Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
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