3 EM Induction - Santa Monica High School Physics · electromagnetic induction. • A transformer...

ElectromagneticInduction

ElectromagneticInduction

• Electriccurrentcanbeproducedinawirebysimplymovingamagnetintooroutofawire• Therelativemotionbetweenthewireandmagneticfieldinducesvoltagewhichproducescurrent.

Electriccurrentcanproducemagneticfields,but(changing)magneticfieldscanalsoproducecurrent!

• Workisdoneinpushing themagnetintotheloop.

• Theinducedcurrentintheloopcreatesamagneticfieldthatrepelstheapproachingmagnet.

• Forexample,inthetopfigure, thenorthpoleofabarmagnetispushed towardasingleloop.Thecurrentinduced intheloopproducesamagneticfield thatrepelstheapproachingbarmagnet.

• Inthebottomfigure, whenthemagnetispulledawayfromtheloop, theinducedcurrentproducesamagneticfieldthatattractstherecedingbarmagnet.

• Bothcasesrequireworkinput.Ifyou trytopushamagnetintoacoilwithmoreloops, itrequiresevenmorework.

• Directionoftheinducedcurrentissuchthatthemagneticfieldresultingfromtheinducedcurrentopposesthechangeinthefieldcausedbytheinducedcurrent.

Lenz’sLaw

Flux• MagneticFlux=theamountofmagneticfieldpassingthroughagivenarea• MeasuredinunitsofWebers (Wb)

• Iftheloopofwireliesparalleltothefield,thefluxthroughtheloopwillbezero.

Φ =ABΦ=flux(Wb)A=areaofloop(m2)B=magneticfield(T)

Ex:EleanorisundergoinganMRIprocedureandisplacedinsideachamberhousingthecoilofalargeelectromagnetthathasaradiusof25.0cm.Afluxof0.290Wb passesthroughthecoilopening.Whatisthemagneticfieldinsidethecoil?

greaternumberofloops=greaterinducedvoltage=greatercurrent

l Faraday’sLaw:Inducedvoltageinacoilisproportionalto:

l Thenumberofloops

l Theareaofeachloop

l Therateatwhichthemagneticfieldchangeswithinthoseloops

V=voltage[V]N=numberofturns/loopst=time∅ = 𝑓𝑙𝑢𝑥

Ex:ThehoodornamentonAbe’ssedanisshapedlikearing8.00cmindiameter.AbeisdrivingtowardthewestsothatEarth’s5.00*10-5Tfieldprovidesnofluxthroughthehoodornament.WhatistheinducedvoltageinthemetalringasAbeturnsfromthisstreetontoonewhereheistravelingnorth,ifhetakes3.0s tomaketheturn?

Generators&Motors• Generator=Amachinethatproduceselectriccurrentbyrotatingacoilwithinastationarymagneticfield

• A motor convertselectricalenergyintomechanicalenergy.• Ageneratorconvertsmechanicalenergyintoelectricalenergy.

Transformers

• Atransformerisadeviceforincreasingordecreasingvoltagethroughelectromagneticinduction.• Atransformerworksbyinducingachangingmagneticfieldinonecoil,whichiscarriedthroughthecoreandinducesanalternatingcurrent(andinducedvoltage)inanearbysecondcoil.• Voltagesmaybesteppedup (V2 >V1)orsteppeddown (V1>V2)withatransformer.• Energy(&power)areconserved.

Transformers

V=voltage[V]N=numberofturnsI=current[A]

PowerTransmission

• Poweristransmittedgreatdistancesathighvoltagesandcorrespondinglylowcurrents,aprocessthatotherwisewouldresultinlargeenergylossesowingtotheheatingofthewires.• Powermaybecarriedfrompowerplantstocitiesatabout120,000voltsormore,steppeddowntoabout2400voltsinthecity,andfinallysteppeddownagainbyatransformertoprovidethe120voltsinourhouses