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1、38QUANTIZATIONConceptualQuestions38.1.(a)Theultravioletlightconsistsofbundlesofenergythatareabsorbedbytheelectronsontheelectroscope.Thefrequencyofthesebundlesofenergyisenoughtoallowtheescapeofelectronsfromthesurfaceoftheelectroscope.(b)Theultravioletlightdoes,infact,ca
2、useafewelectronstobeemittedfromanunchargedelectroscope.Butoncetheelectroscopehasacquiredevenaverytinypositivecharge,furtherphotoelectrons(negativelycharged)cannotescapetheelectricforcethatpullsthembacktothepositiveelectroscope.Thisislikeprojectilesbeinglaunchedfromthee
3、arth’ssurfacewithlessthantheescapespeed.Theverysmallpositivechargethattheelectroscopeisabletoacquireisnotenoughtodeflecttheleavesandthushasnonoticeableeffect.38.2.(a)WhenΔ>V0,alltheemittedelectronsareattractedtoandcollectedbytheanode.Thismeansafurtherincreaseinthevolta
4、gecannotchangethenumberofelectronsarrivingpersecondandthuscannotincreasethecurrent.(b)TheworkfunctionE0istheminimumenergyanelectronneedstoescapefromthemetal.Someelectrons,suchasthoseabitfartherfromthesurface,needmorethanE0toescape.Thereisarangeofescapeenergies,sotheesc
5、apedelectronshavearangeofkineticenergiesandnotasinglekineticenergy.(c)IftheanodepotentialisV,anelectronleavingthecathodewithkineticenergyKarrivesattheanodewithkineticenergyK′=+KeV.AnegativeVcausesadecreaseinkineticenergy.K′cannotbecomenegative,soforeV≤−Ktheelectronisre
6、pelledbytheanodeandturnedbacktowardthecathode.TheemittedelectronshaveamaximumkineticenergyKmax.WheneV=−Kmax,allelectronsareturnedbackandthecurrentdropstozero.IfthecurrentreacheszeroatVV=−stop,thenVKestop=max/.Thestoppingvoltagemeasuresthemaximumkineticenergybycausingth
7、emostenergeticelectrons,thosewithKK=max,tobeturnedbackfromtheanode.38.3.Accordingtoclassicalphysics,thephotoelectriccurrentisnotdependentonthelight’sfrequency.Ifthelightintensityremainsconstant(sameenergypersecondfallingonthemetal),thephotoelectriccurrentshouldbeconsta
8、ntwithnothresholdasthefrequencyischanged.Thus,thegraphwouldbeahorizontalline,startingfromf=0Hz.©Copyright2013PearsonE
