <fir f I

(iii) · m-w if 'fl1T!T wit-{ FcriWl -;rit. � 1 'ff'lllfrr, GT" Jiqff qfff � 'JfH #, m;:r3T<fff � r,:c,; m if 3th rffel 3fqif qfff 'fft:ff m # 3/RfR.qi 'f[lR 'JfGF{ 1wn7T71T f I # m if 31To/fit fc:it rrif rflR if 'ff � f!;<1i 'JfH eft 'qiRf i I

2

(iv) qjflj&2< it � ctr 3lp1frr :,@" � I

(vJ � 3fFWFli -ff} a,rrr. Rl.f@roid 'lllfrrcli f.rlrrfiqff � � cfif � <tir mI:

c = 3 x 108 ms -l h = 6-626 X 10- 34 Jg

e = 1-602 x 10-19 C

µ0

= 4n x 10-7 T mA-1

-1- = 9 x 109 Nm2 c-24m:� cbl � me = 9-1 x 10-31 kg � 9it Wffi � = 1-675 X 10-27 kg

ii()<-2;i1'11-i � k = 1·381 x 10-23 J K-1'

� c@' � NA == 6·022 x 1023 mol-1

�:<b1�==6400km

1. Name the physical quantity _whose S.I. writ ·is J c-1. Is it a scalar or avector quantity ? 1

2.

'3'B" - � qjf '-i'fl1 � � �.a:nt. CS.I.) � J c-1 t I � � mu�1{T�?

A beam of a. particles projected along + x-axis, experiences a force dueto a magnetic field along the + y-axis. What is the direction of themagnetic field ? 1

3

3.

4.

5.

6.

7.

+ x-� iii � � � q;un iii ll<fi fcwl-� "CR, frnm � � iii cfiR11T,

�- ifR, + y-3W iii � :R1@T t I � e}f <h1 � <FiT t ?

Define self-inductan�e of a coil. Write its S.I. units. fcnm � iii �-� <tl- ffifEIT � l � �-�- (S.I.) lff:fq; � I

A converging· lens is kept coaxially in contact with a. diverging lens -both the lenses being of equal focal lengths. What is the focal length of the combination ? ll<fi affemm�q;) ltcJi amnu�ili���WTTrtrf l-ey;TTmllcfil -�-� WlH t I�� <tl- � � � �

Define ionisation energy. What is its value for a hydrogen atom?m � <h1 � � I \;;1�*1\lH � iii � � lJH fcticRT mot W ?

Two conducting wires X and Y of same df ameter but different materialsare joined in series across a battery. If the number density of electronsin X is twice that in Y, find the ratio of drift velocity of electrons in thetwo wires.WlR o!ffif, � 1,r,r-� � iii � qffiqi ijR x am Y, � -q � m �� � , � x it ��¾,.fi qjf � tRc'q Y � � t m � ?J om � ��ct-<;Wiili�WIT<fif3qq@��I

Name the part of electromagnetic spectrum whose wavelength lies inthe range of 10-10 rn. Give its one use.�-� � iii 3lt � <liT 1llf �. � � 10-10 m <h1 q;)flcfi1mm�1����I

1

1

1

1

1

4

: 8.

9.

When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a decrease in the energy carried by the light wave ? Justify your answer. � "5fcfim f<fIB � wr-:i· � � � t or � "q@" qjij m � t 1 cf<l"T "q@" � qjtft c!if -qfturrq � cR1TI � � �, -q qilU t ? am � cfil" � �

Deduce the expression fq_r the magnetic dipole moment of an electron orbiting aro�d the central nucleus. � � c1i1 � � � M � t � fu?¥ � t � �

��(�)�I 10. A spherical conducting shell of inner radius r 1 and outer radius i:2 has

a charge 'Q'. A charge 'q' is placed at the centre of the s�ell. (a) What is the surface charge density on the (i) inner surface, (ii) outer surface of the shel l ?(b) Write the expression for the electric field at a point x > r2 from

1

2

the centre of the shell. 2

Flim � � � � <lil ,furn � r1 am:� � r2 i I � � 'Q'3ffl t I � ciiM ifi � 1l( 'q' � WT t I (a) � 3ffl q;:[rq cp:ir '@TT (i) cnm t ti � '91:, (ii) cliTW t � T8"

-q<: ? (b) iITTf ii,- � � X > r2 � Wffi � . � 1R �a);, if; ffil!: �

�I

11. Draw a sketch of a plane electromagnetic wave propagating along thez-direction. Depict clearly the directions of electric and magnetic fieldsvarying sinusoidally with z.

z-3W t � � W � WTT@ �� 'fR1T qi[���z t m ;wi1q�1l! � � � � � � � � cn1 �au it �

�

2

5

12. Show that the electric field at the surface of a charged conductor is---t

given by E = ; � , where cr is the surface charge density and � is a

unit vector normal to the surface in the outward direction.

� � � M � � t 1fCi3 1R ��, E = ; n mm t. � er..

0

� 3lTW � t 3lR n , 1fCi3 t � � � � 1l � llB[c6 � t I

13. Two identical loops, o:ne of copper and the other of aluminium, arerotated with the. same angular speed in the same magnetic field.Compare (i) the induced emf and. (ii) the current produced in the twocoils. Justify your answer.

Gt wmtJ � (�) -�. � � � qiT am � ��ltlf.w� cfiT �. � "ITT

� � ll, tftlH � � "ij � � � I � � � 1l �en�-� cffiT ct�."Q;li.) am <in� 1:lm3U- � � � , am 3m �ffe� I

14. An a-particle and a proton are accelerated from rest by the samepotential. Find the ratio of their de Broglie wavelengths.

� � (a.) cfiUT am "t:;cfi miR qi\" fcRrif � "ij, "Wl'R � mr � 1wnlfZl'T � I 6'{cbl � � n<•i�Ei-0 cfi"f. � � � I

15. Write two factors justifying the need of modulating a signal.A carrier wave of peak voltage 12 V is used to transmit a · messagesignal. What should be the· peak voltage of the modulating signal in

2

2

2

order to have a modulation index of 75% ? 2

fur.wn q;) � ciiB � ,6{(,wF'ha, cfiT am Gffif.t � � GT CfiR01 � 112 V fir@: � � � otir <!if � � � fur.@ t � t � M1M t I�� 75% � � � � fur.@ � f<mR � fcncR'r m-;fi�")

6

16. Write Einstein's photoelectric equation. State ·clearly· .the three salient ·features observed in photoelectric effect, -which can be explained on thebasis of the above equation. 2 �IFRl$-i �-� t14l<ti{OI <fit � l �-fclWl ™ ll � Tf1l: 6'f � �M q;t � � FiR'c!iT � w1"1qj{u1 � oqJ@TI ctr \l{f mm �-1

17. Draw a plot of potential energy of a pair of nucleons as a function oftheir separation. Write two important conclusions which you can drawregarding the nature of nuclear forces. 2

OR

Draw a plot of the binding energy per nucleon as a function of mass number for a large number of nuclei, 2 s; A s; 240. How do you explain the constancy of binding energy per nucleon in the range 30 < A < 170 using the property that nuclear force is short-ranged ? 2 �31RT � � � ch1 � � cfiT � � � amr cl,l � i tfi"ffi (�) � � ll � I m � G@T ch1 � � � � 31llT � � Ji�=tq'{uf �f.rcn@W@i, ��� l

3lP.fc11

� cfil" � � � 2 �A� 240 t � 11fd �31R aitH � ifil � �. � � m§!lT t � t � � � I � � iii �-'Tlffi t TT (�) ifil om, 3lN 30 <A< 170 iii l{ftm it, Jffu �an;, ffl � chl � <li1 oqJ@TI <tiG iii � � � ?

18. (i) Identify the logic gates marked P and Qin the given logic circuit.

(ii) Write down the output at X for the inputs A = 0, B = 0 andA= 1, B = i. 2

·(i) � � ocfi (�) llITT� T{ p 3TI\ Q 3Wli(f ocfi � (� lfcT) <tt ��I

X

(ii) � A = 0, B = 0 3W: A = 1, B = 1 iii � X 'CJl; f.,tfu � 1

7

19. Which mode of propagation is used by short wave broadcast serviceshaving frequency range from a few MHz upto 30 MHz ? Explaindiagrammatically h_ow long distance communication can be achieved bythis mode. Why is there an upper limit to frequency of waves used in thismode? 3 � MHz � 30 MHz oqi - "tJftm

1

911, �-"ffilT � wuan t �' �

�fcffel'cfiTffl'l'f f<n<lT�t? ·chl�-�m-�fc!i�fq-ftl� � � fern Jlqjf{ '5IJl'.<f mm t , � fcffu � m ij, � c1i1 • ctt �� � cf<U � t

20. Write any two factors on which internal resistance of a cell depends.The reading on a high resistance voltmeter, when a cell is connectedacross it, is 2·2 V

_. _When the terminals of the cell are also connected to

a resistance of 5 Q as shown ip the circuit, the voltmeter reading dropsto 1 ·8 V. Find the internal resistance of the cell.

��cfiRcli�m�fclim�cfif��f.rinqi{ffit 1�� � t q1�vnc:< qif � �- � � 1R � � 2·2 V � 1 � m, t�- qi) � 5 n t � � ,u � wrr � t � � it � lfllT �. m<11�4k< cfiT � � � 1 :8 V m � � � C6r 31RITT<li � � �

8

21. A network of four capacitors each of 12 µF capacitance is connected toa 500 V supply as shown in the figure. Determine (a) equivalentcapacitance of the network and (b) charge on each capacitor. 3

c2

c rlc 1 - 3

---- 500 V ---

� � it -� � (�) it 500 V it; � � Wet � � � � �

� � "ll<lT t I * ffl � <bl 1:1TftlT 12 µF t oT ffl � (a) �

<bl � � � (b) ffl � °Q\ 3lITTl I

c2

c rlc 1 3

--- 500V ----

Draw a neat labelled ray diagram of an astronomical telescope in normal adjustment. Explain briefly its working.

(ii) An astronomical telescope uses two lenses of powers 10 D and1 D. What is its magnifying power in normal adjustment? 3

OR

(i) Draw a neat labelled ray diagram of a compound microscope.Explain briefly its working.

(ii) Why must both the objective and the eye-piece of a compoundmicroscope have short focal lengths ? 3

9

(i) .mtlP-l �q[�'11'1 -q � � � c!if � � � atfus

�l��cn)tja}qit��I

(ii) � � � � � � c!il" 5ffl 10 D am: 1 D � I WiR B�l�\11-ill � awN1 � � t ?

(i) � � � � � � � 300s � I � � cfil�ll��I

(ii) ���zyumrr-��--<hlN�cfilfcFll

��?

23. In Young's double slit experiment, the two slits 0· 15 mm apart areilluminated by monochromatic light of wavelength 450 nm. The screenis l ·O m away from the slits.

(a) Find the distance of the second (i) bright fringe, (ii) dark fringefrom the central maximum.

(b) How will the fringe pattern change if the screen is inoved awayfrom the slits ? 3

� � fu-wfr m -q, -ey;n � t � cfiT � 0· 15- mm t 3lR ef 450 nm

� t � � "ij ffi fcfi'lrr 1l<TT t 1 � � � 1-0 rn � t

(a) m � � (i) � � fiii-ir, {ii) � � � cfil � mo

�

cb) <TR � q;) � � ¥ � m �. m � i -4z-;f 11 q<Jf l!furcR 'ITTlJf?

10

24. State Kirchhoff's rules. Use these rules to write the expressions for thecurrents I

l' 1

2 and I

3 in the circuit. diagram shown.

� t ��I "f-, � t � l ���an�'-�'tffifaTI Ii, 12

afu 13

ii; � � � I

3

·� 25. (a) Write symbolically the p- decay process of �!P.(b)

(a)

(b)

Derive an expression for the average life of a radionuclide. Give its relationship with the half-life. 3 32

· ·

15 P ii, p- � � qiJ Slffi<hli;i-lc!i � Tf � I

fcfim 'tfu;q1�fcR.q '1_cR1!�s qi1 � 3Wj t � � � ('51'Jl'.<f) � I � �-� -« � �

11

26. How does an nnpolarised light get polarised when passed through apolaroid ?Two polaroids are set in crossed positions. A third polaroid is placedbetween the two making an angle e with the pass axis of the firstpolaroid. Write the expression for the intensity of light · transmittedfrom the second polaroid. In what orientations will the transmittedintensity be (i) minimum and (ii) maximum ?� 41ci<1�s "ij" � � -« � "Sfc6m qlf � � m mrr � ?� � � � # � I � � � 1% ffl Cflcl<l�-s � "5!qif{ � �

� fifi � � � � "CJlful" � -« e q;)ur � 1 � tnci<1�s -« ttWfu"5[tfiffi cfil 'ffisrfil i fuQ: � � I � � � 'CfR'fu � � 'ffls;fnfCi) � � cu) � m ?

27. An illuminated object and a screen are placed 90 cm apart. Determinethe focal length and nature of the lens required to produce a clear

3

imag� on the screen, twice the size of the object. 3

� ffl cIBJ 311'{ � i � 90 cm <bl � t I d11 � ch1 -q;mr � am �WI� =m � qlf m am� (a:irrrq) � � "ij" � � � � 'G\GfrlT � I

28. (a) With the help of a diagram, explain the principle and working ofa moving coil galvanometer.

(b) What is the importance of a radial magnetic field and how is it •- '·. produced? (c) Why is it that while using a moving coil galvan�meter as a voltmeter a high resistance in series is required whereas in an ammeter a shunt is used ? 5

OR

(a) Derive an expression for the force between two long parallelcurrent carrying conductors. (b) Use this expression to define S.I. unit of current.

12

(c) A long straight wir� AB carries a current I. A proton P travelswith a speed v, parallel to the wire, at a distance d from it in a

(a)

(b)

(c)

(a)

(b)

(c)

direction opposite to the current as shown in the figure. What isthe force experienced by the proton and what is its direction ? 5

� 3l'Rls � Wfll'oT � � � ��.n412< ��am� 91T ��I���cfil��tam�Fcntr"SlcfiR��\lJ@Tt? m, � ��'11xt72{ qi). �R412{ ChT tjr@ � cfiB � � � � �

· � � <FR WIRT ml t � � � � � cfil � � t ?�

-ey � � � � � � ffl � � � � � oljm�I� � 'lif 3"Cl"llflr����-�- (S.1.) liITTi cfit � � iti��I� � tlW "flR AB�� I� m ® t IM lf � lll; � �mm P, oR t �. � mu <fil � cm mm; ft:ffl �', oR � d � lR, v�-« lffif c!i{fil ! I miR 1R WR cf@1 � fc!itRr t 3TI"< � � �t ?

13

29. State Faraday's law of electromagnetic induction.

Figure shows a rectangular conductor PQRS in which the conductor

PQ is free to move in a uniform magnetic field B perpendicular to

the plane of the paper. The field extends from x = 0 to x = b and is

zero for x > b. Assume that only the arm PQ possesses resistance r.

When the arm PQ is pulled outward from x · =· 0 to x = 2b and is

then. moved backward to x = � with constant speed v, obtain the

expressions for the flux and the induced emf. Sketch the variations of

these quantities with distance 0_ � x s 2b .

OR

Draw a schematic diagram of a step-up transformer. Explain its

working principle. Deduce the expression for the secondary to primary

voltage in terms of the number of turns in the two coils. In an

ideal transformer, how is this ratio related to -the currents in the two

coils ?

How is the transformer used in large scale transmission and

distribution of electric.al energy over long distances ?

14

:ao.

WcfiT�-�?ttulcfiT���

� � 3Tiffl "ij � 6\lllolcfiR q@cfi PQRS � l1m �. � � PQ, �� t � t � M l{cfiB&fl'1 � � B 'q 11@" qiB t � � t l��x=0�x=b(f<ii�t31R�?t"Rx>b"CR�t I

<

-qr., � fcli - � � � PQ cfiT m -� 31R � r t I � � PQ qi)x = O � x = 2b 'ffcfi � ciit am � � � 31R m � � v � x = 0 'ffcfi1fil§ Cfil am rornq;141 � t m l@<ffi � � ftrwl-cfffiqi 6@ t � <${qjm"'<f � I 0�x�2b � t ffl "f,f � t � t � � (�) �I

� M � ?iMi14< cfiT o1.1c1�1($i&; � cm) � 1 � � � qi) � � I ft:ffl 31R � � cfil � ij � t � l1;cfi �.-r-r � 'ij � c1i1 man t � � >l1tij -� , � � ?i{14i14< 'ij, � �zyff � ti fclWl muan � � ){cfiT( �- t ?

� ffi 1R, � � cftfi � � iti � 31R � ti ?iBlfilil-0 qi{ ����i?

31. (a)

(b)

Draw the circuit diagrams of a p-n junction diode in (i) forward bias, (ii) reverse bias. How are these circuits used to study the V - I characteristics of a silicon diode ? Draw the typical V - I characteristics.

What is a light emitting diode (LED) ? Mention two important advantages of LEDs over conventional lamps. 5

OR

15

(a) Draw the circuit · arrangement for stusiying the input andoutput characteristics of an n-p-n transistor in CE configuration.With the help of these characteristics define (i) input resistance,(ii) current amplification factor.

s

(b) Describe briefly with the help of a circuit diagram how an n-p-ntransistor is used to produce self-sustained oscillations.

(a) p-n � � clif � � (i) :,i!:lfef�lcfi atMt afu (ii) q�'clf�focfi � -q� I � tiftqm clii om � sfllts t V - I � it �t��M�t ?��V-1��1·

(b) ·� � ffls (LED) cp;[T t ? � � � � q,1 � � LED

5

it � llt!'tq'iuJ � c1ii � � I

31mn

(a) ,:!''P-IRll:h� (CE) � � \(qi n-p-n � t � � f.rra�t�t���clif��l��<fil � � (i) f.ml � afu (ii) "TIU � � � fflffft � I

(b) � � ams � � � � � cfUR � � � n-p-n � c!iT�. � 51fctq1fcld � � � � � � '5£cnR ��I

16