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1、fin32020_ch02.qxd8/10/015:38PMPage13CHAPTER2PropertiesofFluidsInthischapterwediscussanumberoffundamentalpropertiesoffluids.Anunderstandingofthesepropertiesisessentialforustoapplybasicprinciplesoffluidmechanicstothesolutionofpracticalproblems.2.1DISTINCTIONBETWEENASOLIDANDAFLUIDThemol
2、eculesofasolidareusuallyclosertogetherthanthoseofafluid.Theattractiveforcesbetweenthemoleculesofasolidaresolargethatasolidtendstoretainitsshape.Thisisnotthecaseforafluid,wheretheattractiveforcesbe-tweenthemoleculesaresmaller.Anidealelasticsolidwilldeformunderloadand,oncetheloadisremo
3、ved,willreturntoitsoriginalstate.Somesolidsareplastic.Thesedeformundertheactionofasufficientloadanddeformationcon-tinuesaslongasaloadisapplied,providingthematerialdoesnotrupture.De-formationceaseswhentheloadisremoved,buttheplasticsoliddoesnotreturntoitsoriginalstate.Theintermolecula
4、rcohesiveforcesinafluidarenotgreatenoughtoholdthevariouselementsofthefluidtogether.Henceafluidwillflowundertheac-tionoftheslighteststressandflowwillcontinueaslongasthestressispresent.2.2DISTINCTIONBETWEENAGASANDALIQUIDAfluidmaybeeitheragasoraliquid.Themoleculesofagasaremuchfartherapartth
5、anthoseofaliquid.Henceagasisverycompressible,andwhenallex-ternalpressureisremoved,ittendstoexpandindefinitely.Agasisthereforeinequilibriumonlywhenitiscompletelyenclosed.Aliquidisrelativelyincom-pressible,andifallpressure,exceptthatofitsownvaporpressure,isremoved,thecohesionbetweenmo
6、leculesholdsthemtogether,sothattheliquiddoesnotexpandindefinitely.Thereforealiquidmayhaveafreesurface,i.e.,asurfacefromwhichallpressureisremoved,exceptthatofitsownvapor.Avaporisagaswhosetemperatureandpressurearesuchthatitisveryneartheliquidphase.Thussteamisconsideredavaporbecauseits
7、stateis13fin32020_ch02.qxd8/10/015:38PMPage1414CHAPTER2:PropertiesofFluidsnormallynotfarfromthatofwater.Agasmaybedefinedasahighlysuper-heatedvapor;thatis,itsstateisfarremovedfromtheliquidphase.Thusairisconsideredagasbecauseitsstateisnormallyveryfarfromthatofliquidair.Thevolumeofagas
8、orvaporisgreatlyaffectedby
