《大學(xué)物理簡明教程》 課件 17-2 HEAT FLOW

HeatFlow1Topicsinthislecture:ConductionConvectionRadiationConductionConvectionRadiationMechanismsofHeatFlowEnergytransferredwithinsolidduetocollisionsbetweenvibratingatoms.Energytransferredbymotionoffluid(liquidorgas)duetochangesindensity.Electromagneticwaves.Cantravelthroughavacuum.2ConductionHeatwillflowwhileatemperaturedifferenceexists.3Lagging

=thermalinsulatingmaterialConduction

Q/t=heatflow(J/s)or(W)k=thermalconductivity(W/m.K)A=crosssectionarea(m2)T1–T2=temperaturedifferenceL=lengthofconductor4InsulationInsulatedbar:ThermalcurrentI(dQ/dt)issameatallpointsalongthebar.5Likeahosepipewithholes

LikeelectricityLikeV=IRThermalresistance

6SeriesandParallelSeriese.g.doubleglazingParallele.g.windows,doors,walls,roof,floorQuestion7:

Aironbar0.5mlongandacopperbar1.2mlongarejoinedendtoend.Thefreeendoftheironbariskeptat80°Candthefreeendofthecopperbariskeptat0°C.Bothbarshavecircularcross-sectionofdiameter0.16mandbotharelagged.InthermalequilibriumthejunctionisattemperatureT1.CalculateTherateofheatflowThetemperatureT1

Thermalconductivityofcopper=390Wm–1K–1andiron=75Wm–1K–18NowPAUSEthevideoandtrytosolveQ79

Graphenek≈5000W/mKhasthehighestthermalconductivityofknownmaterials2-DstructureSpacerockettile

silica(SiO2)hasoneofthelowestthermalconductivities.Toprotectcrewfromheatonentrytoearth’satmosphere10UValuesTheseareusedbyheatingengineers

Comparesdifferentmaterials.Valuesobtainedbydirectmeasurement(nottheoreticalcalculations)AllowsforimperfectionsinthematerialsHeatflowperunitarea,for1Cdifference

Lowvaluesgivebetterinsulation.Examples11Whichofthesegraphs……..ABCDEa.Showsalaggedbarb.Showsanunlaggedbarc.Showsalaggedcompositebar(withthehighest

thermalresistanceontheleft)Question8:NowPAUSEthevideoandtrytosolveQ812Whichofthesegraphs……..ABCDEa.Showsalaggedbarb.Showsanunlaggedbarc.Showsalaggedcompositebar(withthehighestthermalresistanceontheleft)Question8:13Convection–influids(liquidsandgases)HeatingelementinliquidConvectioncauseslandandseabreezesP=powerradiated(W)e=emissivity(e=1forblackbody)=Stefan’sconstant(5.67×10-8Wm-2K-4)A=surfaceareaofthebody(m2)T=temperatureofthebody(K)ThermalRadiationAllobjectsemitelectromagneticradiationdependentontheirsurfacetemperature.

FireservicecanfindpeopleinsmokeElectricalsafetyPolicelocatepeopleinwoodInvestigateheatlossTHERMALIMAGINGCAMERAThepeakoftheemittedradiationmovestosmallerwavelengthsathighertemperatures.QuestiononStefan’slawandWienQuestion9:

Asolidmetalsphereishungfromathininsulatingthread,inavacuum.Whenat400Kthesphereradiatesatarateof1W.Whatwouldbetherateofradiationofanidenticalsphereofhalftheradiusat800K?(Ignoreanyheatabsorbedfromthesurroundings).A2WB4WC8WD16WE64WNowPAUSEthevideoandtrytosolveQ9QuestiononStefan’slawandWienQuestion9:

Asolidmetalsphereishungfromathininsulatingthread,inavacuum.Whenat400Kthesphereradiatesatarateof1W.Whatwouldbetherateofradiationofanidenticalsphereofhalftheradiusat800K?(Ignoreanyheatabsorbedfromthesurroundings).A2WB4WC8WD16WE64WAnswer