《大学物理简明教程》 课件 18-1 HEAT and WORK

HeatandWorkTopicsinthislecture:HeatWork1stLawofThermodynamicsThermodynamicProcessesIsobaric,Isochoric,Isothermal,AdiabaticHeatHeatisaflowofenergyfromahotobjecttoacoldobject.Heatflowstopswhentheobjectshavethesametemperature.ThisisTHERMALEQUILIBRIUMAddingheatraisesthetemperatureoftheobject.Itincreasesthekineticenergyofthemolecules.ThisincreasestheINTERNALENERGYWorkWorkismechanicalmovement

(forcethroughadistance)Whatiswork?Workdonebythegas

W=Fd=PAd=P

VWorkismechanicalmovement(forcethroughadistance)ThegasexertspressurePonthepistonofareaA.ThegasexertsaforceFonthepistonF=PAIfweaddheattothegasitmovesthepistonthroughdistanced

(tokeepthepressureconstant).

Workdonebythegas1stLawofThermodynamics

increaseininternalenergyofthesystem heataddedtothesystem networkdonebythesystemThisisjustconservationofenergy

Internalenergy

StateVariablesPV(P1,V1,T1)(P2,V2,T2)P,V,T,UareSTATEVARIABLES

Qand

WareNOTstatevariablesTheyareameasureofenergythatflowsintooroutofthesystem,anddependonthe‘route’taken.DefinedbyapointonthePVdiagram.ThermodynamicProcessesPWecanmoveasystemonanypathonaPVgraph,butforafewspecificpathswecaneasilycalculatethework,heatandchangeininternalenergy.Thesepathsare:IsobaricIsochoricIsothermalAdiabaticP=0V=0T=0

Q=0Isobarice.g.‘open’toatmosphereConstantpressureWork

W=P

V1stlawQ=U+WAllheataddedsplitbetweenincreasinginternalenergyandincreasingvolume.SoQ=U+PVWorkdone=areaunderthegraphIsochorice.g.rigidcontainerLockpistoninpositionConstantvolumeWork

W=01stlawQ=U+WSoQ=UAllheataddedgoesintoincreasinginternalenergy.Noareaundergraph

Heatingatconstantvolumeandconstantpressure

HeatingatconstantvolumeHeatingatconstantpressure

Themolarheatcapacitydependsontheprocess.

W=P

V

W=0

Foridealgas

cp’>cv’asheataddeddoesworkinadditiontoincreasingtheinternalenergy

From1stlaw

Question5Foronemoleofthefollowinggases,He,H2,Ar,N2determineexpressionsfortheinternalenergythemolarheatcapacityatconstantvolumeThemolarheatcapacityatconstantpressureNowPAUSEthevideoandtrytosolveQ5

Question5Foronemoleofthefollowinggases,He,H2,Ar,N2determineexpressionsfortheinternalenergythemolarheatcapacityatconstantvolumeThemolarheatcapacityatconstantpressureHeH2ArN2DoF3535Ucv’cp’IsothermalSLOWprocesstoallowheattoflowinorouttomaintaintemperatureConstanttemperatureT=0Iftemperatureconstant,theninternalenergyisconstant

U=0as1stlawQ=U+WsoQ=0+WAllheataddedgoesintoexpandingthegassoQ=W

Asystemmovesfromstateatostatebalonganyofthe3pathsshown.Ifstatebhasbiggerinternalenergythanstatea,alongwhichpathistheheattransferthelargest?Apath1Bpath2Cpath3D

sameforallthreepathsQuestion6NowPAUSEthevideoandtrytosolveQ6Asystemmovesfromstateatostatebalonganyofthe3pathsshown.Ifstatebhasbiggerinternalenergythanstatea,alongwhichpathistheheattransferthelargest?Apath1Bpath2Cpath3D

sameforallthreepathsQuestion6AnswerUisastatevariable,sosameforallpathsbetweenaandb,

andinthisquestionitincreases,soispositive.1stlaw:

Q=

U+

WSolargest

QisforlargestW,whichisforlargestareaundercurve,whichispath1.Answer:AAdiabatice.g.InsulatedcontainersFastprocessesLargemassesAdiabaticCompressionNoheatinorout.

Q=0FastcompressionFlammablegasAdiabatsaresteeperthanisothermsMeteorites(piecesofrockmovingathighspeedinatmosphere)compresstheairadiabaticallysotemperaturerisestillitglows.AdiabaticExpansionAdiabaticexpansioncoolsthegas–sothatthewatervapourintheaircondensesintoclouds.

Question7Whydoesanaerosolorfireextinguishernozzlegetcoldwhenthegasisreleased?NowPAUSEthevideoandtrytosolveQ7Question7Whydoesanaerosolorfireextinguishernozzlegetcoldwhenthegasisreleased?ANSWERThegasexpandsadiabatically(rapidly)asitleavethenozzle.Adiabaticexpansionreducesthetemperature.SlowtoletheatflowInsulatedtostopheatflow.TrisesforcompressionTfallsforexpansionRigidcontainerVandTabletochangeIsobaricIsochoricIsothermalAdiabaticP=0V=0T=0Q=0U

0

W

W=PV0

W=

Q

W=

UQ

Q=

U+

W

Q=

U

Q=

W0Summaryofthermodynamicprocesses

SummaryHeatisaflowofenergyfromahotobjecttoacoldobject.Addingheatraisesthetemperatureoftheobject,asitincreasesthetranslationalkineticenergyofthemolecules.Addingheatincreasestheinternalenergyoftheobject.Workismechanicalmovementofaforcethroughadistance.1stLawofthermodynamics:

increase