AP_Physics_B_-_Electromagnetic_Induction (1).ppt

ElectroMagneticInduction APPhysicsB WhatisE MInduction ElectromagneticInductionistheprocessofusingmagneticfieldstoproducevoltage andinacompletecircuit acurrent MichaelFaradayfirstdiscoveredit usingsomeoftheworksofHansChristianOersted Hisworkstartedatfirstusingdifferentcombinationsofwiresandmagneticstrengthsandcurrents butitwasn tuntilhetriedmovingthewiresthathegotanysuccess Itturnsoutthatelectromagneticinductioniscreatedbyjustthat themovingofaconductivesubstancethroughamagneticfield MagneticInduction AsthemagnetmovesbackandforthacurrentissaidtobeINDUCEDinthewire MagneticFlux Thefirststeptounderstandingthecomplexnatureofelectromagneticinductionistounderstandtheideaofmagneticflux FluxisageneraltermassociatedwithaFIELDthatisboundbyacertainAREA SoMAGNETICFLUXisanyAREAthathasaMAGNETICFIELDpassingthroughit A B WegenerallydefineanAREAvectorasonethatisperpendiculartothesurfaceofthematerial Therefore youcanseeinthefigurethattheAREAvectorandtheMagneticFieldvectorarePARALLEL ThisthenproducesaDOTPRODUCTbetweenthe2variablesthatthendefineflux MagneticFlux TheDOTproduct HowcouldweCHANGEthefluxoveraperiodoftime Wecouldmovethemagnetawayortowards orthewire WecouldincreaseordecreasetheareaWecouldROTATEthewirealonganaxisthatisPERPENDICULARtothefieldthuschangingtheanglebetweentheareaandmagneticfieldvectors Faraday sLaw FaradaylearnedthatifyouchangeanypartofthefluxovertimeyoucouldinduceacurrentinaconductorandthuscreateasourceofEMF voltage potentialdifference SincewearedealingwithtimeherewereatalkingabouttheRATEofCHANGEofFLUX whichiscalledFaraday sLaw UsefulApplications TheForeverFlashlightusestheFaradayPrincipleofElectromagneticEnergytoeliminatetheneedforbatteries TheFaradayPrinciplestatesthatifanelectricconductor likecopperwire ismovedthroughamagneticfield electriccurrentwillbegeneratedandflowintotheconductor UsefulApplications ACGeneratorsuseFaraday slawtoproducerotationandthusconvertelectricalandmagneticenergyintorotationalkineticenergy Thisideacanbeusedtorunallkindsofmotors SincethecurrentinthecoilisAC itisturningonandoffthuscreatingaCHANGINGmagneticfieldofitsown Itsownmagneticfieldinterfereswiththeshownmagneticfieldtoproducerotation Transformers Probablyoneofthegreatestinventionsofalltimeisthetransformer ACCurrentfromtheprimarycoilmovesquicklyBACKandFORTH thustheideaofchanging acrossthesecondarycoil Themovingmagneticfieldcausedbythechangingfield flux inducesacurrentinthesecondarycoil IfthesecondarycoilhasMOREturnsthantheprimaryyoucanstepupthevoltageandrunsdevicesthatwouldnormallyneedMOREvoltagethanwhatyouhavecomingin WecallthisaSTEPUPtransformer WecanusethisideainreverseaswelltocreateaSTEPDOWNtransformer Microphones Amicrophoneworkswhensoundwavesenterthefilterofamicrophone Insidethefilter adiaphragmisvibratedbythesoundwaveswhichinturnmovesacoilofwirewrappedaroundamagnet Themovementofthewireinthemagneticfieldinducesacurrentinthewire Thussoundwavescanbeturnedintoelectronicsignalsandthenamplifiedthroughaspeaker Example Acoilwith200turnsofwireiswrappedonan18 0cmsquareframe Eachturnhasthesamearea equaltothatoftheframe andthetotalresistanceofthecoilis2 0W Auniformmagneticfieldisappliedperpendicularlytotheplaneofthecoil Ifthefieldchangesuniformlyfrom0to0 500Tin0 80s findthemagnitudeoftheinducedemfinthecoilwhilethefieldhaschangedaswellasthemagnitudeoftheinducedcurrent 4 05V 2 03A WhydidyoufindtheABSOLUTEVALUEoftheEMF Whathappenedtothe thatwasthereoriginally Lenz sLaw Lenz slawgivesthedirectionoftheinducedemfandcurrentresultingfromelectromagneticinduction ThelawprovidesaphysicalinterpretationofthechoiceofsigninFaraday slawofinduction indicatingthattheinducedemfandthechangeinfluxhaveoppositesigns Lenz sLaw Inthefigureabove weseethatthedirectionofthecurrentchanges Lenz sLawhelpsusdeterminetheDIRECTIONofthatcurrent Lenz sLaw Faraday sLaw Let sconsideramagnetwithit snorthpolemovingTOWARDSaconductingloop DOESTHEFLUXCHANGE DOESTHEFLUXINCREASEORDECREASE WHATSIGNDOESTHE D GIVEYOUINFARADAY SLAW DOESLENZ SLAWCANCELOUT Whatdoesthismean Yes Increase Positive NO ThismeansthattheINDUCEDMAGNETICFIELDaroundtheWIREcausedbythemovingmagnetOPPOSEStheoriginalmagneticfield SincetheoriginalBfieldisdownward theinducedfieldisupward Wethenusethecurlingrighthandruletodeterminethedirectionofthecurrent Binduced Lenz sLaw Amagnetisdroppeddownaconductingtube ThemagnetINDUCESacurrentaboveandbelowthemagnetasitmoves TheINDUCEDcurrentcreatesanINDUCEDmagneticfieldofitsowninsidetheconductorthatopposestheoriginalmagneticfield Sincetheinducedfieldopposesthedirectionoftheoriginalitattractsthemagnetupwardslowingthemotioncausedbygravitydownward IfthemotionofthemagnetwereNOTslowedthiswouldviolateconservationofenergy Lenz sLaw Let sconsideramagnetwithit snorthpolemovingAWAYfromaconductingloop DOESTHEFLUXCHANGE DOESTHEFLUXINCREASEORDECREASE WHATSIGNDOESTHE D GIVEYOUINFARADAY SLAW DOESLENZ SLAWCANCELOUT Whatdoesthismean Yes Decreases negative yes Inthiscase theinducedfieldDOESNOTopposetheoriginalandpointsinthesamedirection OnceagainuseyourcurledrighthandruletodeterminetheDIRECTIONofthecurrent Binduced Insummary Faraday sLawisbasicallyusedtofindtheMAGNITUDEoftheinducedEMF ThemagnitudeofthecurrentcanthenbefoundusingOhm sLawprovidedweknowtheconductor sresistance Lenz sLawispartofFaraday sLawandcanhelpyoudeterminethedirectionofthecurrentprovidedyouknowHOWthefluxischanging MotionalEMF TheRailGun Arailgunconsistsoftwoparallelmetalrails hencethename connectedtoanelectricalpowersupply Whenaconductiveprojectileisinsertedbetweentherails fromtheendconnectedtothepowersupply itcompletesthecircuit Electronsflowfromthenegativeterminalofthepowersupplyupthenegativerail acrosstheprojectile anddownthepositiverail backtothepowersupply Inaccordancewiththeright handrule themagneticfieldcirculatesaroundeachconductor Sincethecurrentisinoppositedirectionalongeachrail thenetmagneticfieldbetweentherails B isdirectedvertically Incombinationwiththecurrent I acrosstheprojectile thisproducesamagneticforcewhichacceleratestheprojectilealongtherails Therearealsoforcesactingontherailsattemptingtopushthemapart butsincetherailsarefirmlymounted theycannotmove Theprojectileslidesuptherailsawayfromtheendwiththepowersupply MotionalEmf TherearemanysituationswheremotionalEMFcanoccurthataredifferentfromtherailgun Supposeabaroflength L ispulledtorightataspeed v inamagneticfield B directedintothepage Theconductingroditselfcompletesacircuitacrossasetofparallelconductingrailswitharesistormountedbetweenthem MotionalEMF Inthefigure weareapplyingaforcethistimetotherod DuetoLenz sLawthemagneticforceopposestheappliedforce Sinc