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Discoveryoftheelectron
Excuseme..Howcanyoudiscoverapar6clesosmallthatnobodyhaseverseenone?
ThomsonbecameProfessorofExperimentalPhysics(CavendishLabs,Cambridge)in1884
Oneofthephenomenahewasstudyingwastheconduc&onofelectricitythroughgases
Hewasveryinterestedininvesgangalong-standingpuzzlewhichwasknownasCathodeRays
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CathodeRays
Gasesatatmosphericpressurearebadconductorsofelectricity
Topassanelectriccurrentthroughagasaspecialapparatusisneeded:agas-dischargetube
Agas-dischargetubeisusuallyaglasstubewithelectrodesatitsendssothatavoltagecanbeapplied
Whenvoltageisappliedandthepressureofthegasisreduced(byavacuumpump)eventuallyacurrentwillflow
andthegaswillbegintoglow
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CathodeRays
Gas-dischargetubesfirstdevelopedbetween1857-1870 Nooneknewwhatcausedtheglowsoscienststrieddifferentexperimentstoseeiftheycouldfindout
WilliamCrookesnocedtheglowwasmostintenseoppositethenegaveelectrode(cathode):cathoderays
Chargedparclesorelectromagnecwaves?
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Thomsondiscoversthe
electron Thomsonperformsaseriesofexperimentsin1897anddiscoversthatcathoderaysgetdeflectedwhenpassed
throughanelectricfield
Theraysmovetowardstheposivelychargedplatesotheymusthavenega&vecharge
"Whatarethese
parcles?Arethey
atoms,ormolecules,or
maYerinasllfinerstate
ofsubdivision?"
J.J.Thomson
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Thomsondiscoversthe
electron Thomsonperformsaseriesofexperimentsin1897anddiscoversthatcathoderaysgetdeflectedwhenpassed
throughanelectricfield
Theraysmovetowardstheposivelychargedplatesotheymusthavenega&vecharge
Thenheappliedamagne&cfieldandtheraysweredeflectedintheoppositedirecon
Applyingbothfieldstogetherunltheirrespecveforcescancelledoutallowedhimtocalculatethespeedoftheraysandthecharge-to-massra&o(specificcharge)oftheparcle
Thomsonsexperiment
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Thomson talks about Electrical Engineering andshares his thoughts about the discovery of theelectron:
hYp://www.nobelprize.org/mediaplayer/index.php?id=322 hYp://www-outreach.phy.cam.ac.uk/camphy/electron/electron1_1.htm
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Theelectrongun
Thermionicemission:heangupametalcausesitsfreeelectronstogainK.E.andescapefromitssurface
Electrongun:Adevicethatacceleratesfreeelectronsinavacuum
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Theelectrongun
Thermionicemission:heangupametalcausesitsfreeelectronstogainK.E.andescapefromitssurface
Electrongun:Adevicethatacceleratesfreeelectronsinavacuum
WorkdoneoneachelectronisW=eV e=chargeofelectron=1.610-19coulombs V=p.d.betweencathodeandanode
K.E.ofelectronsatanode=mev2
W =K.E. eV=1
2m
ev
2 v =
2eV
me
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Electronbeaminuniform
electric:ield
Electronsgetdeflectedtowardsthe+veplatewhenpassingthroughanelectricfield
Whendeflected,theyfollowaparabolicpath Forceexperiencedbyeachelectron
+
-
d
F= eE=eV
d=ma a =
eV
med
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y =1
2
eVx2
medv
2
Electronbeaminuniform
electric:ield
Horizontaldistancecoveredint:x=vt Vercaldistancecoveredint:y=at2 Substungweget:
+
-
dv
x
y
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Electronbeaminuniform
electric:ield
Vercalspeedaermet: Direconofbeamatmet:
+
-
dv
x
y
vy = at=eV
medt=
eVx
medv
v
vy vxy
v
vy vxy
tan()=vy
v
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Example
Anelectrongunoperangat3000Visusedtoaccelerate
electronsbetweentwooppositelychargedparallelplates.The
distancebetweentheplatesis50mmandthelengthofthe
platesis100mm.Calculatethedefleconoftheelectronsatthe
pointwheretheyexitthefieldwhenthevoltageacrosstheplateis1000V.Assumethespecificchargeoftheelectrone/
me=1.76x1011Coulombperkilogram.
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Example
Abeamofelectronsmovingwithavelocityof107m/senters
midwaybetweentwohorizontalplatesPandQinadirecon
paralleltotheplatesasshown.PandQare5cmlongandhavea
p.d.Vappliedbetweenthem.CalculateVifthebeamis
deflectedsothatitjustgrazestheedgeofthelowerplateQ.(Assumee/me=1.8x10
11C/kg)
2cm
v
1cm
P
Q
V
5cm
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Electronbeaminuniform
magnetic:ield Thepathelectronstakegetsalteredbythepresenceofamagnecfield
Amagnecfielddeflectstheelectronsbutdoesnotchangetheirspeedortheirkinecenergy
Magne&cforcegivenbyF=Bev e=chargeofelectron B=magnecfieldstrength v=speedoftheelectron
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Electronbeaminuniform
magnetic:ield Themagnecforceactsasthecentripetalforceandelectronsmoveinacircleofradiusr
F= Bve =m
ev2
r r =
mev
Be
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Electronbeaminuniform
magnetic:ield IftheanglebetweenthevelocityofthebeamvandthemagnecfieldBisnot90thenthebeampathisaspiral
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Speci:icchargeoftheelectron
Thespecificchargeoftheelectronisthechargeperunitmass(e/me)
Abeamofelectronsenteringauniformmagnecfieldat90tracesacircleofradiusr
TheelectronbeamdeliverselectronswithK.E.
WhereVistheappliedvoltage(orp.d.betweenanodeandcathode)
1
2
me
v2= eV
r =mev
Be v =
Ber
me
e
me
=
2V
B2
r2
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Example
Posivelychargedparcleswithspecificcharge108C/kgare
rotanginacircularorbitofradiusrinsideauniformmagnec
fieldofstrength0.5T.Showthattheperiodoftheirmoonis
independentofrandcalculatethisperiod.
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Thevelocityselector
dvV
+
-
E
B
Bev =eV
d v =
V
Bd
Fornodefleconmagnecforce=electricforce Speedvfornodeflecon:
Thisarrangementcanbeusedasavelocityselectorifthebeamconsistsofparcleswithdifferentspeeds
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Example
Abeamofelectronsisfiredintoauniformelectricfieldasinthe
previousfigure.Thelengthoftheplatesgenerangthefieldis
100mmandtheirdistanceapartis50mm.Aconstantvoltageof
720Visappliedacrosstheplatesandthebeamisdeflected
vercallybyadistanceof25mm.Thebeamisthenstraightenedoutbyapplyingauniformmagnecfieldofstrength0.65mT
perpendiculartothebeam.Calculate
a) Thespeedofthebeamwhenenteringthefieldsb) Thespecificchargeoftheelectron
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Measurementofchargeof
electron
+
-
Where
Q=thechargeofthedroplet
V=voltageapplied
d=distancebetweentheplates
m=massofthedroplet
Electricforce
Weight
QV
d=mg
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Measuringthemassofatiny
oildroplet Viscosityisameasureofthethicknessofasubstance Viscousforcesactonanybodymovingthroughaviscoussubstance
Becauseoildropletsaresony,whentheyfallthroughair,theyexperienceaviscousforceF
StokesLaw:F=6Rv =viscosityoffluid R=radiusofsphere v=speedofsphere
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Measuringthemassofatiny
oildroplet Terminalspeed=thespeedofabodywhennonetforceisacngonit
Theterminalspeedcanbedeterminedbymingthefallofthedropletoveraknowndistance
Fortheoildropletmovingatterminalspeed:6Rv=mg Dropletisasphereofuniformdensity Volumeofsphereis mass=volumexdensity
4
3R
3
6Rv = 4
3R
3gR =
9v
2g23
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Example
Calculatetheradiusofadropofoil,density900kg/m3,which
fallswithaterminalvelocityof2.9x10-4m/sthroughairof
viscocity1.8x10-5Ns/m2.Ignorethedensityoftheair.Ifthe
chargeonthedropis-3e,whatp.d.mustbeappliedbetween
twoplates5mmapart(e=1.6x10-19
C)tokeepitstaonary?(assumeg=9.8m/s)
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Motionofchargeddropletsin
anelectric:ield Whathappenswhentheelectricforceduetothevoltageapplieddoesnotbalancethegravitaonalforce?
A)Voltageholdingvoltage qV/d=mg+6Rv
Direcon
ofmoon
Fviscous
Weight
Direcon
ofmoonFviscous
Weight25
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Photoelectricity
Aheatedcathode(metalconductor)canbeusedtoproduceasupplyofelectronsbythermionicemission
EnergycanbesuppliedtotheconductoralsointheformofUVlight
H.Hertz1857-1894
Observaons: Emissiononlyoccursifthefrequencyof
incidentlightexceedsaminimum'threshold
frequency
nr.electronsemiYed/sec.proporonaltolightintensity
ElectronsemiYedhavearangeofK.E.uptoamaximumvaluethatdependsonthemetal
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Einsteinandphotoelectricity
In1905Einsteinpublishedapaperexplainingtheeffect Assumedthatlighttravelsinshortpacketscalledwavepackets
Photon=wavepacketoflight=quantumoflight
TheamountofenergyeachphotonhasisgivenbyE=hf=hc/where
f=frequencyofphoton h=Planckconstant=6.63x10-34Js c=speedoflight=3x108m/s =wavelengthofphoton
Reminder:EnergymeasuredinJoules (somemesalsogiveninelectronvolteV) 1eV=1.6x10-19Joules
A.Einsteinin1905
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Einsteinandphotoelectricity
Workfunc&onW theworkneededtoremoveanelectronfromthesurfaceofthemetal
EinsteinsequaonforthephotoelectriceffectMaximumK.E.=hf-W
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Thresholdfrequency
ElectronscanonlyescapeifthemaximumK.E.>0 Thismeansthatthephotonfrequencyf>f0(thresholdfrequency)
Thresholdfrequencyf0=W/h
V
P Q
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Thresholdfrequency
Ifweapplyavoltagebetweenthetwoplateswecanhelporhindertheemissionofelectrons
MaximumK.E.=hfWeV StoppingpotenalVs=(hfW)/e
V
P Q
+ -
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Example
Sodiumhasaworkfunconof2eV.Calculatethemaximum
energyandspeedoftheemiYedelectronswhensodiumis
illuminatedbyradiaonofwavelength150nm.Whatistheleast
frequencyofradiaon(thresholdfrequency)forwhichelectrons
areemiYed?(Assumeh=6.6x10-34
Js,e=-1.6x10-19
C,me=9.1x10-31kg,c=3x108m/s)
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ToDo
Readchapter27[p.552-571Properesoftheelectron] HomeworkAssignmentwk5:quesons27.1,27.2,27.3,27.4,27.9,27.13
Handitinnolaterthan4:00pmnextWednesday-LATEWORKWILLNOTBEACCEPTED
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