Download Module 3 – Ideas to Implementation

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)
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Thegreenishglowiscausedbyelectronscollidingwithatoms.Whenanelectroncollideswithatoms,an
electronfromtheglassatomisblastedintoanexcitedenergystate.Thisstate,however,isunstableso
theelectronspiralbackintolowerenergylevels.Energyintheformoflighthastobegivenupinorderto
theelectrontodoso,givingrisetothegreenishglow
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Explain why the apparent inconsistent behaviour of cathode rays
caused debate as to whether they were charged particles or
electromagnetic waves
PADDLEWHEELEXPERIMENT
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Observation:Wheelmovedonceraysstartedoperating
Showedelectronshavemomentum,aspaddleispropelledbytheelectron(p=mv)
Thus,cathoderayshavemass
ChargedParticle
MAGNET
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Weredeflected/bentinthepresenceofamagneticfield(inaccordancetoLHGrule)
ChargedParticle
MALTESECROSSEXPERIMENTS
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Observation:Shadowwithsharpedgesbehindanode&rayswereblockedbymetal
Showedcathoderayspropagatedinstraightlines,startedfromcathode&cantpenetratemetal
Waves
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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
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Identify that charged particles produce an electric field
Anelectricfieldexistsinanyregioninwhichanelectricallychargedobjectexperiencesaforce.The
observationthatchargedplatesexertaforceonotherobjectsbroughtclosetothemindicatesacharged
particlewillproducearadialelectricalfieldinthesurroundingregion.Thestrengthofthefield
producedisinverselyproportionaltothesquareofthedistance.Greaterdensityoffieldlinesàstronger
ELECTRICFIELDDUETOCHARGEDPARTICLES
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Apairofoppositelychargedparticlesseparatedbysmalldistancewillproduceauniformelectricfield
Aparticleexperiencethesameforceanywhereinauniformfieldasthedensityofthefieldlineisthe
samethroughout
Describe quantitatively the force acting on a charge moving
through an electric field F = Eq
Achargedparticleinanelectricalfieldwillexperienceaforce.Thesizeoftheelectricalforceisgivenby:
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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
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Ifaparticlewithchargeqismovingwithvelocityv,perpendicularlytoamagneticfieldof
strengthB,theparticlewillexperienceamagneticforceF,givenbyF=qvB(V=W/q)
Iftheparticle’sdirectionofmotionisatanangletothemagneticfield,then:
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Forelectrons,usetheLeftHandGripruletofinddirection
Thetaisanglebetweenpathofparticleandmagneticfield
Debate:answernoteasytofindastheparticlesaresmallanddifficulttodetect