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1、37THEFOUNDATIONSOFMODERNPHYSICSConceptualQuestions37.1.(a)Cathoderaysaredeflectedbyamagneticfieldasiftheyarenegativelychargedparticles.Cathodesmadeofanymetalproducecathoderays,suggestingtheraysareafundamentalentity.Iftheyareparticles,thatwouldexplainthedeflectionbyamagneticfield,themotionaway
2、fromotherchargedparticles,andthefluorescencewheretheystrikeglasstubes.(b)Theycouldtravelinastraightlinefor150timesthemeanfreepathofknownparticles.Theycouldpenetratethinfoilsinthesamewaythatradiowavesareknowntopenetratethinfoils.Wavesandcathoderaysbothcancarryenergyandmomentumandalsocausefluor
3、escence.37.2.Theforceduetogravityisverymuchsmallerthantheelectricandmagneticforcesbecauseoftheverysmallmassofthecathode-rayparticles.37.3.Itwasconclusiveproofthatcathoderaysarenegativelycharged.37.4.Cathoderaysemittedbyvarioushotcathodeshavethesamecharge-to-massratioindependentofthematerialfr
4、omwhichthecathodeismade.37.5.(a)Theatomicnumberofanelement,representedbyZ,isthenumberofprotonsinthenucleus.TheatomicnumberofhydrogenisZ=1,heliumisZ=2,andlithiumisZ=3.However,themassesofthesethreeelementsdonotprogressinthislinearmanner.Themassofheliumisfourtimesthemassofhydrogenandthemassoflit
5、hiumisseventimesthemassofhydrogen.IfanucleuscontainsonlyZprotons,thenheliumshouldbejusttwiceasmassiveashydrogenandlithiumthreetimesasmassiveashydrogen.Sotheremustbesomethingelseinthenucleus.(b)ThomsonandhisstudentAstonfoundthatmanyelementsconsistofatomsofdifferingmass.Forexample,neon(Z=10)ato
6、msarefoundtohaveintegralmasses20u,21u,and22u.Potassiumatoms(Z=19)arefoundtohaveintegralmasses39u,40u,and41u.Becausethemassesofthedifferentisotopesdifferby≈1u,themassofneutronmustbe≈1u,thesameasthemassofaproton.37.6.Equation37.6showsthattheradiusrofcurvaturethroughwhichachargedparticleisdeflec
7、tedbyamagneticfieldismvr=qBIfthefieldBandthevelocityvoftheparticlesisthesame,thentheradiusdependsonlyonthemass-to-chargeratiom/q.ThealphaparticleisadoublyionizedHenucleus,soitschargeis+2eanditsmassisapproximately4u.The10−3u.cathode-rayparticl
