《大学物理简明教程》 课件 15-1 Electromagnetic Induction

ElectromagneticInductionMagneticfieldinducedbyelectriccurrentDiscoveredbyHans

Oerstedin1820Electricfield/currentinducedbymagneticfield?MichaelFaraday’sdiscoveryin1831CurrentinducedbychangingmagneticfieldMichaelFaraday(1791–1867)EnglishScientistThereisatransient

currentonlywhenthebatteryisconnected,ordisconnectedFaraday'sfirstexperimentaldemonstration(August29,1831)Faraday’sdisk(1831)movingamagnetinsideandoutsideacoilinducesacurrentonlywhenitismoving!movinginpositivecurrentmovingoutnorthpolefirstnegativecurrentnorthpolefirstCuttingflux-motionalE.M.F.SupposenowthatwehaveaconductingrodmovinginamagneticfieldasshownThepositivechargeswillexperienceamagneticforceupwards,whilethenegativechargeswillexperienceamagneticforcedownwardsChargeswillcontinuetomoveuntilthemagneticforceisbalancedbyanopposingelectricforceduetothechargesthathavealreadymovedInsteadycase,wehaveButwealsohaveElectromotiveforce(e.m.f)

–theequivalentforcefieldthatmoveschargeandbuildspotentialdifferenceSotheinducedpotentialdifferenceacrosstheendsoftherod

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Whatifthemotion,magneticfieldandconductorarenotperpendiculartoeachother?Weonlytaketheperpendicularcomponentsintoaccount

orθ

Whatisthedirectionoftheemf?Thedirectionoftheemfisthedirectionofpotentialincrease–negativechargetopositivechargeWhatistheemfofFaraday’sdisk?B=0.5T

radius

r=0.1mspeed:2revolutionspersecondr

=0.1m

InacircuittworailsandtheresistorRarefixedtheconductorbarismovingatthespeedofvthemagneticforceonthemovingbar?

Itsdirectionopposesthemotionofthebar!MagneticFluxChangingofmagneticflux

CONCLUSION:toproduceemfoneshouldmakeachangeinthemagneticfluxwithtime!Considertheloopshown:

Faraday’sLawofInductionTheinducedemfinaclosedloopequalsthenegativeofthetimerateofchangeofthemagneticfluxthroughtheloopTheinducedemf

opposesthechangethatisoccurring

Theloopis

NOTnecessarilyaclosedcircuit.Itcanbeevenimaginaryxxxxxxxxxxxxxxxxx60Ov=10m/sl=1mxxxxxxxxxxxxxTheresistanceonall3sidesis1Ω/mWhatisthecurrentat

thismoment?

v=10m/s

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Faraday’slawgivesthesameresultasthe“fluxcutting”methodx

lLenz'slaw

xxxxxxxxxxxxxxxxWhatisthedirectionofthecurrent?Whatifthereisnoclosedcircuit?Thedirectionoftheemfinducedbychangingfluxwill(would)produceacurrentthatgeneratesamagneticfieldopposingthefluxchangethatproducedit.

But,ifnoconductoriscuttingtheflux?

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InverselyproportionaltoradiusonlyifitisoutsideofthemagneticfieldregionErWhatiftheloopisinsidethemagneticfieldregion?xxxxxxxxxaalWhatisthe