M38_KNIG7655_03_ITG_CH38

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1、CHAPTER38QuantizationRecommendedclassdays:3BackgroundInformationFromapedagogicalperspective,thephotoelectriceffectisthecriticalexperimentthatpointsthewaytowardquanta.AlthoughPlanckandtheblackbodyspectrumhavehistoricalprecedence,thisstorycannotbepresentedinameaningfulwaytointroduct

2、orystudents.Notonlyarestudentsunsurehowtointerpretacontinuousspectrum,theydon’thavetheknowledgeorbackgroundtounderstandhowthisspectrumwasacrisisforclassicalphysics.Thephotoelectriceffectexperiment,bycontrast,providesastraightforwarddistinctionbetweenclassicalandquantumideas.Hencei

3、tisafocusofthischapter.Thephotoelectriceffectisoneofthefewaspectsofquantumphysicsonwhichtherehasbeenareasonableamountofresearch(Steinberg,Oberem,andMcDermott,1996).Intheirstudyofsophomorestudentsinamodernphysicsclass,theresearchersfoundthatmoststudentshadsignificantconceptualdiffi

4、cultieswiththephotoelectriceffectitselfandwiththephotonmodelusedtoexplaintheresults.Usinginformalsurveys,I’vefoundthatonlyasmallfractionofstudentsinamodernphysicsclassunderstandsthesignificanceortheimplicationsofthephotoelectriceffect.Whenaskedonanexamtoexplainhowthephotoelectrice

5、ffectwasinconsistentwithclassicalphysics,themajorityofstudentswrotethatthemereexistenceofthephotoelectriceffectviolatedclassicalphysics.Onlyaverysmallminoritycouldarticulatehowthephotonmodelsucceedswheretheclassicalmodelfails.Threeissuesthatcauseparticulartroubleare:•Notunderstand

6、ingtheexperimentitself—howitworksorwhatismeasured.•Notunderstandingthebasicideasofthephotonmodeloflight.•Notbeingabletouseeithertheclassicalorthephotonmodeltoreasonabouthowtheexperimentalresultswouldchangeifsomeparameteroftheexperimentisvaried.38-1Theusualpresentationofthephotoele

7、ctriceffectexperimentassumesthatthestudentsunderstandbasiccircuitsandtheuseofpotentialdifference.Asearlierchaptersinthisguidebookhavedocumented,manystudentswhohavecompletedconventionalinstructioncannotreasonwithoraboutpotentialdifference.Forexample,mostcannotcorrectlydeterminethep

8、otentialdifferenceacrossagapinaci