the history of maxwells equations
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Te History Of Maxwell's EquationsLindsay Guilmee (Class of 2012)Sacred Heart University, [email protected]
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[Typetext] [Typetext] [Typetext]
TheHistoryofMaxwellsEquations
Fall
2011
LindsayGuilmette
SacredHeartUniversity
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TheHistoryofMaxwellsEquations 1
JamesClerkMaxwellwasbornin1831inScotlandtoafamilyofFellowsoftheRoyalSociety,an
eliteorganizationofthetopscientistsofalldisciplinesinGreatBritain(TheRoyalSociety,2011).Maxwell
beganhisacademiccareerquiteearly.HepresentedhisfirstpaperOvalCurvestotheRoyalSocietyof
Edinburghwhenhewasfourteen(Forfar,1995).MaxwellbeganhisundergraduatestudiesatEdinburgh
UniversityatagesixteenandenteredgraduateschoolatCambridgeUniversityatagenineteen.After
graduation,hewasafellowandprofessoratavarietyofcollegesintheUnitedKingdom.Maxwellwas
inductedasaFellowofTheRoyalSocietyofEdinburghwhenhewas25,andpromotedtoaFellowofThe
RoyalSocietyatage30.Afterafruitfulcareer,JamesMaxwellpassedawayattheageof48ofstomach
cancer,whichwasoddlythesamecauseandtimingofhismothersdeathwhenMaxwellwaseightyears
old(Forfar,1995).
In1819,twelveyearsbeforeMaxwellwasborn,thefieldofelectromagnetismwasborn.Hans
ChristianOerstedofDenmarkhadsuspectedthatelectricityandmagnetismwererelatedforsometime
andwasfinallyabletocreatesomeevidenceforit.Inhisexperiment,Oerstedheldawireabovea
compass.Whenacurrenttravelledthroughthewire,henoticedthattheneedleofthecompassmoved.
Thislaunchedaninvestigationofhowelectricityandmagnetismwererelated.AndrAmprewasso
excitedafterlearningofOerstedsdiscoverythathecreatedarough
mathematicaltheorythatdescribeditinoneweekin1820,and
publishedhisfinishedequationin1821(Peters,2000).Ampres
equationbecomesthefourthofMaxwellsfamousequations.Ayear
later,thefirstgalvanometerwasmade,whichwasgreatlybeneficial to
themeasurementofelectriccurrent(TheEncyclopediaAmericana
Corporation,1918).AgalvanometerreliesonthefindingsofOersteds
experiment:whenacurrenttravelsthroughacoil,anearby
magnetizedneedleisdeflected.AswasfoundafterOersted,theneedle Agalvanometer1
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isdeflectedinproportiontothestrengthofthecurrent(FloridaStateUniversity,LosAlamosNational
Laboratory,UniversityofFlorida,2011).
ThemostproductivescientistinelectromagnetismbeforeMaxwellwasMichaelFaraday,whowas
actuallyachemist.Inthecourseofhisresearch,whichlastedfrom1831to1855(TheEncyclopedia
AmericanaCorporation,1918),heproducedover16,000perfectlyorganizedentriesinhislaboratory
notes(Peters,2000).TheresultsofhisexperimentsformedthebasisforMaxwellselectromagnetic
theory.AlthoughFaradayhadanamazingphysicalintuitionthatguidedhimthroughhisexperimentsto
successfullycreateanearlycompletemodelofelectromagneticphenomena,sincehewasnotatalla
mathematician,hewasonlyabletodescribethismodelinwordsanddidnotunifyhisresultsintoatheory
(TheEncyclopediaAmericanaCorporation,1918).Sincethephysicistsofthetimeweremathematically
trained,theyhardlypaidFaradaysresultsanyattention.Theysawthelackofmathematicsasalackof
sophistication(Hunt,1983).
AlthoughFaradaycompletedanamazingnumberofexperiments,therearetwoseriesthatare
mostimportanttoconcretelyunderstandhiscontributionstoMaxwellstheory.Thefirstsetof
experimentsallowedFaradaytodiscovertheprincipleofelectromagneticinduction.Hewoundtwo
separatewiresaroundawoodenbobbinandinsulatedthemfromeachother.Tooneheconnecteda
battery,andtotheotherheconnectedagalvanometer.Hediscoveredthatbybreakingorreconnecting
thecircuitconnectedtothebattery,acurrentoftheoppositedirectionwasinducedintheotherwire.
Thisamazingdiscoveryformsthebasisofanydevicethatuseselectricity,includingtelephones,which
wereinventedlaterinthe19th
century.Faradayalsofoundthatsimplymovingamagnetcloserorfarther
awayfromaclosedcircuitwouldinduceacurrent(TheEncyclopediaAmericanaCorporation,1918).This
discoveryallowedFaradaytodesignandbuildageneratoraswellasthefirsttransformerandthefirst
electricmotor.Whenaskedbyapoliticianwhattheuseofthisworkwas,Faradayrepliedatpresent,I
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dontknow,butonedayyouwillbeabletotaxthem(Peters,2000,p.6).Electromagnetic
inductionisatrulyincrediblephenomenonthatredefinedhowthesourcesofelectricitywere
thoughtof.Thesecondexperimentisactuallyonethatmanyelementaryschoolchildren
carryoutintheirclassrooms.Itinvolvesspreadingironfilingsonasurfaceandsimplyplacing
amagnetonthem.Thiscausestheirontobemovedbythemagneticfieldinapatternthatloopsfrom
oneendofthemagnettotheother.Faradaysawthispatternandthoughtthattheselinesofforce,as
hecalledthem,mustbethekeytoelectromagnetism.Whenheinducedanelectriccurrentinawirewith
amagnet,hesawthislineofforcecurvingaroundthewire,aswenowknowisthecase.
ThefirstpaperthatJamesMaxwellpublishedafterearninghisgraduatedegreewasa75page
analysisofFaradaysconceptoflinesofforcepublishedin1855andwassuitablytitledOnFaradays
LinesofForce(Forfar,1995).MaxwelltranslatedFaradaysideasintomathematics.Maxwellcreated
vectorstodescribethemainplayersofelectromagnetism:E,theelectricfieldintensity,H,themagnetic
fieldintensity,B,themagneticfluxdensity,andItheelectriccurrentdensity.EandHareforcesandBandIarefluxes(linesofforce)producedbytheforces(Peters,2000,p.9).Awaytopicturefluxistoimaginehavingasquareloopofwireinaflowingriver.Thefluxofthevelocityofthewaterwouldbelike
consideringhowmuchwaterwillflowthroughtheloop.Thefluxofanelectricfieldisproportionaltothe
numberofelectricfieldlinesthatgothroughsuchaloop(Sciolla,2004).
NoticethatMaxwellhadtransformedFaradayslinesofforceintomathematicalconceptsthatwestill
usetoday.Fieldsareusedinmanydisciplinesacrossmathematics.ThefirstmentionofafieldinMaxwells
ATreatiseonElectricityandMagnetismwasofanelectricfield,whichhedescribedas
Ironfilings2
Asquareloopofwireinflowingwater3
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theportionofspaceintheneighborhoodofelectrifiedbodies,consideredwithreferenceto
electricphenomena.Itmaybeoccupiedbyairorotherbodies,oritmaybeasocalledvacuum,
fromwhichwehavewithdrawneverysubstancewhichwecanactuponwiththemeansatour
disposal.Ifanelectrifiedbodybeplacedatanypartoftheelectricfielditwill,ingeneral,produce
asensibledisturbanceintheelectrificationoftheotherbodies(Maxwell,1892,pp.4748).
Withthesevectors,Maxwellcreatedcountlessequationstodescribehiselectromagnetictheory(Maxwell,
1892).DespitethepublishingofATreatiseonElectricityandMagnetism,whichwasahandbookstarting
fromsquareoneforeverythingonecouldknowaboutelectromagnetismatMaxwellstime,notmuch
attentionwaspaidtoMaxwellandhisrevolutionaryideas.ItwasnotuntilHeinrichHertzfound
experimentalevidenceforMaxwellsconceptofafieldin1887bydiscoveringelectromagneticwavesin
spacedidthescientificcommunitybegintolookatMaxwellstheory.
BeforeweexplorethebasicequationsofMaxwellstheoryofelectromagnetism,wemustlearn
twooperations.Firstrecallthataderivativeofafunctionfistheproportionofthechangeinf(x)and
thechangeinx: .Theproportioniscalledapartialderivativebecauseitisusedwhenafunctionisofmorethanonevariableandissimplythederivativeofthatfunctionwithrespecttox,
treatingallothervariablesasconstants.Whenafunctionisofmorethanonevariable,suchasf(x,y,z),we
have
.Thedeloperator,,isapseudovectorofthepartialderivativesoff:
andisusedinthetwofollowingoperations(Sciolla,2004).
Thefirstoperationiscalleddivergence,andisdenotedby wherevisa
functionofx,y,andz.Thisoperationusesthedotproduct.Recallthatthedotproductoftwovectors
, , and , , is andyieldsascalar.Thedivergenceofafunctionrepresentshowmuchafunctionspreadsaroundapoint(Sciolla,2004).If,,representsan
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electricfield,thenthedivergenceofthefirstfunctionbelowshowsthatanyelectronsinthefieldare
beingrepelledawayfromtheorigin(Sciolla,2004).
Thesecondoperationiscalledcurl,whichdescribeshowmuchafunctioncurlsaroundapoint,
(Sciolla,2004)andisdenotedby .Anexampleofthecalculationofthecurlofafunctionrepresentingamagneticfield,,isshownbelow.Thedeterminantofthematrixbelowisapseudo
crossproduct: 0
0
0 0 0 0 1 1 2 whichissometimeswrittenas2.
Withtheseoperationsinmind,wearereadytolookattheevolutionofMaxwellsequations.
Divergenceoffunctionsrepresentingelectricfields4
Thecurlofafunctionrepresentingamagneticfield5
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In1861,Maxwellpublishedthebeginningsofhisfamousequations.Itwaswhiledevelopingthese
equationsthathediscoveredthatlightissimilartoelectricityandmagnetism.Sincetheseequationswere
ofadifferentformthanweseethemtoday,tobetterappreciateMaxwellsdistillationofFaradays
theory,wewilllookattheforminwhichmathematicianandphysicistOliverHeavisidewrotethema
coupleofdecadesafterthepublicationofMaxwellstreatise.Twoaremostnotable(Darrigol,2005):
/
/
whereMisthemagneticcurrent,whichdescribeshowamagneticfieldseemstoflowfromonepoleto
theother,jistheelectricalcurrent,and/isthedisplacementcurrent,whichisbasicallyapotentialforelectriccurrenttoflow.Theremainingvariablesaredescribedabove.
Electricpotentialwasanewconceptthatgrewtochangeelectromagnetictheorycompletely.It
wasFaradaythathadfirstthoughtofthisparadigmbutcouldnotexpressitmathematically(Peters,2000).
Maxwellactuallycreatedanewformofderivativeinordertoachievethisexpression:DY/Dtisthe
convectivederivativedefinedbyDY/Dt=Y/t(vY)+v(Y),wherevisthevelocityofthestates(Darrigol,2005,p.2)whichareE,H,D,andB.SinceDiselectricflux,DD/Dtrepresentsvaryingelectric
currentpotential.ThisandDB/DtwereaddedbyMaxwelltotheoriginalequationswrittenbyHeinrich
Lenz(thefirst)andAndrAmpre(thesecond)(Darrigol,2005,p.2).
ATreatiseonElectricityandMagnetism,publishedin1873,recordsthetransformationof
Maxwellscompletetheoryofelectromagnetismintotwelveequations(Maxwell,1892).Itisanamazing
featforonescientisttowritetheequationsthatsummarizedanentirefieldofresearch.Theselftaught
mathematicianandphysicistOliverHeaviside,alsofromGreatBritain,furthercondensedMaxwells
theoryintothefourequationsthataremostknowntoday:(Heaviside,1893).
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0
/
Thefirstequationrelateselectricfluxtothevolumetricchargedensity,.Thesecondequationsaysthatamagneticfielddoesnotdivergebecausetherearenomonopoles(Sciolla,2004).Thethird
equationsummarizestheresultfromFaradayselectromagneticinduction:thevoltageinducedina
circuitwasproportionaltothetimerateofchangeofmagneticfluxthroughthecircuit(Peters,2000,p.
9).ThefourthisfromAmpre:themagneticfieldinaclosedlooparoundacurrentwas,infact,equalto
thecurrentthroughtheloop(Peters,2000,p.9).
Themodernpartialderivativeversionoftheseequationsisverysimilar,butconstantshavebeen
addedtosometermsinordertoconformtostandardsystemsofunit.Herewewilllookattheversionin
cgs(centimeters,grams,seconds cgswasactuallyfirstintroducedayearafterthepublicationof
Maxwellstreatise(WolframResearchProducts,2007)):
4
0
1
4 1
wherecisthespeedoflight(Sciolla,2004).
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Today,wealsosometimesfinditconvenienttouseMaxwellsequationsintheirintegralform.The
followingareinSIunits(Fowler,2009):
/
0
whereqistheelectriccharge,istheelectricconstant,andisthemagneticconstant.Thefirstequationsaysthattheintegraloftheoutgoingelectricfieldoveranareaenclosingavolumeequalsthe
totalchargeinside(Fowler,2009).Thesecondisstillanotherwayofsayingthattherearenomagnetic
monopoles.Thethirdandfourthequationshaveapathintegralthatsimplydenotesintegrationarounda
closedpath.Inthiscase,thatpathisusuallyawire.Theleftsideoftheequationgivesthetotalvoltage
changearoundthecircuit,whichisgeneratedbyavaryingmagneticfieldthreadingthroughthecircuit
(Fowler,2009).Thefourthequationgivesthetotalmagneticforcearoundacircuitintermsofthecurrent
throughthecircuit,plusanyvaryingelectricfieldthroughthecircuit(thatsthedisplacementcurrent)
(Fowler,2009).
Maxwellsequationsareamazingbecausetheysummarizethefundamentalphenomenarelated
toelectromagnetisminfourconciseequations.However,hisequationshadmoreofaneffecton
electromagnetismthanadditionalconvenience.Maxwellextended,connectedandtranslatedFaradays
conceptsofelectromagnetismintoafullfledgedmathematicaltheory,whichputitonthetablefor
comparisontotheleadingelectromagnetictheoryatthetime.Wewillcomparethesetheoriesusingthe
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LeydenJar,adevicestillusedinMaxwellstime,aspictured.The
LeydenJarhastwocoatingsofaconductivemetalthatdonot
toucheachother,oneontheinsideofaninsulatingglass,andthe
otherontheoutside.Theinnercoatingischargedwithametalrod
andthenthemetalrodisremoved.Whenascientisttouchesthe
outsideofthejar,thescientistreceivesanelectricshock(Wolfram
ResearchProducts,2007).Thissurprisedscientistsbecausethey
thoughtonlytheinnercoatinghadbeencharged.Thedifferingexplanationsofthisphenomenon
encompasstheheartofthedifferencebetweenMaxwellstheoryandtheleadingelectromagnetictheory
(TheEncyclopediaAmericanaCorporation,1918).
Thisprevalenttheorywascalledactionatadistance.Thistheoryassumesthatinsulators
completelyblockelectriccurrent.Therefore,thereasonthatachargecouldbereceivedfromtheouter
coatingisthatelectricityworksatadistance,asgravitydoes.Whileelectricitywasbuildinguponthe
innercoating,anelectricityofanoppositekindwasattractedtotheoutercoating.Therefore,when
touched,theoutercoatingcoulddeliverashock(TheEncyclopediaAmericanaCorporation,1918).
MaxwellsawthephenomenonoftheLeydenJarandwasinspiredbyFaradayslinesofforce.
MaxwellexpandeduponFaradaysconceptionofelectricitytoassertthatnotonlydoeselectricitytravel
alonglinesofforce,butalsorequiresamedium.Hesawinsulatorssuchasglassasbeingresistiveto
electriccurrent,notimmunetoit.Hesawtheprocessofchargingtheinnercoatingtobelikedisplacing
electricityinthewaythatyouwoulddisplaceaspringbystretchingitawayfromitsequilibrium.The
resistiveforceoftheglassblockstheelectricityfromreturningtoitsequilibriumuntilthechargingforce
overcomesit.Inthespringexample,thechargingforcewouldbetheforceofthespringandtheresistive
forcewouldbetheforceofthehandsthatarepullingthespringapart.Whentheforceofthespring
TheLe denJar6
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overcomestheforceexertedbythehands,thespringsnapsbackintoequilibrium.ThisishowMaxwell
sawthephenomenonoftheLeydenjar:whenthechargingforceovercomestheresistiveforce,the
electricitysnapsbackintoequilibriumthroughanelectriccurrent(TheEncyclopediaAmericana
Corporation,1918).Thischangeinconceptionwouldgreatlysimplifythemathematicsinvolvedinthe
studyofelectromagnetismaswellasmoreaccuratelyexplainelectromagneticphenomena(Darrigol,
2005).
Itisamazinghowquicklyelectromagnetismdevelopedinsuchashorttimefromitsbeginnings
withOerstedsexperimentin1819.However,thisquickpacewasnotjustcoincidence,butwasrelatedto
theculturalchangesthatGreatBritainwasgoingthroughduringthe19th
century.Duringthemid19th
century,theideathattheprincipleoffreetradewouldbringpeaceandprosperitybegantopercolate
throughEuropeansociety.Forsometime,thedominatingeconomicphilosophyhadbeenmercantilism,
whichwasadoctrineofstrictprotectionofastatesresources,hightariffsontrade,andcolonial
domination.Theseideasofthebenefitsoffreetradeintheintellectualculturehadaplaceinthescientific
cultureaswell.Ratherthanworkingindependentlyononesreputationandtheory,scientistsbegantosee
thebenefitsofdevelopingupontheirpeersideasanddiscussingtherelativemeritofscientifictheoriesas
acommunity(Turner,1980).Maxwellspentthemajorityofhisworkinelectromagnetismsimply
convertingFaradaystheoryintomathematics,butthecombinationoftheirrespectivetalentscreatedan
incrediblecontributiontoscience.ItwasthisideaoffreetradethatmadeMaxwellwanttosharehis
theorywithEurope,notjusttodominatethefield,buttopeacefullyofferacomparisontotheleading
theory(Forfar,1995).Duringthemid19thcentury,theBritishscientificculturealsofullyequatedsocietys
progresswithscientificprogress.Theyhadseensciencecreatethetelegraph,seenthetelegraph
revolutionizecommunicationandEuropeansociety,andseenthescientificadvancementofthetelegraph
continuetorevolutionizesociety.Theysawthepowerofsciencetochangetheworld(Turner,1980). For
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physicistsspecifically,itwasthepowerofthetelegraphthatwasamajormotivationtounderstand
electromagnetismcompletelytoadvancecommunication(Maxwell,1892).
Itwasbecauseofthisbeliefthatscientificadvancementwasimportantforsocietysprogressthat
scientistsbeganwantingtoinfluencepublicdecisionsandhavepublicfunding.However,thepeoplewho
wereinthepublicforefrontatthetime,politiciansandbusinessmen,dismissedtheideathatscientific
workhadanythingtodowiththeirwork.Inanattempttowinsocietyover,manyscientistsbeganto
emphasizehowimportantsciencewastothesecurity,economiccompetency,andstabilityofGreat
Britain.ThisstrategywasdrawingonthefactthatBritishscienceandBritishpoliticsandbusinesshada
commonenemy:Germany.Bythelate19th
century,GermanybecameanindustrialthreattoGreat
Britainseconomicdominance,andwasslowlybecomingamilitarythreataswell.Germanyhadalsobeen
athorninthesidesofBritishscientistsforyears.Germanyspublicfundingandpublicrecognitionoftheir
scientistsresearchhadmadeGermanscienceflourishinthe19thcentury,andwasmuchmoreinfluential
andprosperousthanBritishscience(Turner,1980).Infact,theleadingtheoryofelectromagnetism,
actionatadistance,wasdevelopedinGermanyanditwastheseGermanscientistswholookeddown
uponandrefusedtoacknowledgeMaxwellstheory.Frustratedwiththelackofresponseofthescientific
communitytohispaperOnFaradaysLinesofForce,Maxwelltransformedhiscomprehensivetheory
intotwelveequations,agreatfeatevenfortodayspaceofscientificprogress(Forfar,1995).However,the
Germanscientistsstillrefusedtorelinquishtheirmonopolyonelectromagnetism,andsimplydismissed
Maxwellstheoryassomethingthatjustworksoutonpaper(Turner,1980).OliverHeavisideseditingof
Maxwellsequationsbegantoturnthetide,butitwasnotuntilHeinrichHertz,aGermanphysicist,
demonstratedin1887theexistenceofafieldofelectromagnetismbyfindingelectromagneticwavesin
spacethatGermanscientistsbegantoacceptMaxwellstheory(Hunt,1983).
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Maxwellstheoryofelectromagnetismwasrevolutionary.Itcompletelychangedthestudyof
electromagnetismaswellasEuropeansociety.AfterHertzdiscoveredelectromagneticwavesinspace,the
wirelesstelegraphwasinvented,whichmadefastcommunicationpossibleacrossoceansforthefirsttime.
Maxwellstheoryalsomadelongdistanceelectricalwireseffective,somethingtheactionatadistance
theorywasnotabletodo(Hunt,1983).Stilltoday,weoweanenormousamountoftechnologyto
Maxwellstheoryofelectromagnetismandhisperseverancethroughculturalobstaclestoadvocatehis
talentofmathematicalinterpretation.
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WolframResearchProducts.(2007).cgs.RetrievedDecember7,2011,fromEricWeisstein'sWorldof
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Endnotes1. http://hyperphysics.phyastr.gsu.edu/hbase/magnetic/galvan.html2.
http://www.cyberphysics.co.uk/My_Notes/Magnets1.htm
3. http://ocw.mit.edu/courses/physics/8022physicsiielectricityandmagnetismfall2004/lecturenotes/lecture2.pdf
4. http://ocw.mit.edu/courses/physics/8022physicsiielectricityandmagnetismfall2004/lecturenotes/lecture1.pdf
5. http://ocw.mit.edu/courses/physics/8022physicsiielectricityandmagnetismfall2004/lecturenotes/lecture1.pdf
6. http://electronics.howstuffworks.com/capacitor.htm/printable