英国物理竞赛

1PhysicsChallenge2012Mark-schemePreamble:Pleaseawardmarksasindicatedbelow.Equivalentvalidreasoningshouldgainequalcredittothesolutionspresentedhere.Errorcarriedforwardmarksmaybeawardedwhereanincorrectanswerisusedaspartofthedataneededforasubsequentquestion,providingthattheresultinganswerisnotplainlyridiculous.Ifincorrectunitsareusedmorethanoncethenonemarkshouldbedeductedfromthetotal.Ifaninappropriatenumberofsignificantfiguresaregivenmorethanonceinfinalanswersthenonemarkshouldbedeductedfromthetotal.SectionA–MultipleChoiceQuestions12345678910EDCACBDDABSectionB–ShortAnswerQuestionsMarksforthesetwoquestionsshouldbeawardedforaclearexplanationoftheunderlyingphysicalprinciplesusingcorrectscientificterminology.Answersthatareincomplete,containerrorsinphysicsoruseterminologyincorrectlycannotbeawardedfullcredit.Award0marks:NovalidattemptmadetoanswerquestionAward1mark:Validpointpresentedbutother-wiseincorrectorincompleteanswerAward2marks:PartiallycorrectanswerbutmajorerrororomissioninreasoningAward3marks:Mostlycorrectanswer,onlyminorerrorsoromissionsinreasoningAward4marks:Completelycorrectanswer,noerrors,omissionsofreasoningorincorrectuseofterminology211.Dispersion•Dndnd•Whitelightismadeupofmanydifferentcolours•Thelightrefractstowardsthenormalwhenitenterstheprismandawayfromthenormalasitleaves•Theamountofrefractiondependsonthespeedofthelightintheglass•Eachcolourhasadifferentwavelengthandtravelsatadifferentspeedintheglassandthereforerefractsbyadifferentamount•Redlighthasalongerwavelengthandslowsdowntheleastandsoisrefractedtheleast•Whilstbluelighthasashorterwavelengthandissloweddownmoreandsoitrefractsmore•Differentcolourschangedirectionbydifferentamountsandsowhitelightissplitupintothevariouscolours12.Transformer•Electromagneticinductionoccursduetoachangingmagneticfield•Achangingmagneticfieldinthesecondarycoilinducesavoltageacrossthesecondarycoil•Thischangingmagneticfieldinthesecondarycoiliscreatedbyachangingmagneticfieldintheironcoreorthereisachangingfieldlinkingthesecondarycoil.•Achangingmagneticfieldintheironcoreisproducedbyachangingcurrentintheprimarycoil•Thereforea.c.isrequiredSectionC–ShortAnswerQuestions13.BouncingBalla)UseofCoE:mgh=0.5mv2orsuvat[1]givesv=√(10/0.5)=4.47m/s[1]b)v=4.47x0.9=4.0m/s[1]useofCoEorsuvat[1]givesheight=0.81m[1]c)Showingheightreducing[1]Appropriateshape–exponentialcurve[1]Nottouchingx-axis[1]d)Ballhaslessvelocityaftereachbounce[1]andsoballachieveslessheightoneachbounce[1]andsotakeslesstimetorise/fall[1]314.PlanetaryMotiona)Useofspeed=distance/timeanddistance=2πr[1]Correcttranslationofunitstogivev=29700m/s[1]b)Useofk=T2/R3attempted[1]togive3.00x10-19[1]withunitss2/m3[1]Note:correctvaluesinotherunitsacceptablee.g.k=4.03x10-20days2/km3c)Useofcalculatedvalueofkoruseofratiosattempted[1]togiveT=691days(5.97x107seconds)[1]TusedwithRtogivev=24000m/s[1]d)EasttoWestbecauseMarsistravellingmoreslowlyandappearstofallbehindthemotionofEarth,thereforeitspositionintheskyappearsfurthertotheWesteachnight(orwordstothateffect).Note–markawardedforcorrectanswerandexplanation.[1]15.PotentialDividera)UseofRTotal=20kΩandV=IR[1]toshowthatI=12/20k=0.6mA[1]b)UseofV=IRoranyequivalentmethod[1]toclearlyshowV=6.0V[1]c)12V-5.5V=6.5V[1]thereforeIA=6.5/10k=0.65mA[1]d)IB=5.5/10k=0.55mA[1]e)Currentthroughvoltmeter=0.65mA–0.55mA=0.1mA[1]ThereforeRv=5.5V/0.1mA=55kΩ[1]f)UseofVA=10VandRA=200ΩgivingI=50mA[1]socurrentthroughRBis30mAandp.d=2V[1]thusR=67Ω[1]