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MaximumMarks:70

TimeAllowed:3hours

GeneralInstructions:

i. Allquestionsarecompulsory.Thereare37questionsinall.

ii. Thisquestionpaperhasfoursections:SectionA,SectionB,SectionCandSectionD.

iii. SectionAcontainstwentyquestionsofonemarkeach,SectionBcontainssevenquestions

oftwomarkseach,SectionCcontainssevenquestionsofthreemarkseach,andSectionD

containsthreequestionsoffivemarkseach.

iv. Thereisnooverallchoice.However,internalchoiceshavebeenprovidedintwo

questionsofonemarkeach,twoquestionsoftwomarks,onequestionofthreemarksand

threequestionsoffivemarksweightage.Youhavetoattemptonlyoneofthechoicesin

suchquestions.

SectionA

1. AparticleofmassmandchargeqisreleasedfromrestinauniformelectricfieldE.

Thekineticenergyattainedbytheparticleaftermovingadistancexis

a.

b.

c.

d.

2. Whichofthefollowingistrueaboutequipotentiallines.Electricfieldlinesare

a. paralleltoequipotentiallines

b. oppositetoequipotentiallines

c. tangentialtoequipotentiallines

d. perpendiculartoequipotentiallines

3. Potentiometermeasuresthepotentialdifferencemoreaccuratelythanavoltmeter,

CBSE Class 12 PhysicsSample Paper - 10 (2019-20)

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because

a. Ithasawireoflowresistance.

b. Itdrawsaheavycurrentfromexternalcircuit.

c. Itdoesnotdrawcurrentfromexternalcircuit.

d. Ithasawireofhighresistance.

4. TheequivalentresistanceoftworesistancesPandQwhichareinparallelis

a.

b.

c. P+Q

d.

5. Twoequalelectriccurrentsareflowingperpendiculartoeachotherasshowninthe

figure.ABandCDareperpendiculartoeachotherandsymmetricallyplacedwith

respecttothecurrents.Wheredoweexpecttheresultantmagneticfieldtobezero?

a. onAB

b. onbothABandCD

c. onbothODandBO

d. onCD

6. Abeamoflighthasawavelengthof650nminvacuum.Whatisthewavelengthof

thesewavesintheliquidwhoseindexofrefractionatthiswavelengthis1.47?

a. 452nm

b. 442nm

c. 472nm

d. 462nm

7. Lightofwavelength5000Åfallsonaplanereflectingsurface.Whatarethe

wavelengthandfrequencyofthereflectedlight?Forwhatangleofincidenceisthe

reflectedraynormaltotheincidentray?

a.

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b.

c.

d.

8. Whenlightispassedthroughaprism,thecolourwhichdeviatesleastis:

a. Red

b. Blue

c. Green

d. Violet<

9. Magnitudeoftheelectricandthemagneticfieldsinanplaneelectromagneticwave

arerelatedby

a. =

b. =

c. =

d. =

10. Wavelengthoflightincidentonaphotocellis3000 ,ifstoppingpotentialis2.5volts,

thenworkfunctionofthecathodeofphotocellis

a. 1.64eV

b. 1.41eV

c. 1.56eV

d. 1.52eV

11. Fillintheblanks:Themagneticforcerequiredtodemagnetizethematerialis________

12. Fillintheblanks:Thelinesjoiningtheplacesofequaldiporinclinationarecalled

________lines.

13. Fillintheblanks:

Faraday'slawgivesusthe________ofinducedemf.

OR

Fillintheblanks:Theelectricfieldscreatedbytime-varyingmagneticfields

havingnon-vanishingloopintegralsarecalled________fields.

14. Fillintheblanks:Densityofnucleusmatteristheratioof________ofnucleusandits

volume.

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15. Fillintheblanks:Theangleofincidencewhenarayoflightfallsnormallyona

mirroris________.

16. Writetheequationofdecayoftheradioactivenuclei.

17. Nametheabsorbingmaterialusedtocontrolthereactionrateofneutronsina

nuclearreactor.

18. Nameoneimpurityeach,whichwhenaddedtopureSi,produces

i. n-typeand

ii. p-typesemiconductor.

19. Writetheexpressionforthede-Brogliewavelengthassociatedwithachargedparticle

havingchargeqandmassm,whenitisacceleratedbyapotentialV.

20. Nameoneimpurityeach,whichwhenaddedtopureSi,produces

i. n-typeand

ii. p-typesemiconductor.

OR

Namethetypeofbiasingofap-njunctiondiodesothatthejunctionoffersveryhigh

resistance.

SectionB

21. UseKirchhoff'srulestodeterminethevalueofthecurrentI1flowinginthecircuit

showninthefigure.

22. Whyareelectricfieldlinesperpendicularatapointonanequipotentialsurfaceofa

conductor?

23. Aprotonandadeuteronareacceleratedthroughthesameacceleratingpotential.

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Whichoneofthetwohas

i. greatervalueofde-Brogliewavelengthassociatedwithitand

ii. lessmomentum?

Givereasonstojustifyyouranswer.

24. AcapacitorofcapacitanceCisbeingchargedbyconnectingitacrossaDCsource

alongwithanammeter.Willtheammetershowamomentarydeflectionduringthe

processofcharging?Ifso,howwouldyouexplainthismomentarydeflectionandthe

resultingcontinuityofcurrentinthecircuit?Writetheexpressionforthecurrent

insidethecapacitor.

25. Ashortbarmagnetplacedwithitsaxismakinganangle withauniformexternal

fieldBexperienceatorque.Whatisthemagneticmomentofthemagnet?

26. Findoutthewavelengthoftheelectronorbitinginthegroundstateofhydrogen

atom.

OR

Inthegroundstateofhydrogenatom,itsBohrradiusisgivenas5.3 10-11m.The

atomisexcitedsuchthattheradiusbecomes21.2 10-11m.Find(i)thevalueofthe

principalquantumnumberand(ii)thetotalenergyoftheatominthisexcitedstate.

27. Drawthecircuitdiagramofanilluminatedphotodiodeinreversebias.Howis

photodiodeusedtomeasurelightintensity?

OR

Namethedevice,Dwhichisusedasavoltageregulatorinthegivencircuitandgive

itssymbol.

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SectionC

28. Inameterbridge,thenullpointisfoundatadistanceof60cmfromA.Ifaresistance

of isconnectedinserieswithS,thennullpointoccursat50.0cmfromA.

DeterminethevaluesofRandS.

29. Drawalabelleddiagramofcyclotron.Explainitsworkingprinciple.Showthat

cyclotronfrequencyisindependentofthespeedandradiusoftheorbit.

30. Aninductor ,capacitor ,resistor areconnectedinserieswitha

100V,variablefrequencya.c.source.

Calculate:

i. frequencyatwhichthepowerfactorofthecircuitisunity

ii. currentamplitudeatthisfrequency

iii. Q-factor

31. AparallelplatecapacitormadeofcircularplateseachofradiusR=6.0cmhasa

capacitanceC=100pF.Thecapacitorisconnectedtoa230Vacsupplywitha

(angular)frequencyof300rads-1.

a. Whatisthermsvalueoftheconductioncurrent?

b. DeterminetheamplitudeofBatapoint3.0cmfromtheaxisbetweentheplates.

32. Howisresolvingpowerofamicroscopeaffectedwhen

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i. wavelengthofilluminatingradiationsisdecreased.

ii. thediameterofobjectivelensisdecreased?Justify.

OR

WhenoneoftheslitsinYoung'sexperimentiscoveredwithatransparentsheetof

thickness thecentralfringeshiftstoapositionoriginallyoccupiedby

the30thbrightfringe.If ,findtherefractiveindexofthesheet.

33. A12.5eVelectronbeamisusedtobombardgaseoushydrogenatroomtemperature.

Whatseriesofwavelengthswillbeemitted?

34. AZenerdiodeisfabricatedbyheavilydopingbothp-andn-sidesofthejunction.

Explain.

BrieflyexplaintheuseofZenerdiodeasaDCvoltageregulatorwiththehelpofa

circuitdiagram.

SectionD

35. a. UsingGauss'theorem,obtainanexpressionfortheelectricfieldintensityata

pointatadistancerfromaninfinitelylonguniformlychargedstraightwire.

b. AnelectricdipoleABconsistsofcharges±5nCandseparatedbyadistanceof2

10-3m[Fig].

Thedipoleisplacednearalonglinechargehavinglinearchargedensity4.5 10-4

Cm-1,suchthatthenegativechargeisatadistanceOA=2 5cmfromtheline

charge.Findtheforceactingonthedipole.

OR

TwoisolatedpointchargesAandBareseparatedbyadistanceof30 0cm,asshown

infig.

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ThechargeatAis+3 6 10-9C.ThevariationwithdistancexfromAalongABofthe

potentialVisasshowninfig.

i. Statethevalueofxatwhichthepotentialiszero.

ii. Useyouranswerin(i)todeterminethechargeatB.

iii. AsmalltestchargeisnowmovedalongthelineABfromx=5 0cmtox=27cm.

Stateandexplainthevalueofxatwhichtheforceonthetestchargewillbe

maximum.

36. Acurrentof10Aisflowinginalongstraightwiresituatedneararectangularcoil.

Thetwosides,ofthecoil,oflength0.2mareparalleltothewire.Oneofthemisata

distanceof0.05mandtheotherisatadistanceof0.10mfromthewire.Thewireisin

theplaneofthecoil.

i. Calculatethemagneticfluxthroughtherectangularcoil.

ii. Ifthecurrentuniformlytozeroin0.02s,findtheemfinducedinthecoiland

indicatethedirectioninwhichtheinducedcurrentflows.

OR

Ifapieceofmetalhasacharge andisplacedinsideahollowmetalsphereof

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radius20cm(withouttouchingit),whatisthepotentialofthesphere?Whatwillthe

potentialofthespherebecome,if

a. thesphereistemporarilyearthedandthenleftinsulated,

b. themetalsubsequentlytouchedtheinsideofthesphere?

37. a. Apointobjectisplacedontheprincipalaxisofaconvexsphericalsurfaceof

radiusofcurvatureR,whichseparatesthetwomediaofrefractiveindicesn1and

n2(n2>n1).Drawtheraydiagramanddeducetherelationshipbetweentheobject

distance(u),imagedistance(v)andtheradiusofcurvature(R)forrefractionto

takeplaceattheconvexsphericalsurfacefromrarertodensermedium.

b. Aconverginglenshasafocallengthof20cminair.Itismadeofamaterialof

refractiveindex1·6.Ifitisimmersedinaliquidofrefractiveindex1·3,findits

newfocallength.

OR

FigureshowsaconvexsphericalsurfacewithcentreofcurvatureCseparatingthe

twomediaofrefractiveindicesµ1andµ2.Drawaraydiagramshowingtheformation

oftheimageofapointobjectOlyingontheprincipalaxis.Derivetherelationship

betweentheobjectandimagedistanceintermsofrefractiveindicesofthemediaand

theradiusofcurvatureRofthesurface.

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Answer

SectionA

1. (c) Explanation:IfachargeqplacedinuniformelectricfieldEexperiences

force,F=qE,displacement=xW=ForceXdisplacementW=qEx

Accordingtowork-energytheorem,Networkdone=changeinKineticenergy,Hence

Kineticenergyattained=qEx.

2. (d)perpendiculartoequipotentiallines

Explanation:Thepotentialatallpointsonanequipotentialsurfaceisconstantand

workdoneinmovingachargeonanequipotentialsurfaceiszero.Electricfieldlines

showthedirectionoftheelectricfieldatthepoint.Iftheelectricfieldlineswere

tangential,paralleloroppositetotheequipotentialsurface,atangentialfieldwillexist

onthesurfaceandworkdoneinmovingachargeonthesurfaceisnotzero.

Thereforeelectricfieldlinesarealwaysperpendiculartotheequipotentialsurface.

3. (c)Itdoesnotdrawcurrentfromexternalcircuit.

Explanation:Potentiometermeasuresthepotentialdifferenceusingnulldeflection

method,wherenocurrentisdrawnfromthecell;whereasvoltmeterneedsasmall

currenttoshowdeflection.So,accuratemeasurementofp.disdoneusinga

potentiometer.

4. (b) Explanation:Thereciprocaloftheequivalentresistanceoftwo

resistancesconnectedinparallelisequaltothesumofthereciprocalsofthetwo

resistances.

5. (a)onAB

Explanation:onAB

6. (b)442nm

Explanation:

CBSE Class 12 PhysicsSample Paper - 10 (2019-20)

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sowavelengthis442nm.

7. (b) Explanation:Thereisnochangeinwavelengthoflight

inreflection.

use

8. (a)Red

Explanation:Refractingindexisgivenby, ,whereA&Bareconstant.

Wavelengthifredcolorismaximum,hencerefractiveindexofmaterialofprismfor

redcolorlightisminimumhenceredcolordeviatestheleast.

9. (a) =

Explanation: ,wherecisspeedoflight

10. (a)1.64eV

Explanation:Vo=2.5V,henceeVo=2.5eV

11. Coercivity

12. Isoclinical

13. Magnitude

OR

Non-conservative

14. Mass

15. Zerodegree

16.

17. Heavywater

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18. i. As(Arsenic)

ii. In(Indium)

19. de-Brogliewavelength,

Hint:W=K=qV= or

20. i. As(Arsenic)

ii. In(Indium)

OR

Reversebiasing.

SectionB

21. GustavKirchhoff’sCurrentLawisoneofthefundamentallawsusedforcircuit

analysis.Hiscurrentlawstatesthatforaparallelpaththetotalcurrententeringa

circuitjunctionisexactlyequaltothetotalcurrentleavingthesamejunction.Thisis

becauseithasnootherplacetogoasnochargeislost.

InotherwordsthealgebraicsumofALLthecurrentsenteringandleavingajunction

mustbeequaltozeroas:ΣIIN=ΣIOUT

Inclosed-loopABEFCDA,

-80+20I2-30I1=0

20I2-30I1=80...(i)

Inclosed-loopBEFCB,

-80+20I2-20+20I1=0

20I2+20I1=100...(ii)

OnsolvingEqs.(i)and(ii),weget

22. Electricfieldisalwaysnormaltotheequipotentialsurface,becausenoworkisdone

tomoveatestchargeq0ontheequipotentialsurface,

i.e.W=q0(VA-VB)=0 VA-VB=0

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23. i. de-Brogliewavelengthisgivenby [V0andqaresame,

becauseprotonanddeuteronhavebeenacceleratedbysamepotentialandhave

samecharge].

Since,massofprotonislessascomparedtoadeuteron.So,itwillhavehigher

valueofde-Brogliewavelengthassociatedwithit.

ii. de-Brogliewavelengthisgivenby

As,

So,

24. Yes,theammeterwillshowthemomentarydeflection.

Thismomentarydeflectionoccursduetothefactthattheconductingcurrentflows

throughconnectingwiresduringthechargingofcapacitor.Thisleadstodepositionof

chargeattwoplatesandhence,varyingelectricfieldofincreasingnatureisproduced

betweentheplateswhichinturnproducesdisplacementcurrentinspacebetween

twoplates,whichmaintainsthecontinuitywiththeconductioncurrent.

Expressionforthecurrent where istherateofchangeofelectricflux

withtime.

25.

26. Radiusofgroundstateofhydrogenatom .

AccordingtodeBroglierelation2πr=nλ

Forgroundstaten=1

therefore,

OR

i. Accordingtothequestion,

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Given,r1=5.3 10-11mandr2=21.2 10-11m

n1=1

Weknowthat,

ii. Weknowthat,

27. Circuitdiagramofilluminatedphotodiodeinreversebiasisshownbelow.

Achangeinthephotocurrentindicatesachangeinthelightintensity,ifareverse

biasisapplied.

OR

Device-Zenerdiode.

Symbolis

P(anode) N(cathode

SectionC

28. ThemeterbridgeworksontheprincipleofWheatstonebridge.Atbalancecondition:

where isdistancemeasuredfromAtotheNullpoint.

Inameterbridge,thenullpointisfoundatadistanceof60cmfromA.

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Theconditionofbalancedmeterbridgeis

given

...(i)

Again,applyingtheconditionwhenSand areconnectedinseries

with

and

...(ii)

FromEquations,(i)and(ii),weget

substitutingvalueof'S'in(i)

29. Cyclotronisadevicebywhichthepositivelychargedparticleslikeprotons,deutrons,

etc.canbeaccelerated.ThelabelleddiagramofcyclotronisshownintheFigure.

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WorkingPrinciple:Thecyclotronusescrossedelectricandmagneticfieldswhich

increasesthekineticenergyofachargedparticlewithoutchangingitsfrequencyof

revolution.

Here,MagneticlorentzforceisequaltotheCentripetalforce.Thus,

Fc=Fm

=qvB

Thus, =

Now,timetakentodescribeasemi-circleis,

Thus,periodofrotation,T=2t

Thefrequencyofrotationofparticleknownascyclotronfrequencyis,

Fromtheaboveequationwecansee,cyclotronfrequencyisindependentofthespeed

andradiusoftheorbit.

30. i. Powerfactor

R=Z

or

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ii. Thecurrentamplitudeatthisfrequency,

iii. TheQ-factor,

31. a. Here,a=6.0cm

C=100pF=

Erms=230V

b. Since,I=IDwhetherIissteadyd.c.ora.c.Thisisshownbelow:

Or

Weknowthat

ThisformulagoesthroughevenifID(andthereforeB)oscillatesintime.The

formulashowsthattheyoscillateinphase.SinceID=I,wehave

IfI=I0,themaximumvalueofcurrent,thenamplitudeofB=maximumvalueof

B

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32. Foramicroscope,

Resolvingpower

i. When isdecreased,resolvingpowerincreases.

ii. Whendiameterofobjectivelensisdecreased, decreases, decreases.Hence

resolvingpowerofmicroscopealsodecreases.

OR

Thepositionofthe30thbrightfringeisgivenby

Hencetheshiftofthecentralfringeis

But

or

33. Ingroundstate,energyofgaseoushydrogenatroomtemperature=-13.6eV.

Whenitisbombardedwith12.5eVelectrobeam,theenergybecomes-13.6+12.6=

-1.1eV

So

Theelectronwouldjumpfromn=1ton=3,

where

.

Onde-excitation,theelectronmayjumpfromn=3ton=2givingrisetoBalmer

series.Itmayalsojumpfromn=3ton=1,givingrisetoLymanseries.

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So,numberofspectralline

=3spectrallinesappear.

34. Zenerdiodeisfabricatedbyheavilydopingofp-nsideofjunctionsoastooperate

continouslywithoutgettingdamagedintheregionofreversebreakdownvoltage.

ThecircuitdiagramofavoltageregulatorusingaZenerdiodeisshowninfigure.

Zenerdiodeasvoltageregulator:Zenerdiodeisusedinregulatingfluctuatingvoltage

asshown.ItisconnectedincircuitthroughresistanceRdependingonvoltageor

powerratingR1isconnectedinparallelandoutputisreceived.Onanabrupt

increaseofvoltageacrossdiodebecomesconstant,equaltobreakdownvoltagebut

currentrisessharply.Hence,thereisanincreaseinvoltagedropR.AsRLisin

parallelsovoltageacrossRLissame.

SectionD

35. a. Considerathininfinitelylongstraightlinechargehavingauniformlinearcharge

density placedalongYY'.Drawacylindricalsurfaceofradiusrandlengthl

aboutthelinechargeasitsaxis.

IfEisthemagnitudeofelectricfieldatpointP,thenelectricfluxthroughthe

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gaussiansurfaceisgivenby

areaofthecurvedsurfaceofacylinderofcylinderradiusrandlengthl

or

AccordingtoGauss'theorem,wehave

Now,chargeenclosedbythegaussiansurface,q= l

Thus,

or

b. Electricfieldatadistancerfromthelinecharge,

Tocalculateforceoncharge-qatpointA:

Here,OA=2 5cm=2 5 10-2m

ElectricfieldatpointA,

=3 24 108NC-1

Forceoncharge-qatpointA,F1=qE1=5 10-9 3 24 108=1 62N(towards

thelinecharge)

Tocalculateforceoncharge+qatpointB:

Here,OB=2 5 10-2+2 10-3=2 7 10-2m

ElectricfieldatpointB,

=3 108NC-1

Forceoncharge+qatpointB,

F2=qE2=5 10-9 3 10-8=1 5N(awayfromthelinecharge)

Hence,netforceonelectricdipole,

F=F1-F2=1.62-1.5=0 12N(towardsthelinecharge)

OR

i. Atx=18cm,thepotentialiszero.

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ii. Now,atx=18cm,netpotentialduetothetwochargesAandBiszeroi.e.

whereristhedistancebetweenthetwocharges.

Here,QA=3 6 10-6C,x=18cm=0.18mand

r-x=30-18=12cm=0 12m

orQB=-2 4 10-6C

iii. Forceonthetestchargeatanypoint=-potentialgradientatthatpoint charge

Fromthegraph,itfollowsthatthepotentialgradient(i.e.slopeofthegraph)and

hencetheforceonthetestchargeismaximumatthepointx=27cm.

36. i. Considerastripofwidthdratadistancerfromthestraightwire.Magneticfieldat

thelocationofthestripduetothewire,

z

Areaofstrip,dA=ldr

Magneticfluxlinkedwiththestrip,

Totalmagneticfluxlinkedwiththecoil,

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ii. Inducedemf

Magneticfield,duetowire,atthelocationofthecoilisperpendiculartotheplane

ofthecoilanddirectedinwards.Whencurrentisreducedtozero,thismagnetic

fielddecreases.Toopposethisdecrease,inducedcurrentshallflowclockwise,so

thatitsmagneticfieldisalsoperpendiculartotheplaneofthecoilanddownward.

OR

Ontheinnerandoutersphere'ssurfacechargesof and are

inducedrespectively.Thepotentialofthesphererelativetoearthisdeterminedsolely

bytheoutersurfacechargeq.

whereristheradiusofthesphere

Now,

=4500V

a. Thepotentialofthesphereismomentarilyreducedtozerowhenitisearthed.The

positivechargeontheoutersurfacedisappearsbuttheinducednegativecharge

insideremains.Therefore,thepotentialofthesphereiszero.

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b. Theinducednegativechargeisneutralizedwhenthemetaltouchesthesphere

andnochargeremainsonthemetalorsphere.Bothareatthesamepotentiali.e.

zeropotential.

37. a. Arefractingsurfacewhichformsapartofasphereoftransparentrefracting

materialiscalledasphericalrefractingsurface.

Theabovefigureshowsthegeometryofformationofimage/ofanobjectOand

theprincipalaxisofasphericalsurfacewithcentreofcurvatureCandradiusof

curvatureR.

Assumptions:

i.Theapertureofthesurfaceissmallcomparedtootherdistanceinvolved.

ii.NMwillbetakentobenearlyequaltothelengthoftheperpendicularfromthe

pointNontheprincipalaxis.

For NOC,iistheexteriorangle.

...(i)

Similarly,

i.e., ...(ii)

BySnell'slaw,

n1sini=n2sinr

Forsmallangles,

n1i=n2r

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Substitutingthevaluesofiandrfromequations(i)and(ii),weobtain

...(iii)

ApplyingnewCartesiansignconventions,

OM=-u,MI=+v,MC=+R

Substitutingtheseinequation(iii),weobtain

...(iv)

Aboveequationistherequiredequation.

b. CaseI:Lensinair

LetthefocallengthoflensinairbeFair

Giventhat:Fair=20cm,n1=1(air),n2=1.6

Accordingtolensmaker'sformula:

...(i)

CaseII:Lensinliquid

LetthefocallengthoflensinliquidbeFliquid

Giventhat:n1=1.3(liquid),n2=1.6

Accordingtolensmaker'sformula:

...(ii)

Diving(i)by(ii),weget

Thus,Fliquid=52.0156cm

OR

Theraydiagramisshowninthefigure:

Let,NM=h

TheconvexsphericalrefractingsurfaceformstheimageofobjectOatI.Theradiusof

curvatureisR

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Here

In ...(i)

In

...(ii)

Forsmallangles and andassumingMisveryclosetoP,wehave

BySnell'slaw,

Forsmalliandr,

[FromEqs.(i)and(ii)]