Physics 212 – lecture 4 Electric Flux and Gausss Law212–物理學(xué)講座4電通量高斯定律_第1頁
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Physics108–Lecture24:

Faraday’sLaw,Motors,Generators,andTransformersMagneticFluxMagneticfluxrepresentshowcloselytogethermagneticfieldlinesarepacked.Magneticfluxishigherforstrongermagneticfields.Strongmagnet=morelines.Equationformagneticfluxthroughanarea:

isanglebetweenmagneticfieldBandthedirectionAfaces=0degrees:head-onandcos=1.Allgoesthrough.=90degrees:edge-onandcos=0.Nonegoesthrough

DiagramBA1Faradaywasabletosuccessfullyrelatehowmagneticfieldsaffectelectriccircuits.Faraday’slawofinductionrelatesthevoltagecreatedinacircuitduetochangesinthemagneticfield.Faraday’slawofinductionisasfollows:

Faraday’sLawE=voltageinducedincircuitwithchangingBthroughitN=numberofloopsincircuit=changeinmagneticfluxpassingthroughcircuitloopst=timeduringwhichthemagneticfluxchanges

Howdoesthiswork?1.Putcurrentinloop12.CurrentmakesBfieldthatgoesthroughloop23.ChangingBinloop2inducescurrenttocounteractchangingB2Example(Faraday’sLaw):2.At20mphthewiresturnrotateabout200timespersecond.Themagnetsurroundingthecoilhasastrengthof2000Gauss.Whatisthevoltagegeneratedinthecircuittopowerthelightbulb?Given

Path

Want

Conversions/Equations3ElectricDCMotorsandACGeneratorsFaraday’slawcanbeusedtoexplainhowelectricmotorsandgeneratorswork.ElectricDCmotorsandACgeneratorshavethreebasiccomponentsarotatingshaftacircuitloopattachedtotheshaftamagneticfieldgoingfromNtoSonthesidesofthecircuitloopDCMOTORS:CURRENTPROVIDEDtoloop,RESULTISROTATIONGENERATORS:ROTATIONPROVIDEDtoshaft,RESULTISCURRENT

4SomeQuestions1.HowdoyoukeeptheDCmotorturning?2.WhyaremostgeneratorsAC?

3.Howdoesanalternatorfitintotheseconcepts?5Transformers·

Transformersaresimplyvoltagechangers·

Transformersappearalloverinourdailyliveso

Adaptersforsmallelectricalequipment(120VdowntoafewV)o

Cylindricalcanistersneartopsoftelephonepoles(20kVdownto120V)oSomehighvoltageindustrialequipment(120Vuptothousandsofvolts)·

TransformersworkbyFaraday’slawwhereonecoilofwireinducesavoltageinanothercoilofwire,andviceversa.·Keyconcept:theamountofmagneticfluxanditsrateofchangearethesame.Diagramandtransformerequation:NpNsVpVs6Example(Transformers):Aportabletabletopradiooftenrequiresabout12V.Ifatransformerwith800turnsontheprimarysideisusedwhenpluggingintothewall,howmanyturnsshouldbeonthesecondaryside?Note:thisproblemisabitsimplifiedbecausetherad

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