《大学物理简明教程》 课件 10-1 Quantum Physics 1

QuantaandAtomsBlackbodyradiationQuantizationofenergyPhotonmodelPhotoelectriceffectBohr’satommodellinespectrumExcitationofatomsMatterwaveMichelson-Morleyexperiment

“zeroresult”“Ultravioletcatastrophe”inBlackbodyradiation

QuantummechanicsSpecialRelativityNineteenth-CenturyCloudsovertheDynamicalTheoryofHeatandLight(LordKelvin,1899)BlackbodyRadiation

Blackbody-Thebodythatabsorbs

ALLincidentelectromagneticwaveanddoesn’treflect-anidealmodelofthermalradiation.Maximumabsorbingability=strongestemittingabilityDevicetoMeasuretheDistributionofEnergyinBlackbodyRadiation:lenseaparallellighttubeadisperseprismathermocouple

BlackbodyattemperatureT

BlackbodyradiationspectrumIntensity-powerradiatedfromthesurfaceoftheblackbodyperunitwavelengthintervalperunitsurfaceareaj(

)

0

0+

Integratedradiativepowercarriedbyelectromagneticwaveswithwavelengthsbetween

0and

0+

IntensityStefan-BoltzmannlawtheTOTAL

energy

radiatedperunit

time

perunit

surfacearea

ofa

blackbody

across

ALLwavelengths

isdirectly

proportional

tothefourthpoweroftheblackbody's

thermodynamictemperature

T

the

blackbodyradiation

curvefordifferenttemperaturespeaksata

wavelength

inverselyproportionaltothethermodynamictemperature.Withtheincreaseoftemperature,decreases.IntensityWien’sDisplacementLawIncandescentlightbulbTungsten(W)filamentmeltingpoint3695KlampIncandescentFluorescentLEDTypicalefficiency2%10%20%Question–Pleaseestimatethetemperatureofthesurfaceofthesun.ExperimentaldataRayleigh-Jeansformula

Ultravioletcatastrophe!infiniteenergy??Planckintroducedtheconceptofa“quantumofaction”inanattempttoexplainthespectraldistributionofblackbodyradiation,whichlaidthefoundationsforquantumtheory.In1918,hewasawardedtheNobelPrizeinphysicsforthediscoveryofthequantizednatureofenergy.MaxPlanck(1858-1947)Thequantizationofenergy

energyofoscillationPlanckFormulaelectromagneticenergycouldbeemittedonlyin

quantized

form,inotherwords,theenergycouldonlybeamultipleofanelementaryunitWherenisapositiveintegercalledaquantumnumber,hisPlanck’sconstant.ElectromagneticwaveOscillationsintheblackbodyradiateArbitraryenergy??CertainnaturalfrequencyfQuantizedenergy

ThekeypointofPlanck’stheoryistheradicalassumptionofquantizedenergystates.Energyofoscillator01234n

0quantumnumber

PlanckFormulaofBlackbodyRadiation0Goodmatch!

Planck’stheorymarkedthebirthofthequantumtheory.(14December1900)Next–Quantizationoflight-PhotonmodelbyAlbertEinstein(1879

–1955)German-born

theoreticalphysicist.AlbertEinsteinPhotonthe

quantum

ofthe

electromagneticfield

including

electromagneticradiation

suchas

light.Thephotonhaszero

restmass

andalwaysmovesatthe

speedoflight

withina

vacuum.AtypeofelementaryparticleTheenergyofaphoton

Planck’sconstantFrequencyofelectromagneticwaveAgain,lightsaremadeupofparticles…Speedofphoton

massofphoton

momentum

WavelengthofelectromagneticwaveThetypicalvalueofthepowerfromsunradiationpersquaremeteratnoonis1000W.Theaveragewavelengthofsunlightis510nm.Pleaseestimatehowmanyphotonsarriveat1squaremetersurfacein1second.Whatisthepressurebythesunlight?Eachphoton–

1squaremeterin1second

Totalmomentum

Thepressure

metalExperimentsshowthatlightshoneoncertainmetalsurfacescauseselectronstobeemittedfromthesurfaces.theemittedelectronsarecalledphotoelectronsPhotoelectricEffectelectronsThe

electron

isanelementaryparticle,withanegative

elementary

electriccharge.Mass

me

=9.10938356(11)×10−31

kgCharge

-e=−1.6021766208(98)×10−19

CoulombASchematicDiagramofaPhotoelectricEffectApparatusTwoelectrodesAandKareinevacuatedtube,whenlightisshiningonthemetalsurfaceofK.ThevoltagebetweenAandKismeasuredbyavoltmeter.PhotoelectronsemittedfromKthatcanreachAcontributestothecurrentinthecircuit.ParametersWavelengthofincidentlight

LightintensityVoltagebetweenAandKVCurrentILightvoltagesourcevacuumHighintensityLowintensitycurrentAppliedvoltagePhotoelectricCurrentversusAppliedVoltageThecurrentincreaseswithvoltagebutreachesasaturationleveleventually.HelpingphotoelectronsreachtheothersideStoppingvoltage-acertainwavelengthoflight!theinitialkineticenergyofaphotoelectronescapedfromtheplateKisThelowestvoltagetomakethecurrenttodropjusttozeroisU0(stoppingvoltage)StoppingvoltageelectricforceonelectronsThephotoelectronwithmaximumkineticenergywillbejuststoppedbyU0

f0CsKCuf01f02f03(frequencyoftheincidentelectromagneticwave)StoppingvoltageCutofffrequencyNophotoelectronsifthelightfrequencyissmallerthanthecutofffrequency!HighintensityLowintensitycurrentStoppingvoltagef0CsKCuf01f02f03CutofffrequencyClassicaltheoryCANNOTexplainWhythereisacutofffrequencyWhythestoppingvoltagedoesn’tdependonlightintensityWhythestoppingvoltagehasalinearrelationshipwithlightfrequencyPhotonmodelcanexplainIncidentphotonEscapingelectronElectroninthemetalAbsorbsONEphotonPhotonenergyE=hfElectronenergyincreasedbyhfBreakstheconfinementItcostsacertainamountofenergy!Someenergyremainingforescapedphotoelectron

MinimumenergycosttoescapeMaximumkineticenergyofphotoelectron

Workfunction:electronbindingenergy,dependingonthematerialWhyisthereacutofffrequency?Whythestoppingvoltagedoesn’tdependonlightintensityWhythestoppingvoltagehasalinearrelationshipwithlightfrequency

Lightfrequencymustbegreaterthanf0inordertoexcitephotoelectrons