电子信息与通信工程专业英语lesson4-1.ppt

Chapter4ElectromagneticFieldandWave Outline4 1ElectromagneticFieldsTheory4 2Antenna 4 1ElectromagneticFieldsTheory OutlineMaxwell sequationsCurrentcontinuityequationConstitutiverelationsBoundaryconditionsPowerandenergy Terminology vectorn 数 向量 矢量 带菌者vt 无线电导引scalarn 数量 标量adj 分等级的 数量的 标量的positionvectorn 位矢 方位向量 位置矢量fluxn 物 流量 通量 涨潮 变迁vi 熔化 流出vt 使熔融 用焊剂处理contourn 轮廓 周线 等高线emfabbr 电动势 electromotiveforce Stockes theoremn 斯托克斯定理divergencetheoremn 散度定理conductioncurrentn 传导电流displacementcurrentn 位移电流 solenoidalfieldsn 螺线管磁场 管形场continuityequationn 连续性方程constitutiverelationn 本构关系isotropicadj 等方向性的 各向同性的dielectricconstantn 介电常数relativepermeabilityn 相对磁导率conductivityn 传导性 传导率anisotropicadj 各向异性的tensorn 张量 张肌 In1864 JamesClerkMaxwellproposedoneofthemostsuccessfultheoriesinthehistoryofscience InafamousmemoirtotheRoyalSociety hepresentednineequationssummarizingallknownlawsonelectricityandmagnetism Bypostulatingtheneedforanadditionaltermtomakethesetofequationsself consistent Maxwellwasabletoputforthwhatisstillconsideredacompletetheoryofmacroscopicelectromagnetism Toformacompletefieldtheoryweneedasourcefield amediatingfield andasetoffielddifferentialequations Thisallowsustomathematicallydescribetherelationshipbetweeneffect themediatingfield andcause thesourcefield Introduction Texttour InMaxwell sequations thesourcefieldconsistsofthevectorfield thecurrentdensity andthescalarfield thechargedensity themediatingfieldistheelectromagneticfieldconsistingofthesetoffourvectorfields and electricfieldintensity vector volts meter V m magneticfieldintensity vector ampere meter A m electricfluxdensity vector coulombs meter2 C m2 magneticfluxdensity vector webers meter2 Wb m2 electriccurrentdensity vector amperes meter2 A m2 electricchargedensity scalar coulombs meter3 C m3 Alltheabovequantities intime varyingfields arearbitraryrealfunctionsofspace i e thepositionvectorr meters andtimet seconds whiletheyarefunctionsofjustrinthestaticfields Twonewconceptsshouldbebuilt theelectricfieldcanbeproducedbychangingmagneticfieldandthemagneticfieldproducedbychangingelectricfield Sowecallit electromagneticfield Maxwell sequations Itsleftsideistheclosed looporlineintegral theintegralalongtheclosed pathline ofamagneticfieldstrength Itsrightsideisthesurfaceintegralofthetotalcurrentdensity Thislawstatesthattheemf inducedintheclosedpathbyachangingmagneticfieldisequaltothetimerateofdecreaseofthemagneticfluxlinkingtheloop Thelawsaysthatthemagneticfluxthroughanyclosedsurfaceisequaltozero ThelawtellsusthattheoutwardfluxofdensityDthroughaclosedsurfaceisequaltothechargeenclosedwithinthesurface ThedifferentialformsofMaxwell sequationsarelistedbelow Thesefourequationsexplainthetotalrelationshipsbetweenelectromagneticsourcesandthefields Theconductioncurrentandthedisplacementcurrentarethecurlsourcesofthemagneticfields Timevaryingmagneticfieldproducinganelectricfield Themagneticfieldsaresolenoidalfields Theelectricchargesarethedivergencesourcesoftheelectricfields Althoughitisn tgiveninMaxwell sequations wecangetitfromMaxwell sequations Theequationofcontinuitycanbeconcluded Currentcontinuityequation Constitutiverelations Toallowauniquedeterminationofthefieldvectors Maxwell sequationsmustbesupplementedbyrelationsdescribingthebehaviorofthemediumundertheinfluenceofthefield Thesesubsidiaryrelationsarecalledconstitutiverelationsthatareestablishedbyexperimentationordeducedfromatomictheory Ifthefieldvectorsarelinearlyrelatedsothattheprincipleofsuperpositionapplies themediumissaidtobelinear AcombinationofMaxwell sequationsandlinearconstitutiverelationsformsthebasisoflinearelectrodynamics Amediumissaidtobeisotropiciftheelectricalandmagneticpropertiesatagivenpointareindependentofthedirectionofthefieldatthepoint If andhavethesamevaluesateverypointinspace themediumissaidtobehomogeneous Ontheotherhand iftheelectricalandmagneticpropertiesofamediumdependuponthedirectionsoffieldvectors themediumiscalledanisotropic Boundaryconditions InordertocontinuethesolutionofMaxwell equationsfromoneregiontoanothersothattheresultingsolutionisuniqueandvalideverywhere weneedboundaryconditionstoimposeonthefieldvectorsandtheinterface TheboundaryconditionsbetweenthesetwosetsoffieldsattheinterfacecanbederivedfromtheintegralformofMaxwell equations Denotetheunitvectorofasthenormaldirectionfrommedium1tomedium2andtakethesurfaceintegralinMaxwell sequationsovertheclosedsurfaceofasmallcylinder Thecontributionfromthecurvedsurfaceofthecylinderisdirectlyproportionalto and forasufficientlysmallcross sectionalareaofthecylinder maybeconstantovereachend Inthelimitas theendsofthecylinderliejustoneithersideofandthecontributionfromthecurvedsurfacebecomesvanishinglysmall Asaspecialcase ifmedium1isaperfectelectricconductor surfaceconductionelectriccurrentandchargedensitiesmayexist Notethatthesurfaceelectricchargedensityandelectriccurrentdensityarealsogovernedbythecontinuityequation Powerandenergy ThenbyperformingavolumeintegraloverVboundedbytheclosedsurfaceandGausstheorem weobtainthefollowingrelation Therefore thisequationisnothingbuttheconservationofenergywithinavolumeVandisknownastheintegralformofPoyntingtheorem Letusassumethatintheremotepast thereisnostoredenergybecausethefieldsarezero PoyntingtheoremItisapowertheorem Thetimerateofincreaseoftheelectromagneticenergyoveranarbitraryvolumeisequaltot