heat and thermodynamics - ucla热学和热力学-加州大学洛杉矶分校

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1、HeatandThermodynamics3g*Energycannotbecreatedordestroyed,althoughinmanyprocessesenergyistransferredtotheenvironmentasheat.Asabasisforunderstandingthisconcept:3.g*Studentsknowhowtosolveproblemsinvolvingheatflow,work,andefficiencyinaheatengineandknowthata

2、llrealengineslosesomeheattotheirsurroundings.Introductiontorealheatengines.AsstatedinStandard3b,agoodwaytobeginastudyofheatenginesistodiscussexamplesofactualengines.ModelslikethosethatcanbeobtainedfromCynmar(Petrol022-39327andDiesel022-39329www.cynmar.c

3、om)couldhelpthediscussion.Evendiscussingasteamturbinewouldgivearealreferencetohowworkcanbeextractedfromaheatengineastheheatflowsfromahightemperaturesourcetoalowtemperaturesource.Understandinghemeaningoftheareaunderapressurevs.volumegraph.Althoughnotspec

4、ifiedinthestatementofthestandards,muchofthephysicsofhowheatenginesworkrequiresanappreciationthattheareaunderapressurevs.volumegraphiswork.Asimplediscussionofwhathappenswhenapistonconfiningavolumeofgasmovesoutwardundertheforceexertedbythepressureofthegas

5、canhelptounderstandthisconcept.Thepistonshownontherightconfinesavolumeofairinsideofacylinderunderpressure,P.Theairisheatedinsuchawayastocausethepistontomovetotherightandforsimplicity,thepressureiskeptconstantasthepistonmoves.TheareaofthepistonisAandfrom

6、thedefinitionofpressure,theforceexertedonthepistonbytheairisF=PA.Asthepistonmovesinthecylinderitsweepsoutavolume∆V=Adwheredisthedistancethepistonmoves.SinceworkequalsFd=PAd=P∆VwecanseethattheworkdoneinmovingthepistonPV∆VFFcanberepresentedonthepressurevs

7、.volumegraphillustratedaboveastheareaunderthegraph.Wehavechosentouseagraphforwhichthepressureremainsconstant,however,theareaunderanygraphofpressurevs.volume(whichcanbeapproximatedbyalargesumofP∆Vrectangles)willalwaysequalworkorenergy.Ifpressureismeasure

8、dinN/m2(orPascal)andvolumeismeasuredinm3theproductofpressureandvolumewillbeNm,which,aswealreadyknow,isajoule,theunitofworkorenergy.RealenginesandthePVgraph.Ifyoudonothaveamodelofaninternalcombustionenginetousetodemonstratewithyou