Q1.Amagnetismovingtowardsacoilwithauniformspeedasshowninthe Figure. State Thedirection

SAMPLEQUESTION PAPER PHYSICS(042)

CLASS-XII–(2012-13)

Q1.Amagnetismovingtowardsacoilwithauniformspeedasshowninthe figure. State thedirection of the induced current inthe resistorR. 1

Q2.Asquarecoil,OPQR,ofsidea,carryingacurrentI,isplacedintheY-Zplaneas shownhere. Find themagnetic momentassociated with this coil. 1

Q3.Give one example each ofa ‘system’ that uses the

(i)Skywave(ii)Space wave

modeof propagation1

Q4.Aconcavemirror,ofaperture4cm,hasapointobjectplacedonitsprincipalaxis atadistanceof10cmfromthemirror.Theimage,formedbythemirror,isnot likely tobe a sharp image. State thelikelyreason for the same. 1

Q5.Twodipoles,madefromchargesqandQ,respectively,haveequaldipole moments.Give the(i)ratiobetweenthe‘separations’ofthethesetwopairsof

charges (ii)angle between the dipole axisof these twodipoles.1

Q6.Thegraph,shownhere,representstheV-Icharacteristicsofadevice.Identifythe region, ifany, overwhich thisdevice hasa negativeresistance. 1

Q7.Define the term ‘Transducer’ fora communication system.1

Q8.State the steadyvalueof the reading of the ammeterinthe circuit shownbelow.

Q9.The following table gives data about the single slit diffraction experiment:

Wave length of Light / HalfAngularwidthof the principal maxima
p / q

Findtheratio of thewidths of theslitsusedinthetwocases.Wouldtheratio of thehalfangularwidthsofthefirstsecondarymaxima,inthetwocases,bealso equaltoq? 2

Q10.N spherical droplets, eachof radiusr, have been charged tohaveapotential V each. If all these droplets were tocoalesce toform a single large drop, what would be the potential of this largedrop?

(Itisgiven that the capacitance of a sphere of radiusx equals4kx.)

Two point charges, q1 andq2,are located at points (a, o, o) and (o, b, o)

respectively. Find the electric field, due to both these charges, at the point,

(o,o, c).2

Q11.When a given photosensitive material is irradiated with light of frequency, the maximum speed of the emitted photoelectronsequals max.Thesquareof max, i.e., max2,isobservedtovarywith ,asper

the graph shownhere.Obtain expres

(i)Planck’s constant, and

(ii)The workfunction of the given photosensitive material,

intermsof the parametersn and the mass,m, of the electron.

Q12.Forthe circuit shownhere, wouldthe balancing length increase,2 decrease or remainthesame, if

(i)R1 isdecreased

(ii)R2 isincreased

withoutanyotherchange,(ineachcase)intherest of the circuit.Justify youranswersineach case.

Q13.FindtheP.E.associatedwithacharge‘q’ifitwerepresentatthepointPwith respect to the ‘set-up’of twocharged spheres, arranged as shown. Here Oisthe

mid-point of the line O1 O2.2

Q14.Anathletepeddlesastationarytricyclewhosepedalsareattachedtoacoil having100turnseachofarea0.1m2.Thecoil,lyingintheX-Yplane,isrotated, inthisplane,attherate of50rpm,abouttheY-axis,inaregionwherea uniform magnetic field, =(0.01) tesla, ispresent. Find the

(i)maximum emf(ii)average e.m.f

generated inthe coil over one completerevolution.2

Q15.Amonochromaticsource,emittinglightofwavelength,600nm,hasapower output of 66W. Calculate the number of photons emitted by this source in

2minutes.2

Q16.Forthe circuit shownhere, findthe current2 flowing through the 1 resistor. Assume

that the twodiodes, D1andD2,are ideal diodes.

Q17.The galvanometer, in each of the two given circuits, does not show any deflection. Find the ratio of the resistors R1 and R2, used in these two circuits. 3

Q18.Theelectron,inahydrogenatom,initiallyinastateofquantumnumbern1 makesa transition to a state whose excitation energy, with respect to the groundstate,is10.2eV.Ifthewavelength,associatedwiththephotonemitted inthistransition, is487.5 mm, find the

(i)energyinev,and(ii)valueofthequantumnumber,n1 oftheelectroninits initial state. 3

Q19.ThreeidenticalpolaroidsheetsP1,P2,andP3 areorientedsothatthe(pass)axis ofP2 andP3 areinclinedatanglesof600 and900,respectively,withrespectto the(pass)axisofP1.Amonochromaticsource,S,ofintensityI0,iskeptinfront of the polaroid sheet P1.Find the intensity of this light, as observed by observers O1,O2,andO3,positioned as shown below.

Q20.Afinepencilof-particles,movingwithaspeed,entersaregion(regionI), whereauniformelectricandauniformmagneticfieldarebothpresent.These

-particlesthenmoveintoregionIIwhereonlythemagneticfield,(outofthe

twofieldspresentinregionI),exists.Thepathofthe-particles,inthetwo regions, isas shown in the figure.

(i)State the directionof the magnetic field.

(ii)State the relation between ‘E’ and‘B’ inregion I.

(iii)Drivetheexpressionfortheradius of thecircularpathofthe-particlein region II.

Ifthemagnitudeofmagnetic field, inregionII,ischanged to n times its earlier value, (without changing the magnetic field in region I)

findthe factorby which the radiusof this circularpathwould change. 3

Q21.Drawanappropriate raydiagramtoshowthepassageofa‘whiteray’,incident ononeofthetworefractingfacesofaprism.Statetherelationfortheangleof deviation, foraprism of small refractingangle.

Itisknownthattherefractiveindex,,ofthematerialofaprism,dependson the wavelength , , of the incident radiation asper the relation

=A +

whereAandBareconstants.Plotagraphshowingthedependenceofon andidentifythepairofvariables,thatcanbeusedhere,togetastraightline graph. 3

Q22.Definetheterms(i)massdefect(ii)bindingenergyforanucleusandstatethe relationbetween the two.

ForagivennuclearreactiontheB.E./nucleonoftheproductnucleus/nucleiis more than that for the original nucleus/nuclei. Is this nuclear reaction exothermicorendothermicinnature? Justify yourchoice.

(a)Thenumberofnuclei,ofagivenradioactivenucleus,attimest=0andt=T, areN0 and(N0/n)respectively.Obtainanexpressionforthehalflife(T1/2)of thisnucleusintermsof nandT.

(b)Identifythenatureofthe‘radioactiveradiations’,emittedineachstepofthe

‘decaychain’ given below:3

A-4A-4A-4

AXYYW

ZZ-2Z-2Z-1

Q23.Drawthe waveformsfor

The (i) InputAMwave atA, (ii)output, B, of the rectifierand (iii) outputsignal, C, of the envelope detector. 3

Q24.ThecapacitorsC1,and C2,havingplatesofareaAeach,areconnectedinseries, asshown.CamparethecapacitanceofthiscombinationwiththecapacitorC3,

againhavingplatesof area A each, but‘made up’ as showninthe figure.3

3
Q25 / (a) / Writetheformulaforthevelocityoflightinamaterialmediumofrelative permittivityrandrelative magneticpermeability r. 1
(b) / Thefollowingtablegivesthewavelengthrangeofsomeconstituentsof the electromagnetic spectrum.
S.No. / Wavelength Range
1. / 1mm to700nm
2. / 0.1mto1mm
3. / 400nmto1nm
4. / 103 nm

Selectthewavelengthrange,andnamethe(associated)electromagneticwaves, thatare used in

(i)Radarsystems forAircraftnavigation

(ii) Earth satellitestoobserve growthof crops.2

Q26. Suhasini’s uncle,was advised by his doctor to have an MRIscan of his chest. Heruncledid not know much about thedetailsand significanceofthistest.Healsofeltthatitwastoo expensiveand

thought ofpostponingit.

WhenSuhasinilearntaboutheruncle’sproblems,sheimmediatelydecidedtodosomething aboutit. Shetookthehelpofherfamily,friendsandneighborsandarrangedforthecostofthetest. Shealsotoldherunclethat anMRI(MagneticResonance Imaging)scanofhischestwouldenablethedoctorstoknowoftheconditionofhisheartandlungswithoutcausingany(test related)harm tohim. This testwasexpensivebecauseofits setupthatneededstrongmagneticfields(0.5T to 3T)andpulsesof radio wave energy.

HerunclewasconvincedandhadtherequiredMRIscanofhischestdone.The resulting informationgreatlyhelpedhisdoctorstotreathimwell.

(a)Whataccordingtoyou,arethevaluesdisplayedbySuhasiniandher(2)

family, friends andneighbourstohelpheruncle

(b)AssumingthattheMRIscanofheruncle’schestwasdonebyusinga magneticfieldof1.0T,findthemaximumandminimumvaluesofforce thatthismagneticfield could exertonaproton(charge=1.6x10–19)that wasmovingwithaspeedof104 m/s.Statetheconditionunderwhichthe force hasits minimum value. 2

Q27.A conducting rod XY slides freely on two parallel rails, A and B, with a

uniformvelocity‘V’.Agalvanometer‘G’ is connected,asshowninthefigureand theclosedcircuithasatotalresistance‘R’. Auniform magneticfield,perpendicular totheplanedefinedbytherailsAandB and the rod XY (which are mutually perpendicular),ispresentovertheregion, as shown.

(a)With keykopen:

(i)Find the nature of charges developed atthe endsof the rod XY.

(ii)Whydotheelectrons,intherodXY,(finally)experiencenonetforce eventhroughthemagneticforceisactingonthemduetothemotion

of the rod?

(b)Howmuchpowerneedstobedelivered,(byanexternalagency),tokeep the rod movingatitsuniform speed whenkeykis(i) closed (ii) open?

(c)Withkeykclosed,howmuchpowergetsdissipatedasheatinthecircuit?

State the source of thispower.

‘ Box’ A, in the set up shown below, represents an electric device often

used/needed tosupply, electricpowerfrom the(ac)mains, toa load.

It isknownthat Vo Vi.

(a)Identify the device A anddrawits symbol.

(b)Drawaschematicdiagramofthiselectricdevice.Explainitsprincipleand working.Obtainanexpressionfortheratiobetweenitsoutputandinput voltages.

(c)Findtherelationbetweentheinputandoutputcurrentsofthisdevice assuming it tobe ideal. 5

Q28.Definetheterms‘depletionlayer’and‘barrierpotential’foraP-Njunction diode. Howdoesan increase in thedoping concentration affect thewidth of the depletion region?

Drawthe circuit of a full wave rectifier. Explain its working.

Whyis the base region of a transistorkept thin andlightly doped?

Drawthecircuitdiagramofthe‘set-up’usedtostudythecharacteristicsofa npntransistorinitscommonemitterconfiguration.Sketchthetypical(i)Input characteristicsand (ii) Outputcharacteristics for thistransistorconfiguration.

Howcanthe outputcharacteristicsbeusedtocalculatethe‘current gain’of the

transistor?

Q29.(i)Athinlens,havingtwosurfacesofradiiofcurvaturer1 andr2,made fromamaterialofrefractiveindex ,iskeptinamediumofrefractive index . Derive the LensMaker’sformula forthis‘set-up’

(ii)Aconvexlensisplacedoveraplanemirror.Apinisnowpositionedso thatthereis noparallaxbetweenthepinanditsimageformedbythis lens-mirrorcombination. Howcanthisobservationbeusedtofindthe focallengthoftheconvexlens?Give appropriatereasonsinsupportof youranswer.

Thefigure,drawnhere,showsamodifiedYoung’sdoubleslitexperimentalset

up. If SS2SS1,= /4,

(i)state the condition for constructive and destructiveinterference

(ii)obtain an expression for the fringe width.

(iii) locate the position of the central fringe.5

MARKINGSCHEME

Q.No. / Valuepoint/ expectedpoints / Marks / Total
1. / From XtoY / 1 / 1
2. / Themagneticmoment,associatedwiththe
coil, is
m=Ia2 / 1 / 1
3. / (i) Shortwave broadcastservices
(ii) Television broadcast (ormicrowave linksorSatellite communication) / ½
½ / 1
4. / The incident rays are not likely to be
paraxial. / 1 / 1
5. / Asqa = Qa’, we have
=
and=0o / ½
½ / 1
6. / Region BC / 1 / 1
7. / A‘transducer’isanydevicethatconverts
one form of energyintoanother / 1 / 1
8. / Zero / 1 / 1
9. / Letdandd’ bethewidthoftheslitsinthe
two cases.
andq =
=
Yes,thisratio would alsoequalq / ½+½
½
½ / 2
10. / Total (initial) charge on all the droplets
=N x (40krV) AlsoN xr3 =R3
R=r
IfV’isthepotentialofthelargedrop,we
have
4R xV’ =N x 4krxV
V’=V =V
OR We havenet =1+2
=1 +2
where 1=-a +c and 2 =-b +c
net = / ½
½
½
½
½
½
1 / 2
2
11. / AccordingtoEinestein’sEquation:
Kmax=mmax=–
Thisistheequationofastraightlinehaving aslope2h/mandanintercept(onthemax
axis)ofComparingthese,withthe given graph, we get
=orh =and=or= / ½
½
½+½ / 2
12. / (i) decreases
(The potential gradient would increase) / ½+½
(ii)increases
(The terminal p.d across the cell would increase) / ½+½
13. / r1=O,P=
r2=O2P= V =
P.E of charge , q, atP=qV
= / ½
½
½
½ / 2
14. / (i)Themaximumemf‘ ’generatedinthe
coil is,
=N B A
=N B A 2 f
=[100 x0.01 x0.1 x2] V
=V0.52 V
(ii)Theaverageemfgeneratedinthecoil overone complete revolution =0 / ½
1
½ / 2
15. / Energy ofone photon=E = E =
3.3x10-19J
E1 =energyemittedbythesourceinone second = 66J
numberofphotonsemittedbythesource in1s=n = =2 x1020
Total number of photons emitted by / ½
½
½
source in 2 minutes
=N = n x 2 x60
=2 x1020x120=2.4 x1022photons / ½ / 2
16. / DiodeD1 isforwardbiasedwhileDiodeD2
isreverse biased
Hencetheresistances,of(ideal)diodes,D1 and D2, canbetakenaszeroandinfinity, respectively.
Thegivencircuitcan,therefore,beredrawn as shownin the figure.
Using ohm’s law,
I=A =2A
current flowinginthe 1resistor, is 2A. / ½
½
½
½ / 2
17. / Forcircuit 1, we have,(from theWheatstone
bridge balance condition),
R1 =6
Incircuit2,theinterchangeofthepositions ofthe batteryandthegalvanometer,does notchangethe(wheatstoneBridge)balance condition.
= / ½
½
½
½
orR2 =4
== / ½
½ / 3
18. / In a hydrogen atom, the energy (En) of
electron,inastate,havingprincipal
quantumnumber‘n’, isgiven by
En =eV
E1 =-13.6eV andE2 =-3.4 eV
Itfollowsthatthestaten=2hasan excitation energy of 10.2 eV. Hence the electronismakingatransitionfromn=n1 to n=2 where (n12).
NowEn1 –E2 = But
En1=(-3.4 +2.55)eV
≃-0.85 eV
Butwe alsohave En1 =eV
we getn1 =4 / ½
½
½
½
½
½ / 3
19. / Intensity observed by
(i)ObserverO1=
(ii)ObserverO2=cos2 60o
=
(iii) ObserverO3=cos2 (90o-60o) / ½
½
½
1 / 3

152

=x= / ½ / 3
20. / (i) The magnetic field is perpendicular to
the plane of page and isdirected inwards
(ii)In region I
=
qE = q B E =B
(iii)In region II
=q Br=
Substituting the valueof, we get r=
LetB’(=nB)denotethenewmagneticfield inregionII.Ifr’istheradiusofthecircular path now, we have
r1==
Hence radius of the circular path, would decrease by a factorn. / ½
½
½
½
½
½ / 3
21. / See (fig 9.25, Page332PartIINCERT)
Forasmallangledprism, of refractingangle
:
Angleofdeviation=whereis therefractiveindexofthematerialofthe prism. / 1
½
Togetastraightlinegraph,weneedtouse andasthe pair of variables. / 1
½ / 3
22. / (i)Massdefect( M),ofanynucleus AX,is
Z
the difference in the mass of the nucleus (=M)andthesumofmassesofits constituent nucleons(= M’).
M= M’–M
=[ Zmp+(A- Z) mn]–M
wheremp andmn denotethemassofthe proton and the neutron respectively.
(ii)Bindingenergyistheenergyrequiredto separateanucleusintoitsconstituent nucleons. The relation between the twois:
B.E.= (massdefect) c2
(iii) There is a release of energy i.e., the reaction isexothermic.
Reason: Increase in B.E/nucleon implies that more mass has been converted into energy. This would result in release of energy. / 1
½
½
½
½ / 3
OR
(a)Accordingtothe(exponential)lawof radioactive decay.
N= No
Given N=No/n and t=T
=No
orn =
=
half life==
2
(b)
(i)–rays (ii)–rays (iii)–rays / ½
½
½
½
½
½ / 3
23. / 1
1
1 / 3
24. / Wehave C1=
andC2 =
Ceq ==
Now, capacitor C3 can be considered as madeupoftwocapacitorsC1 andC2,each ofplateareaAandseparationd,connected in series.
Wehave : C1’=
andC2’=
C3 ==
=1
Hencenetcapacitanceofthecombinationis equalto that of C3. / ½
½
½
½
½
½ / 3
25.(a)
(b) / Wehave=
(i)Wavelength range : [0.1mto1mm] (Microwaves)
(ii) Wavelength range: [1mm to700 nm]
(Infrared waves) / ½+½
½+½
½+½ / 1
3
26. / (a)(i)Presence of mind
High degree of generalawareness Ability to take promptdecisions Concern forheruncle (Any two)
(ii) Empathy;Helpingandcaring / (½+½)
nature
(b) Maximum force =qVB
=1.6x10–19x104x1 N
=1.6x10–15N Minimum force =zero

Condition: whenVisparallel toB / (½+½)
½
½
½
½ / 4
27. / (a)
(i) X : negative , Y: positive
(ii)Magneticforce,Fm,experiencedbythe moving electrons, gets balanced by the electricforceduetotheelectricfield,caused bythechargesdevelopedattheendsofthe rod.Hencenetforceontheelectrons,inside the rod, (finally) become zero.
(b)Thepower,thatneeds tobedeliveredby the externalagency, whenkeykis closed, is
P=FmV= (I l B)V =.lBV
=B2l2V2/R
Whenkisopen,thereisaninducedemf, butno inducedcurrent.Hencepowerthat needstobe deliverediszero.
(c) Power, dissipated asheat
=i2 R=
Thesourceofthispoweristhemechanical workdone bythe externalagency.
OR / ½
1+½
½+½
½
½+½
½ / 5
(a)Step down transformer.
(b) Diagram Principle Working
Obtaining the expression
(c)Inputpower= outputpower
VpiP=Vsis
== / ½
½
½
½
½
2
½ / 5
28. / Thespacechargeregion,oneithersideof
thejunction(takentogether),isknownas the depletion layer.
Thep.dacrossthedepletionlayerisknown asthe barrierpotential
Thewidthofthedepletionregiondecreases withanincreaseinthedoping concentraction.
Thecircuitofafull-waverectifierisshown below. / ½
½
½
1½

158

Workingdetails
OR
Thebaseregionofatransistoristhinand lightlydopedsothatthebasecurrent(IB)is verysmall compared toemittercurrent (IE).
The current gainof a transistor in common emitterconfiguration is / 2
1
1
1
1 / 5
=
IC andIB canbeobtained,fromthetwo curves, inthe outputcharacteristics. / 1 / 5
29. / (i)Diagram
Derivation
(ii)Theraysmustfallnormallyonthe planemirrorsothattheimageof the pin coincideswithitself
Hence rays, like CA and DB, form a parallel beam incidenton the lens.
Pisthe position of the focusof the lens DistanceOPequalsthefocallengthof the lens
OR
=initialpathdifferentbetweenS1 andS2
=SS2 –SS1 =
= S2P–S1P = path difference between / 1
2½
½
½
½
1
½ / 5
disturbance from S1 and S2,atpointP
=
=Totalpathdifferencebetweenthetwo disturbances at P
=+=+
Forconstructive interference:
==n ; n = 0, 1, 2,….
=(n-)…(i)
Fordestructiveinterference
=(2n-1). . . (ii)
–
=fringe width =yn+1–yn =
ThepositionYo ofcentralfringeisobtained by puttingn=oinEqn (i). Therefore,
yo =-
[Negativesignshowsthatthecentralfringe is obtained at a point below the (central) point O.] / ½
1
1
1 / 5