电磁场与电磁波第8讲边界条件电容静电能量-wb课件.ppt

《电磁场与电磁波第8讲边界条件电容静电能量-wb课件.ppt》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、FieldandWaveElectromagnetics电磁场与电磁波2012.3.1311.ElectricPotentialReview22.ConductorsinStaticElectricField3.DielectricsinStaticElectricFieldInsideaconductor(understaticconditions)BoundaryConditions(ataConductor/FreeSpaceInterface)34.ElectricFluxDensityandDielectricConstant4Maintopic1.BoundaryCon

2、ditionsforElectrostaticFields2.CapacitanceandCapacitors3.ElectrostaticEnergy51.BoundaryConditionsforElectrostaticFieldsElectromagneticproblemsofteninvolvemediawithdifferentphysicalpropertiesandrequiretheknowledgeoftherelationsofthefieldquantitiesataninterfacebetweentwomedia.Forinstance,wemaywi

3、shtodeterminehowtheEandDvectorschangeincrossinganinterface.wenowconsideraninterfacebetweentwogeneralmedia6(1)thetangentialcomponentofELetusconstructasmallpathabcdawithsidesabandcdinmedia1and2respectively,bothbeingparalleltotheinterfaceandequaltow.Theintegralformisassumedtobevalidforregionscon

4、tainingdiscontinuousmedia,isappliedtothispath.Ifweletsidesbc=da=happroachzero,theircontributionstothelineintegralofEaroundthepathcanbeneglected.ThuswehaveE1E221atwhacdban27Thetangentialcomponentsoftheelectricfieldintensitiesinbothsidesoftheinterfacebetweentwodielectricsareequal.Orsay,thet

5、angentialcomponentsoftheelectricfieldintensitiesarecontinuous.Forlinearisotropicdielectrics,wehaveWherethereferenceunitnormalan2isourwardfrommedium②to①.8(2)thenormalcomponentofDInordertofindarelationbetweenthenormalcomponentsofthefieldsataboundary,weconstructasmallpillboxwithitstopfaceinmedium

6、1andbottomfaceinmedium2.ThefaceshaveanareaS,andtheheightofthepillboxhisvanishinglysmall.ApplyingGauss’slawtothepillbox,wehavehS21an2D1D2s9Thedirectionofthenormaltotheboundaryisspecifiedasthatfromdielectric②to①.whereSisthesurfacedensityofthefreechargeattheboundary.Theaboveequationstates

7、thatthenormalcomponentofDfieldisdiscontinuousacrossaninterfacewhereasurfacechargeexists--theamountofdiscontinuitybeingequaltothesurfacechargedensity.10讨论(1)、BoundaryConditionsforDielectric-conductorInterface讨论(2)、Intheabsenceofnetsurfac