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片断： PROLOGUE AUnifiedViewof PhysicalProcesses Heraclides(550-480) Everythingflows.WaterandairflowonthesurfaceoftheEarth,wheretheycre- atethemultitudeofphenomenaweknowfromeverydaylife.Windscanimpart theirmotiontothewateroftheoceans,andinafar-awayplace,thismotioncan bepickedupagainthroughtheactionofthewaves.Theseprocessesaremain- tainedbytheradiationpouringoutfromthesurfaceofthesun;lightflowsfrom therethroughspace,andsomeofitisinterceptedandabsorbedbyourplanet. Bothinnatureandinmachines,heatisproducedandtransportedfromplaceto place.Inelectricalmachines,wemakeelectricityflowinanimitationofitsflow intheatmosphere,andinreactors,chemicalsubstancesflowwhileatthesame timeundergoingchange.Today,weevenseelifeasgovemedbyflowprocesses. Weshalltakethisobservationasthestartingpointofourinvestigationofnatural andmanmadephenomena.Itleadstooneofthemostgeneraldescriptionofna- tureweknowtoday.Thereareafewphysicalquantitieswhichcanflowintoand outofsystems,whichcanbeabsorbedandemitted,andwhichcanbeproduced anddestroyed.Electricalchargeistransportedinelectricalprocesses,andmass andsubstancellowingravitationalandchemicalphenomena,respectively.In continuummechanics,motionisseenastheexchangeoflinearandangularmo- mentum.Thermalphysicsisthescienceofthetransportandtheproductionof heat.Oneofthegreatadvantagesofthisdescriptionofnatureisthatitrelatesthe differentphenomena,whichleadstoaneconomicalandunifiedviewofphysical processes.Ittumoutthatclassicalcontinuumphysicsisaprecisemethodofex- pressingthispointofviewformacroscopicsystems(seeSectionP.5). Inthischapter,weshallpresentsomeexamplesofintroductoryphysics,mostof whichyoushouldbefamiliarwith.Weshalluseasthemaintooltheimagesand thelanguagefoundincontinuumphysics.Inthisway,wehopetopreparethe groundfortheapproachtothennodynamicswhichyouwillfindinthisbook. Note,however,thatthischapterisacondensedoverview,notatext.Afterread- ingSectionP.1youmaywanttoventuredirectlyintothemainbodyofthebook, inwhichcaseyoumightwishtoreturntothischapterlateron.Eitherway,we believeyouwillfinditadvantageoustodrawcomparisonsbetweendifferent fieldsofphysicsasoftenaspossibleduringyourjoumeythroughthennodynam- ics. Whatisthisunifiedapproachtophysics?Inshort,itisbasedonananalogywith continuumphysics.First,wehavetoagreeonwhichphysicalquantitiesweare goingtouseasthefundamentalorprimitiveones;ontheirbasisotherquantities aredefined,andlawsareexpressedwiththeirhelp.Second,therearethefunda- mentallawsofbalanceofthequantitieswhichareexchangedinprocesses,such asmomentum,charge,oramountofsubstance;wecallthesequantitiessubstan- celike.Third,weneedparticularlawsgovemingthebehaviorof,ordistinguish- ingbetween,differentbodies;theselawsarecalledconstitutiverelations.Last butnotleast,weneedameansofrelatingdifferenttypesofphysicalphenomena. Thetoolwhichpennitsustodothisisenergy.Weusetheenergyprinciple,i.e., thelawwhichexpressesourbeliefthatthereisaconservedquantitywhichap- pearsinallphenomena,andwhichhasaparticularrelationshipwitheachofthe typesofprocesses. Tointroducetheelementsoftheoseslistedabove,weshallbeginwithacom- parisonoftheflowofwaterandAelectricalcharge. P.lTheFlowofaterandCharge Weallarefamiliarththeflowofwaterinsimplesettings,suchasthefilling orthedischargingcontainersthroughpipes(Figure1).Bylookingataspecial examplewewillbeabletoidentiiytheelementsofaphysicaltheorywhichal- lowustocalculatesuchthingsasthecnrrentofwaterthroughapipe,thepres- sureatvariouspointsinthefluid,andthetimerequiredtodischargeacontainer. Theanalysisalsowilltellusthatthesystemandtheprocessesitmayundergo areverysimilartowhatweknowfromelectricity.Bycomparinghydraulicand electricalsystemsweshallleamaboutthepowerofanalogiesbetweendifferent fieldsofphysics. P.l.lPhyslcalQuantltles Ourfirstquestionmustbewhichphysicalquantitieswecanuseasbasisfora quantitativedescriptionoftheflowofwaterintoandoutofcontainers.Weshall havetodothesamefortheelectricalsystem.ThechoiceofFundamentalorprim- itivequantitiesisnotunique.Wesimplyhavetobeginsomewhere,insomeway. Wecertainlyneedameasureoftheamountofwaterinacontainer.Therearesev- eralpossiblechoices.Thesimplestoftheseisthevolumeofthewater.Another thatcomestomindquicklyisthemassofthewater.Finally,chemistsmightbe inclinedtomeasuretheamountofwateronthebasisofitsamountofsubstance (SectionP.2.7).