浙江大学大学物理甲下-chapter-34.ppt

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1、Chapter34Faraday’sLawofInduction134-1Faraday’sExperimentsTheexperimentabouttheinduction:***Inducedcurrentisproduced.Itissetupbyaninducedelectromotiveforce(inducedemfs).2Thecommonfeature:motionorchange34-2Faraday’sLawofInductionCommonpoint:thenumberoflinesofthemagneticfieldpassing

2、throughtheleft-handcoilchanges.Therateofchangeinthenumberoffieldlinespassingthroughtheloopdeterminetheinducedemfs.1.MagneticfluxΦBWhatischanged?32.Faraday’slawofinductionTherearemanywaysofchangingthefluxthroughaloop.Bcanbechanged.θcanbechanged.Acanbechanged.434-3Lentz’LawThefluxo

3、fthemagneticfieldduetotheinducedcurrentopposesthechangeinfluxthatcausestheinducedcurrent.Flux(cause)Flux(result)increaseopposeincrease,thatisthisfluxisoppositedirectionoftheoriginalflux.decreaseopposedecrease,thatisthisfluxisthesamedirectionoftheoriginalflux.Thedirectionoftheindu

4、cedemfsandcurrentsalwaysmakethemagneticfieldcausedbythemopposesthechangeofthefluxthatproducesthem.5Whataboutthiscase?Whataboutthiscase?6Istheenergyconservativewithinducedcurrents?Conservationofenergyconsiderations:Claim:Directionofinducedcurrentmustbesoastoopposethechange;otherwi

5、seconservationofenergywouldbeviolated.Why???Ifcurrentreinforcedthechange,thenthechangewouldgetbiggerandthatwouldinturninducealargercurrentwhichwouldincreasethechange,etc..7SignsinFaraday’slawsignofε:Whataboutthiscase?Definethedirectionofthepathfirst:8Whataboutthiscase?9signofε:si

6、gnofdA:signoffluxΦB:WhenBisincreasing,…Onthetextbook:Right-handrulewithianddA10Sampleproblem34-2:r,R(resistance)areknown,Bz=-4.0-5.6t+2.2t2.Findinducedcurrentintheloopatt=1sandt=2s.Solution:ChoosedAintothepage,t=1s,Bz=-7.4T;t=2s,Bz=-6.4T,thusinthetimeinterval,Bz<0.BzdA11“+”meanst

7、hecurrentisclockwise.Note:Itisthedirectionofthechangeinthefieldratherthanthedirectionofthefielditselfthatdeterminesthedirectionoftheinducedcurrent.“-”meansthecurrentiscounterclockwise.12QuickQuiz-1TheFigureshowsagraphicalrepresentationofthefieldmagnitudeversustimeforamagneticfiel

8、dthatpassesthroughafixedloopandisoriente