信息科学与电子工程专业英语Lecture03Unit3电磁场,天线和微波EMFields,AntennaandMicrowavesNewWordsTheelectromagneticfieldisaphysicalfieldproducedbyelectricallychargedobjects.Itaffectsthebehaviorofchargedobjectsinthevicinityofthefield.Theelectromagneticfieldextendsindefinitelythroughoutspaceanddescribestheelectromagneticinteraction.Itisoneofthefourfundamentalforcesinthenature(theothersaregravitation,theweakinteraction,andthestronginteraction)Thefieldcanbeviewedasthecombinationofanelectricfieldandamagneticfield.Theelectricfieldisproducedbystationarycharges,andthemagneticfieldbymovingcharges(currents);thesetwoareoftendescribedasthesourcesofthefield.ThewayinwhichchargesandcurrentsinteractwiththeelectromagneticfieldisdescribedbyMaxwell’sequationsandtheLorentzforcelaw.Fromaclassicalpointofview,theelectromagneticfieldcanberegardedasasmooth,continuousfield,propagatedinawavelikemanner,whereasfromaquantummechanicalpointofview,thefieldcanbeviewedasbeingcomposedofphotons.Theelectromagneticfieldmaybeviewedintwodistinctways.Continuousstructure:Classically,electricandmagneticfieldsarethoughtofasbeingproducedbysmoothmotionsofchargedobjects.Forexample,oscillatingchargesproduceelectricandmagneticfieldsthatmaybeviewedina“smooth”,continuous,wavelikemanner.Inthiscase,energyisviewedasbeingtransferredcontinuouslythroughtheelectromagneticfieldbetweenanytwolocations.Forinstance,themetalatomsinaradiotransmitterappeartotransferenergycontinuously.Thisviewisusefultoacertainextent(radiationoflowfrequency),butproblemsarefoundathighfrequencies(seeultravioletcatastrophe).Thisproblemleadstoanotherview.大家应该也有点累了，稍作休息Discretestructure:Theelectromagneticfieldmaybethoughtofinamore“coarse”way.Experimentsrevealthatelectromagneticenergytransferisbetterdescribedasbeingcarriedawayinphotonswithafixedfrequency.Planck’srelationlinkstheenergyEofaphotontoitsfrequencynthroughtheequation:E=hnwherehisPlanck’sconstant,namedinhonorofMaxPlanck,andnisthefrequencyofthephoton.Forexample,inthephotoelectriceffect–theemissionofelectronsfrommetallic