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Physics–Module3–IdeastoImplementation 1. Increasedunderstandingsofcathoderaysledtothedevelopmentoftelevision Explain that the cathode ray tubes allowed the manipulation of a stream of charged particles CathodeRay:Areastreamofelectronsthatareobservablewhenalargepotentialdifferenceisapplied acrossahighlyevacuatedglasstube(cathoderaytubes)–achievedthroughavacuumpump o Theglassmustbehighlyevacuatedwithlittleobstructionfromthefewremaininggasparticles o Atlowerpressures,electronscanacceleratetofasterspeedbeforecollidingwithglassparticles o Whensignificantamountsofgasarepresent,electroncollidewithgasparticleswhichare ionizedcausingthegastofluoresceresultinginstriationpatterns Itwasfoundthatwhenalargepotentialdifferencewasappliedbetweenthetwoelectrodeswhichare sealedinsideaglasstube,andthepressureisreducedsufficiently,atransportofelectricitytakesplace andagreenishglowisobservedontheglassoppositethecathode(thenegativeelectrode) • Thegreenishglowiscausedbyelectronscollidingwithatoms.Whenanelectroncollideswithatoms,an electronfromtheglassatomisblastedintoanexcitedenergystate.Thisstate,however,isunstableso theelectronspiralbackintolowerenergylevels.Energyintheformoflighthastobegivenupinorderto theelectrontodoso,givingrisetothegreenishglow • Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether they were charged particles or electromagnetic waves PADDLEWHEELEXPERIMENT • • • • Observation:Wheelmovedonceraysstartedoperating Showedelectronshavemomentum,aspaddleispropelledbytheelectron(p=mv) Thus,cathoderayshavemass ChargedParticle MAGNET • • Weredeflected/bentinthepresenceofamagneticfield(inaccordancetoLHGrule) ChargedParticle MALTESECROSSEXPERIMENTS • • • Observation:Shadowwithsharpedgesbehindanode&rayswereblockedbymetal Showedcathoderayspropagatedinstraightlines,startedfromcathode&cantpenetratemetal Waves • • Showedthatelectronsweredeflectedtowardspositiveelectrode ChargedParticle(negative) ELECTRICPLATES CHARGEDPARTICLESvs.WAVES Cathoderaysareparticlesasoftoday.Howeverearlyexperimentsprovidedinconsistentresults, suggestingbothparticleandwavelikenature EvidenceforParticleNature: → Suggestedbythefactthatcathoderaysweredeflectedbyamagnet → Couldmoveapaddlewheelimplyingtheyhadmomentumandhencemass → Travelledconsiderablyslowerthanlight → PerrinfurthershowedthattheCathodeRaytubeshadchargeusinganelectroscopeattachedto aFaradayCylinder EvidenceforWave-LikeNature: → Propagatedinstraightlines:ashadowofanopaqueobjectappearedifplaceinray’spath(Malt.) → Whenhitfluoroscreen,thescreenemittedlight.Thusittravelsinstraightlines&haveenergy → Couldpassthroughthinmetalfoilswithoutdamagingit(unlikeordinaryparticles)–Rutherford testedwithgoldfoil → ErroneousexperimentofHertz:failedtoshowthatcathoderaysweredeflectedbyelectric plates. o EMwaveswereunaffectedbyelectricfields,leadingtotheinferencecathodewereEM o HoweverHertzcriticalerrorwasthatheplacedtheelectricplateoutsidetheglasstube, andwithintheglasstube,theCRionisedthegas.Theseionswereattractedtotheopp. plates,effectivelyneutralisingthechargesuchthatCRcouldpassthruunaffected ThedebatewasfinallysilencedwhentheAmericanPhysicistJ.J.Thompsonmeasurecharge-to-massratio ofcathoderays(anelectricfieldcandeflectchargedparticles) NB:Shouldstayatleast1mawayfromCRTastheycancauseionisationoftissues,leadingtocancers • Identify that charged particles produce an electric field Anelectricfieldexistsinanyregioninwhichanelectricallychargedobjectexperiencesaforce.The observationthatchargedplatesexertaforceonotherobjectsbroughtclosetothemindicatesacharged particlewillproducearadialelectricalfieldinthesurroundingregion.Thestrengthofthefield producedisinverselyproportionaltothesquareofthedistance.Greaterdensityoffieldlinesàstronger ELECTRICFIELDDUETOCHARGEDPARTICLES • • Apairofoppositelychargedparticlesseparatedbysmalldistancewillproduceauniformelectricfield Aparticleexperiencethesameforceanywhereinauniformfieldasthedensityofthefieldlineisthe samethroughout Describe quantitatively the force acting on a charge moving through an electric field F = Eq Achargedparticleinanelectricalfieldwillexperienceaforce.Thesizeoftheelectricalforceisgivenby: • • Identify that moving charged particles in a magnetic field experiences a force and describe quantitatively the force acting on a charge through a magnetic field F = qvbsinθ • Ifaparticlewithchargeqismovingwithvelocityv,perpendicularlytoamagneticfieldof strengthB,theparticlewillexperienceamagneticforceF,givenbyF=qvB(V=W/q) Iftheparticle’sdirectionofmotionisatanangletothemagneticfield,then: • Forelectrons,usetheLeftHandGripruletofinddirection Thetaisanglebetweenpathofparticleandmagneticfield Debate:answernoteasytofindastheparticlesaresmallanddifficulttodetect