M34_KNIG7655_00_ITG_CH34

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1、CHAPTER34ElectromagneticFieldsandWavesRecommendedclassdays:3BackgroundInformationThere’slittleresearchonstudentdifficultieswiththemoretechnicalandmathematicalaspectsofelectromagnetism.Inaninvestigationoflightwaves,McDermott’sgroupattheUniversityofWashingtonfoundthatstudentsofteninterpreta“picture

2、”ofalightwaveasifthewavehasspatialextent,likeawaterwave.Thusalightwavethatis“toobig”foranapertureistruncatedasitpassesthrough.Ifyouaskwhetherornotitispossibleforalightwavewithwavelengthλtopassthroughanapertureofwidtha<λ,manystudentsreply,“No,becauseitwon’tfitthroughtheopeningifλislessthana.”These

3、responsessuggestthatmoststudentsdonothaveacorrectmentalimageofanelectromagneticwaveandcannotinterpretthestandardtextbookpictureofatransverseelectromagneticwave.Astudyofstudents’organizationofknowledgeinthedomainofelectromagnetism(BagnoandEylon,1997)foundthatmoststudents—attheendofinstruction—coul

4、dnotidentifythemainideasofelectromagnetism(alargefractionidentifiedOhm’slawasoneofthemostimportantideas),norcouldtheyidentifywhattheprimaryequations(Maxwell’sequationsandtheLorentzforcelaw)tellus.Whilestudentsmaybecomeadeptatmanipulatingequations,formostitisamathematical“game”withlittleornoconnec

5、tiontophysicalphenomena.34-1StudentLearningObjectives•Tounderstandthatelectricandmagneticfieldsareinterdependent.There’sjustasingleddelectromagneticfieldthatpresentsdifferentfaces,intermsofEandB,todifferentobservers.•Electromagneticfieldsobeyfourgenerallaws,calledMaxwell’sequations.•Electromagnet

6、icfieldscanexistwithoutsourcechargesorcurrentsintheformofaself-sustainingelectromagneticwave.•Maxwell’sequationspredictthatallelectromagneticwavestravelatthesamespeed.•Electromagneticwavescanbepolarized.PedagogicalApproachChapter34assumesthatyou’vecoveredGauss’slawandAmpère’slaw.Muchofthischapter

7、isfairlystandard.Earlierchaptersemphasizedelectricandmagneticphenomenaandaconceptualunderstandingofelectricandmagneticfields.Nowit’stimeformathematicalsophistication.Theonenon-standardtopicinthischapteristheGalileanfie