电磁场与电磁波第6讲基本假设库仑定律高斯定理电位-wb

FieldandWaveElectromagnetics电磁场与电磁波2012.3.611.ProductsofVectors2.OrthogonalCoordinateSystemsReviewCartesianCoordinatesPositionvector:ArbitraryVectorA:2Dotproduct:Crossproduct:Differentiallength:Differentialvolume:Differentialsurface:33.GradientofaScalarField4.DivergenceofaVectorField5.DivergenceTheorem46.CurlofaVectorField7.Stokes’sTheorem8.TwoNullIdentitiesifthenifthen59.Helmholtz’sTheoremHelmholtz’sTheorem:Avectorfield(vectorpointfunction)isdeterminedtowithinanadditiveconstantifbothitsdivergenceanditscurlarespecifiedeverywhere.6MaintopicStaticElectricFields1.FundamentalPostulatesofElectrostaticsinFreeSpace2.Coulomb’sLaw3.Gauss’sLawandApplications4.

ElectricPotential

71.FundamentalPostulatesofElectrostaticsinFreeSpace1.1.ElectricfieldintensityElectircfieldintensityisdefinedastheforce

perunitchargethataverysmallstationarytestchargeexperienceswhenitisplacedinaregionwhereanelectricfieldexists.Thatis,TheelectricfieldintensityEis,thenproportionaltoandinthedirectionoftheforceF.IfFismeasuredinnewtons(N)andchargeqincoulombs(C),thenEisinnewtonspercoulomb(N/C),

whichisthesameasvoltspermeter(V/m).AninverserelationofaboveEq.givestheforceFonastationarychargeqinanelectricfieldE:81.2.FundamentalPostulatesThetwofundamentalpostulatesofelectrostaticsinfreespacespecifythedivergenceandcurlofE.Theyare

isthevolumechargedensityoffreecharges(C/m3),and0

isthepermittivityoffreespace,auniversalconstant.Equationassertsthatstaticelectricfieldsareirrotational(conservative)andimpliesthatastaticelectricfieldisnotsolenoidalunless=0.Differentialform9DivergenceTheoremWhereQisthetotalchargecontainedinvolumeVboundedbysurfaceS.EquationisaformofGauss’slaw,whichstatesthatthetotaloutwardfluxoftheelectricfieldintensityoveranyclosedsurfaceinfreespaceisequaltothetotalchargeenclosedinthesurfacedividedby0.

Stokes’sTheoremwhichassertsthatthescalarlineintegralofthestaticelectricfieldintensityaroundanyclosedpathvanishes.Thescalarproductintegratedoveranypathisthevoltagealongthatpath.ThisEq.isanexpressionofKirchhoff’svoltagelawincircuittheorythatthealgebraicsumofvoltagedropsaroundanyclosedcircuitiszero.101.2.FundamentalPostulatesDifferentialformIntegralformPostulatesofelectrostaticsinfreespace112.Coulomb’sLaw2.1.Electricfieldduetoapointcharge12Example3-1p78场点P

(x,y,z)y源点Q(x’,y’,z’)zxO132.2Coulomb’sLawWhenapointchargeq2isplacedinthefieldofanotherpointcharge

q1attheorigin,aforceF12isexperiencedbyq2duetoelectricfieldintensityE12ofq1atq2.wehave2.3ElectricfieldduetoasystemofdiscretechargesSinceelectricfieldintensityisalinearfunctionof(proportionalto)aRq/R2,theprincipleofsuperpositionapplies,andthetotalEfieldatapointisthevectorsumofthefieldscausedbyalltheindividualcharges.WecanwritetheelectricintensityatafieldpointwhosepositionvectorisRas14Letusconsiderthesimplecaseofanelectricdipolethatconsistsofapairofequalandoppositecharges+qand–q,separatedbyasmalldistance,d,asshowninFig.Electricdipolemoment,p:15电偶极子的电场线和等位线16PV’2.4ElectricfieldduetoacontinuousdistributionofchargesTheelectricfieldcausedbyacontinuousdistributionofchargecanbeobtainedbyintegrating(superposing)thecontributionofan

elementofchargeoverthechargedistribution.dv’R(V/m)17Example3-4p85-87183.Gauss’sLawandApplicationsGauss’slawassertsthatthetotaloutwardfluxoftheelectricfieldintensityoveranyclosedsurfaceinfreespaceisequaltothetotalchargeenclosedinthesurfacedividedby0.Gauss’slawisparticularlyusefulindeterminingtheE-fieldofchargedistributionswithsomesymmetryconditions,suchthatthenormalcomponentoftheelectricfieldintensityisconstantoveranenclosedsurface.TheessenceofapplyingGauss’slawliesfirstintherecognitionofsymmetryconditionsandsecondinthesuitablechoiceofasurfaceoverwhichthenormalcomponentofEresultingfromagivenchargedistributionisaconstant.SuchasurfaceisreferredtoasaGaussiansurface.1920Example3-5p8821xzyr21rO例4求长度为L，线密度为的均匀线分布电荷的电场强度。

令圆柱坐标系的z轴与线电荷的长度方位一致，且中点为坐标原点。由于结构旋转对称，场强与方位角

无关。因为电场强度的方向无法判断，不能应用高斯定律，必须直接求积。22

因场量与无关，为了方便起见，可令观察点P

位于yz平面，即，那么xzyr21rO考虑到23求得当长度L

时，1

0，2，则24Example3-6p8925Example3-7p9026274.ElectricPotentialWewanttomaketwomorepointsaboutEq.First,theinclusionofthenegativesignisnecessaryinordertoconformwiththeconventionthatingoingagainsttheEfieldtheelectricpotential

Vincreases.Second,whenwedefinedthegradientofascalarfield,thatthedirectionofVisnormaltothesurfacesofconstantV.hencethefieldlinesorstreamlinesareeverywhere

perpendiculartoequipotentiallinesandequipotentialsurfaces.28Electricpotentialdoeshavephysicalsignificance,anditisrelatedtotheworkdoneincarryingachargefromonepointtoanother.Aswedefinedtheelectricfieldintensityastheforceactingonaunittestcharge.Therefore,inmovingaunitchargefrompointP1

topointP2

inanelectricfield,workmustbedoneagainstthefield

andisequaltoAnalogoustotheconceptofpotentialenergyinmechanics,AboveequationrepresentsthedifferenceinelectricpotentialenergyofaunitchargebetweenpointP2andpointP1.

DenotingtheelectricpotentialenergyperunitchargebyV.theelectricpotential,wehave29Wehavedefinedapotentialdifference(electrostaticvoltage)betweenpointsP2andP1.Itmakesnomoresensetotalkabouttheabsolutepotentialofapointthanabouttheabsolutephaseofphasorortheabsolutealtitudeofageographicallocation;areferencezero-potentialpoint,areferencezero(usuallyatt=0),orareferencezeroaltitude(usuallyatsealevel)mustfirstbespecified.Inmost(butnotall)casesthezero-potentialpointistakenatinfinity.Whenthereferencezero-potentialpointisnotatinfinity,itshouldbespecificallystated.30ElectricPotentialduetoaChargeDistributionForasystemofndiscretepointchargesq1,q2,…,qnTheelectricpotentialdueto

onepointcharge31ForavolumechargedistributionForasurfacechargedistribution

Foralinechargedistribution

Asanexample,letusagainconsideranelectricdipoleconsistingofcharges+qand–qwithasmallseparationd.Calculatetheelectricfieldintensityproducedbytheelectricdipole.32TheelectricpotentialatPproducedbyanelectricdipolecanbewrittendowndirectly:Solution:33Makeatwo-dimensionalsketchoftheequipotentiallinesandtheelectricfieldlinesforanelectricdipole.34Example3-9P98-9935TheprecedingexampleillustratestheprocedurefordeterminingEbyfirstfindingVwhenGauss’slawcannotbeconvenientlyapplied.However,weemphasizethatifsymmetryconditionsexistsuchthataGaussiansurfacecanbeconstructedoverwhichE·dsisconstant,itisalwayseasiertodetermineEdirectly.ThepotentialV,if