《大学物理简明教程》 课件 英文 17-2 HEAT FLOW.pptx

HeatFlow1Topicsinthislecture:ConductionConvectionRadiation

ConductionConvectionRadiationMechanismsofHeatFlowEnergytransferredwithinsolidduetocollisionsbetweenvibratingatoms.Energytransferredbymotionoffluid(liquidorgas)duetochangesindensity.Electromagneticwaves.Cantravelthroughavacuum.2

ConductionHeatwillflowwhileatemperaturedifferenceexists.3Lagging

=thermalinsulatingmaterial

Conduction??Q/t=heatflow(J/s)or(W)k=thermalconductivity(W/m.K)A=crosssectionarea(m2)T1–T2=temperaturedifferenceL=lengthofconductor4

InsulationInsulatedbar:ThermalcurrentI(dQ/dt)issameatallpointsalongthebar.5Likeahosepipewithholes?

LikeelectricityLikeV=IRThermalresistance??6

SeriesandParallelSeriese.g.doubleglazingParallele.g.windows,doors,walls,roof,floor

Question7:Aironbar0.5mlongandacopperbar1.2mlongarejoinedendtoend.Thefreeendoftheironbariskeptat80°Candthefreeendofthecopperbariskeptat0°C.Bothbarshavecircularcross-sectionofdiameter0.16mandbotharelagged.InthermalequilibriumthejunctionisattemperatureT1.CalculateTherateofheatflowThetemperatureT1 Thermalconductivityofcopper=390Wm–1K–1andiron=75Wm–1K–18NowPAUSEthevideoandtrytosolveQ7

9?

Graphenek≈5000W/mKhasthehighestthermalconductivityofknownmaterials2-DstructureSpacerockettile

silica(SiO2)hasoneofthelowestthermalconductivities.Toprotectcrewfromheatonentrytoearth’satmosphere10

UValuesTheseareusedbyheatingengineers?Comparesdifferentmaterials.Valuesobtainedbydirectmeasurement(nottheoreticalcalculations)AllowsforimperfectionsinthematerialsHeatflowperunitarea,for1?Cdifference?Lowvaluesgivebetterinsulation.Examples11

Whichofthesegraphs……..ABCDEa.Showsalaggedbarb.Showsanunlaggedbarc.Showsalaggedcompositebar(withthehighest

thermalresistanceontheleft)Question8:Now