2021 FJS 4E Phy Prelim ans

Answers/ MarkingScheme Sec4EExpressPrelim2021 Physics Paper 1 1 D 11 B 21 C 31 C 2 A 12 C 22 B 32 D 3 B 13 D 23 B 33 C 4 D 14 D 24 A 34 B 5 B 15 C 25 D 35 B 6 D 16 A 26 A 36 A 7 C 17 B 27 B 37 B 8 C 18 A 28 C 38 C 9 C 19 A 29 B 39 C 10 B 20 A 30 C 40 B

Paper 2SectionA1 Scale CorrectvectordiagramwitharrowsNetforce=2.4to2.6N 1 113m 2 (a)(i) KEgained=½x40x1.5 2 –0J=45J 1(a)(ii) WD=30x6=180JWD=gaininKE+WDagainstresistiveforce180–45=fx6.0mf=22.5N 1 11(b) WD=Areaunderthegraph=30x6+½x30x4=240JAveragepower=240÷10.0=24W 116m 3 (a) Whenrandomlymovinggasmoleculeshitthewallsofthecontainerathighspeed,theyexertaforceonthewall.Theforceperunitareaexertedonthewallsofthecontaineristhepressure. 1 1(b)(i) Aspaintisusedup,thevolumeofairinthecanincreases.Thusthenumberofgasparticlesperunitvolumedecreases.Thefrequencyofcollisionbetweenthegasparticlesandthewallsdecreases.Thustheforceexertedperunitareadecreases. 1 1(b)(ii) Theaveragekineticenergyoftheremaininggasmoleculesinsidethecandecreaseshencethetemperatureofthespraycandecreases. 15m 4 (a) 3 (b) Forabodyatequilibriumaboutapivot,theprincipleofmomentsstatesthatsumofclockwisemomentsaboutthepivotisequaltosumofanti-clockwisemomentsaboutthepivot. 1 (c) Takingmomentaboutknee,

F(85)=600(50)F=353N 21(d) Sumofupwardforces=SumofdownwardforcesF+N=W353+N=600N=247N 18m 5 (a) arrangement:Ice:inlattice/regular/arranged/orderly/fixedinplaceWater:random/irregular/notarranged/notorderly motion:Ice:vibrateWater:move(around)orslideovereachother 1 1(b)(i) m=1800×0.025×920=41000kg 11 (b)(ii) Q=mc Δθ=2.1×10 3 ×41000×3.5=3.0×10 8 J 116m 6 (a) Microwavesmicrowaveovens/handphonesignaltransmission 11(b) T=1/f=1/(12x10 9 )=8.33x10 -11 s 11(c) v=fλ3.0x10 8 =(12x10 9 )(λ)λ=0.025m 11(d)(i) Anyoneofthefollowingdifferences:● Laserlightisatransversewavewhileultrasoundisalongitudinalwave.● Laserlightcantravelinvacuumbutultrasoundcannottravelinvacuum.● Laserlighttravelsatalmost3x10 8 m/sinairwhileultrasoundtravelsatabout340m/sinair. 1 (d)(ii) Ultrasoundcannottravelthroughspaceastherearenoparticlesthatenabletransmissionofultrasound. 1 8m 7 (a) A(fixed)resistorBthermistorCL.E.D.ORlightemittingdiode(2correct1m;3correct2m) 2 (b) anyfourfromsix:● ifcold/hotresistanceofthermistorhigh/low● ifcold/hotvoltage(across)thermistorhigh/low● ifcold/hotvoltageofinputtoLEDhigh/low● ifcold/hotthereiscurrent/nocurrentinLED● ifcoldLEDlights/brighter● ifhotLEDdoesnotlight/dimmer 46m

8 (a) Whenthecurrentexceedsthefuserating,thewireinthefusemelts(acceptfuseblown) Thisopensthecircuitandprotectsthecircuit. 1 1(b) operatingcurrentoffan=1.25Aoperatingcurrentofheater=10.4ATotalI=11.7A Fusewillblowundernormaloperatingcondition.(onlyawardifthereiscalculation) 1 1 (c) Fuseisconnectedtotheneutralwire.Itshouldbeconnectedtolivewire.Earthwireisconnectedtheheatercoil,insteadofthemetalcasing.Earthwireshouldbeconnectedtometalcasingofheatercoil. 1 1(d) TotalIdrawn=1.25+10.42=11.67AEffectiveR=240/11.67=20.6Ω ORRcoil=23.04ΩRlamp=192ΩR=(1/(240^2/2500)+1/(240^2/300))^-1=20.6Ω[1] 11 8m SectionB 9 (a)(i) f=1/[(1.5-1)(2/0.2)]=0.2m=20cm 1(a)(ii) O lens F F 1mforobjectwithinfocallength1mforvirtualimageonthesamesideasobject1mforlightraysandarrows. 3 (a)(iii) Theradiiofcurvatureisdirectlyproportionaltothef. 1

(b)(i) Speedoflightchangesindifferentmedia. 1(b)(ii) Thehighertherefractiveindex,thesmallerthefocallength. 1(b)(iii) Inahighrefractiveindexmaterial,thelightbendsmoretowardsthenormal/smallerangleofrefraction.Lightraywillconvergetowardseachotherinashorterdistance. 1 1(b)(iv) Athinner/lighterlenscanbeused. 110m 10 (a) Speedisascalarquantitywhilevelocityisvectorquantity(acceptdirectionvsnodirection) 1 (b) Fromt=0to0.88s, velocitydecreasesconstantly/constantnegativeacceleration/constantdeceleration.Att=0.88s,ballismomentarilyat rest. Fromt=0.88to1.8s,ball acceleratesuniformlyintheoppositedirection. 1 1 1 (c) (i) 0.88s 1(c)(ii) Distance=0.5x8.8x0.88=3.87m 1(d) Theballhitsgroundorshorttimefordeceleration 1(e) Kineticenergyofballconvertedtothermalenergy+soundenergy 1(f) Any2: ● shortertimetodroptozerovelocity● linenotstraight orvelocitydoesnotchangeuniformlyorgradientnotconstant● smallerareaunder(firstpartof)graphorlessdistancetravelled● slowerfinalvelocity● initialdownwardgradientsteeper 2 10m 11E (a)(i) 7.5V 1 (a)(ii) R=V/I=7.5/4.0=1.9Ω 11(a)(iii) P=VI=6.5×4.0P=26W 11(a)(iv) resistanceincreaseshencereadingofammeterdecreases 1(b) atleasttwolinesonleftandtwolinesonrightofcoreandcorrectshapeatleastonearrowNtoS(primarilyupwards)andnonewrong 1 1(c) Thecylinderismagnetised(byinduction)withtop(ofcylinder)isinducedwithaSouthpole.Sinceunlikepolesattract,thecylinderisattractedtothecore 1 110m 11O (a)(i) Analternatingcurrentiswheretheflowofchargechangesdirectionatregularintervals 1(a)(ii) Whenthemagnetrotates,thereisachangeinthemagneticfluxwithinthecoil.Thisinducesane.m.f.thatproducesaninducedcurrent. 1

ORthemagneticfieldlinescutthecoil(a)(iii) Whenthemagnet rotatestowardsthecoil,aninducedacurrent isproducedinonedirection, andwhenit rotatesawayfromthesoftiron,aninducedcurrentisproducedintheoppositedirection. 1 1 (a)(iv) Any1way:IncreasethespeedofrotationofmagnetUseastrongerpermanentmagnetUseahighernumberofturnofthecoilDecreasedistancebetweenthecoilandmagnetUsealowerresistancelamp 1 (a)(v) Ithelpstoconcentratethemagneticfluxthroughthecoils. 1(b)(i) Highervoltagereducesthecurrent,andthisreducespower lossinthetransmissioncables. 1 (b)(ii) P=IxV=100x23000=2300000Wor2300kWor2.3MW 1(b)(iii) Vp/Vs=Is/Ip23000/660000=Is/100Is=3.48A 1110m