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Electromagnetism
InthelasttutorialaboutMagnetismwelookedbrieflyathowpermanentmagnetsproduceamagneticfieldaround
themselvesfromtheirnorthpoletotheirsouthpole.Whilepermanentmagnetsproduceagoodandsometimesvery
strongstaticmagneticfieldinsomeapplicationsthestrengthofthisfieldisstilltooweakorweneedtobeabletocontrol
theamountofmagneticfluxthatispresent.
Soinordertoobtainamuchstrongerandmorecontrollablemagneticfieldweneedtouseelectricity.Byusingcoilsof
wirewrappedorwoundaroundasoftmagneticmaterialsuchasanironcorewecanproduceverystrong
electromagnetsforuseinmaydifferentapplications.Thisthenproducesarelationshipbetweenmagnetismand
electricitythatgivesusanotherformofmagnetismcalledElectromagnetism.
Electromagnetismisproducedwhenanelectricalcurrentflowsthroughasimpleconductorsuchasapieceofwireor
cable.Asmallmagneticfieldiscreatedaroundtheconductorwiththedirectionofthismagneticfieldwithregardstoits
"North"and"South"polesbeingdeterminedbythedirectionofthecurrentflowingthroughtheconductor.Therefore,itis
necessarytoestablisharelationshipbetweencurrentflowingintheconductorandtheresultantmagneticfieldproduced
bythiscurrentflowandtherebydefiningthedefiniterelationshipthatexistsbetweenElectricityandMagnetisminthe
formofElectromagnetism.
Whenanelectricalcurrentflowsthroughaconductoracircularelectromagneticfieldisgeneratedaroundit.Thedirection
ofrotationofthismagneticfieldisgovernedbythedirectionofthecurrentflowingthroughtheconductorwiththe
correspondingmagneticfieldproducedbeingstrongerneartothecentreofthecurrentcarryingconductorandweaker
fartherawayfromitasshownbelow.
MagneticFieldaroundaConductor
Asimplewaytodeterminethedirectionofthemagneticfieldaroundtheconductoristoconsiderscrewing
anordinarywoodscrewintoasheetofpaper.Asthescrewentersthepapertherotationalactionis
CLOCKWISEandtheonlypartofthescrewthatisvisibleabovethepaperisthescrewhead.Ifthewood
screwisofthepozidrivorphilipstypeheaddesign,thecrossontheheadwillbevisibleanditisthiscross
thatisusedtoindicatecurrentflowing"into"thepaperandawayfromtheobserver.
Likewise,theactionofremovingthescrewisthereverse,anti-clockwise.Asthecurrententersfromthe
topitthereforeleavestheundersideofthepaperandtheonlypartofthewoodscrewthatisvisiblefrom
belowisthetiporpointofthescrewanditisthispointwhichisusedtoindicatecurrentflowing"outof"the
paperandtowardstheobserver.
LeftHandRuleofElectromagnetism
Thenthephysicalactionofscrewingintoandoutofthepaperindicatesthedirectionofthecurrentinthe
conductorandtherefore,thedirectionofrotationoftheelectromagneticfieldarounditasshownbelow.
ThisconceptisknowngenerallyastheRightHandScrewAction.
TheRightHandScrewAction
Amagneticfieldimpliestheexistenceofpolesandthepolarityofacurrentcarryingconductorcanbeestablishedby
drawingthecapitallettersSandNandthenaddingarrowheadstothefreeendofthelettersasshownabovegivinga
visualrepresentationofthemagneticfielddirection.
Anothermorefamiliarconceptwhichdeterminesboththedirectionofcurrentflowandtheresultingdirectionofthe
magneticfluxaroundtheconductoriscalledthe"LeftHandRule".
Thedirectionofthemagneticfieldisfromnorthpoletosouthpoleand
canbededucedbyholdingthecurrentcarryingconductorinyourleft
handwiththethumbextendeditwillbepointinginthedirectionof
theelectronflowfromnegativetopositive.Thepositionofthefingers
laidacrosstheconductorwillnowpointinthedirectionofthemagnetic
linesofforceasshown.
Ifthedirectionoftheelectronflowingthroughtheconductorisreversed,
thelefthandwillneedtobeplacedontotheothersideoftheconductor
withthethumbpointinginthenewdirectionoftheelectroncurrentflow.
Alsoasthecurrentisreversedthedirectionofthemagneticfield
producedaroundtheconductorwillalsobereversed.
This"LeftHandRule"canalsobeusedtodeterminethemagneticdirectionofthepolesinanelectromagneticcoil.This
time,thefingerspointinthedirectionoftheelectronflowfromnegativetopositivewhiletheextendedthumbindicating
thedirectionofthenorthpole.Thereisavariationonthisrulecalledthe"righthandrule"whichisbasedonso-called
conventionalcurrentflow,(positivetonegative).
Whenasinglestraightpieceofwireisbentintotheformofasingleloopasshownbelow,thecurrentwillbeflowingin
oppositedirectionsthroughthepapersuchthataclockwisefieldandananticlockwisefieldareproducednexttoeach
other.Theresultingspacebetweenthesetwoconductorsbecomesan"intensified"magneticfieldwiththelinesofforce
spreadingoutinsuchawaythattheyassumetheformofabarmagnetgeneratingadistinctivenorthandsouthpoleat
thepointofintersection.
ElectromagnetismaroundaLoop
LinesofForcearoundtheLoopThecurrentflowingthroughthetwoparallelconductorsofthelooparein
oppositedirectionsasthecurrentthroughtheloopexitsthelefthandside
andreturnsontherighthandside.Thisresultsinthemagneticfield
aroundeachconductorinsidetheloopbeinginthe"SAME"directionto
eachother.
Theresultinglinesofforcegeneratedbythecurrentflowingthroughthe
loopopposeeachotherinthespacebetweenthetwoconductorswhere
thetwolikepolesmeettherebydeformingthelinesofforcearoundeach
conductorasshown.
However,thedistortionofthemagneticfluxinbetweenthetwoconductors
resultsinanintensityofthemagneticfieldatthemiddlejunctionwerethe
linesofforcebecomeclosertogether.Theresultinginteractionbetween
thetwolikefieldsproducesamechanicalforcebetweenthetwoconductorsastheytrytorepelawayfromeachother
producingmotion.
However,astheconductorscannotmove,thetwomagneticfieldsthereforehelpeachotherbygeneratinganorthanda
southpolealongthislineofinteraction.Thisresultsinthemagneticfieldbeingstrongestinthemiddlebetweenthetwo
conductors.Theintensityofthemagneticfieldaroundtheconductorisproportionaltothedistancefromtheconductor
andbytheamountofcurrentflowingthroughit.
Themagneticfieldgeneratedbyastraightlengthofcurrent-carryingwireisveryweakevenwithahighcurrentpassing
throughit.However,ifseveralloopsofwirearewoundtogetheralongthesameaxisproducingacoil,theresultant
magneticfieldwillbecomeevenmorestrongerthanthesingleloopproducinganelectromagneticcoilmorecommonly
calledaSolenoid.Theneverycoilofwireusestheeffectofelectromagnetismwhenanelectricalcurrentflowsthrough
itandwewilllookatthiseffectinmoredetailinthenexttutorial.
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