Introduction
Fromthefollowingtwopoints,itisexplainedthatworkandheatareequivalent,notequal.
Oneisthatthesamechangeintheinternalenergyofthesystemcanbedoneeitherbydoingworkorbytransferringheat.Thetwoareequivalentonlyintermsoftheeffectonthesystem,andtheymustneverbethesame.
Thesecondisthattheconversionbetweenworkandheatcanonlybecompletedthroughchangesintheinternalenergyofthesystem.Itisnotappropriatetotalkaboutthedirectconversionbetweenworkandheatwithoutthesystem.Althoughmacroscopicallyitmayreflectthattheinternalenergyofthesystemhasnotchanged,itisimpossibletodrawasimpleconclusionthatheatcanchangeworkorworkcanchangeheat.Ifintheconversionprocess,theoutsidesupplysystemheat,sothatthesystemisusedforexternalwork,infact,theexternalsupplysystemheat,sothattheinternalenergyofthesystemincreases,atthesametimethesystemdoesworkexternally,consumingtheenergyobtainedfromtheoutsideworld.
Theworkheatequivalentistheinverseoftheheatworkequivalent,whichisequalto0.24cal/joule.
Significance
Beforeunderstandingthenatureofheat,therelationshipbetweenheat,work,andenergywasnotclear,sotheywereexpressedindifferentunits.Theunitofheatiscalorie,referredtoascalorie.Attheendofthe18thcentury,peoplerealizedthatheatisrelatedtosports.ThisopenedthewayforJouletostudytherelationshipbetweenheatandwork.Joulebelievesthatheatandworkshouldhaveacertainequivalentrelationship,thatis,thereisacertainquantitativerelationshipbetweentheunitcalorieofheatandtheunitjouleofwork.From1840to1878,heconductedalargenumberofexperimentsusingelectrothermalmethodandmechanicalcalorimetrymethodfornearly40years,andfinallyfoundouttheequivalentrelationshipbetweenheatandwork.IfWisusedtorepresentelectricalworkormechanicalwork,andQisusedtorepresenttheheatcorrespondingtoallofthese,thentherelationshipbetweenworkandheatcanbewrittenasW=JQ,andJistheequivalentofthermalwork.
In1843,theJvaluemeasuredbytheelectricheatingmethodofJoulewasapproximately4.568J/cal;theJvaluemeasuredbythemechanicalmethodwasapproximately4.165J/cal.Later,Jouleannouncedtheresultsofhisfurtherdeterminationsin1845,1847,and1850respectively,andthefinalresultpublishedin1878wasJ=4.157joules/cal.Withthefurtherdevelopmentofscientificinstrumentsinthefuture,otherscientistshavedonealotofverification.Therecognizedthermalequivalentvalueis:inphysicsJ=4.1868joules/cal(the"card"iscalledtheinternationalsteammetercard);inchemistryJ=4.1840joules/cal(the"cal"is"It'scalledathermochemicalcard).
Theinternationalunithasuniformlystipulatedthattheunitsofwork,heat,andenergyareallinjoules,andtheheatequivalentdoesnotexist.However,thethermalpowerequivalentexperimentanditsspecificdatahavealwaysplayedaroleinthehistoryofthedevelopmentofphysics.Joule'sexperimentlaidthefoundationforthelawofenergyconversionandconservation.
Discoveringpeople
Backgroundknowledge
Inthe18thcentury,people’sresearchonthenatureofheattookadetour,andthe"thermalmasstheory"wasinthehistoryofphysics.Ruledformorethanahundredyears.Althoughsomescientistshaddoubtsaboutthiswrongtheory,peoplehavebeenunabletosolvetheproblemoftherelationshipbetweenheatandwork.ItwastheBritishself-taughtphysicistJamesPrescottJoulewhofinallysolvedthisproblem.Theproblempointstheway.
Personalresume
Joule(JamesP.Joule,1818-1889),Britishphysicist.Hewasthefirsttoestablishthelawofenergyconservationandtransformationthroughscientificexperiments.JoulewasbornonDecember24,1818inSoft,Manchester,England.Hisfatherwasabreweryowner.Joulefollowedhisfathertoparticipateinwine-makinglaborsincehewasachildandhadnoformaleducation.
Inhisyouth,undertheintroductionofothers,JoulemetthefamouschemistDalton.DaltongaveJouleanenthusiasticinstruction.Joulelearnedmathematics,philosophy,andchemistryfromhimhumbly.ThisknowledgelaidthetheoreticalfoundationforJoule'slaterresearch.Moreover,DaltontaughtthescientificresearchmethodofcombiningJoule'stheoryandpractice,whichinspiredJoule'sinterestinchemistryandphysics.
Introductionexperience
Joule’sinitialresearchdirectionwaselectromagneticmachines.Hewantedtoreplacethesteamengineusedinhisfather’sbrewerywithelectromagneticmachinestoimproveworkefficiency.In1837,Jouleinstalledabattery-drivenelectromagneticmachine,butbecausethecurrentsupportingtheelectromagneticmachine'soperationcomesfromzincbatteries,andzincisexpensive,itisnotascost-effectivetouseanelectromagneticmachineasasteamengine.AlthoughJoule'soriginalgoalwasnotachieved,hediscoveredfromexperimentsthatelectriccurrentcandowork,whichinspiredhisinterestinin-depthresearch.
Testprocess
In1840,Jouleputthetoroidalcoilintoatesttubefilledwithwatertomeasurethewatertemperatureatdifferentcurrentintensitiesandresistances.Throughthisexperiment,hefoundthattheheatemittedbyaconductorinacertainperiodoftimeisproportionaltotheproductoftheconductor'sresistanceandthesquareofthecurrentintensity.Fouryearslater,theRussianphysicistLenzpublishedalargenumberofexperimentalresults,thusfurtherverifyingthecorrectnessofJoule'sconclusionontheheatingeffectofelectriccurrent.Therefore,thislawiscalledJoule-Lenz'slaw.
AfterJoulesummeduptheJoule-Lenzlaw,itisfurtherassumedthattheheatgeneratedbythebatterycurrentandtheheatgeneratedbytheinducedcurrentoftheelectromagneticmachineshouldbeessentiallythesame.In1843,Jouledevisedanewexperiment.Asmallcoiliswoundontheironcore,theinducedcurrentismeasuredwithanammeter,thecoilisplacedinacontainerwithwater,andthewatertemperatureismeasuredtocalculatetheheat.Thiscircuitiscompletelyenclosed,thereisnoexternalpowersupply,theincreaseinwatertemperatureisonlytheresultoftheconversionofmechanicalenergyintoelectricalenergyandelectricalenergyintoheat.Thereisnotransferofheatandmassinthewholeprocess.Theresultofthisexperimentcompletelynegatedthetheoryofthermalmass.
TheaboveexperimentalsomadeJoulethinkoftheconnectionbetweenmechanicalworkandheat.Afterrepeatedexperimentsandmeasurements,Joulefinallymeasuredthethermalworkequivalent,buttheresultwasnotaccurate.AttheBritishAcademicConferenceonAugust21,1843,Joulereportedhispaper"OntheThermalEffectofElectromagnetismandtheMechanicalValueofHeat".Inhisreport,hesaidthat1kcalofheatisequivalentto460kilogramsofwork.Hisreportdidnotreceivesupportandstrongresponse,andatthistimeherealizedthatheneededtoconductmorepreciseexperiments.
In1844,Joulestudiedthetemperaturechangesofairduringexpansionandcompression,andhehasmademanyachievementsinthisregard.Throughthestudyoftherelationshipbetweenthevelocityofgasmoleculesandtemperature,Joulecalculatedthevalueofthethermalvelocityofgasmolecules,theoreticallylayingthefoundationforBoyle-MarriottandGuy-Lussac'slawsandexplainingTheessenceofthepressureofthegasonthewall.ManyofJoule'sexperimentsintheresearchprocessweredonejointlywiththefamousphysicistWilliamThomson(laternamedLordKelvin).Ofthe97scientificpaperspublishedbyJoule,20aretheresultoftheircollaboration.Whenthefree-diffusiongasentersthelow-pressurecontainerfromthehigh-pressurecontainer,thetemperatureofmostofthegasandtheairwilldrop.Thisphenomenonwasdiscoveredbythetwo.ThisphenomenonwaslatercalledtheJoule-Thomsoneffect.
Whetherintermsofexperimentsorintheory,Jouleisoneofthepioneersofin-depthresearchfromthestandpointofmoleculardynamics.
Whileengaginginthesestudies,Joulehasnotinterruptedthemeasurementofthermalpowerequivalent.In1847,Jouledidwhatisconsideredtobethemostingeniousexperimentofdesignthinkingsofar:Hefilledthecalorimeterwithwater,installedarotatingshaftwithbladesinthemiddle,andthenletthefallingweightdrivethebladestorotate,duetothefrictionbetweenthebladesandthewater.,Thewaterandthecalorimeterhavebecomehot.Accordingtotheheightatwhichtheweightfalls,theconvertedmechanicalworkcanbecalculated;accordingtotheincreasedtemperatureofthewaterinthecalorimeter,theincreaseintheinternalenergyofthewatercanbecalculated.Bycomparingthetwonumbers,theexactvalueofthethermalequivalentcanbeobtained.
Joulealsousedwhaleoilinsteadofwaterasanexperiment,andmeasuredtheaveragethermalpowerequivalentof423.9kg·m/kcal.Hethenreplacedwaterwithmercuryandcontinuedtoimprovetheexperimentalmethodsuntil1878,whenhestartedthisworknearly40yearsago.Hehascarriedoutmorethan400experimentswithvariousmethodsbeforeandafter.Heusedfrictiontoheatwaterin1849andtheresultwasthesameasin1878,whichwas423.9kg·m/kcal.Themeasurementofanimportantphysicalconstantcanbemaintainedfor30yearswithoutmajorcorrections,whichisalsoextremelyrareinthehistoryofphysics.Thisvaluewasrecognizedasthethermalpowerequivalentatthetime.
Measurementmethod
Purposeandrequirements
Usetheelectrothermalmethodtomeasurethethermalpowerequivalenttodeepenstudents’understandingofthethermalpowerequivalentUnderstandingofphysicalmeaning.
Instruments
Calorimeter(J2251type),thermometer(0-100°C),studentpowersupply(J1202typeorJ1202-1type),DCammeter(J0407typeorJ0407-1),DCvoltmeter(J0408orJ0408-1),slidingrheostat(J2354-1),stopwatch,studentbalance(J0104),resistancewire(about6ohms),liquid(waterorKerosene),single-poleswitch(J2352type),severalwires.
Experimentalmethod
1.Useabalancetoweighoutthetotalmassoftheinnercylinderofthecalorimeterandtheagitatormcylinder.Thenwater(orkerosene)isinjectedintotheinnertubeofthevectorheatexchanger,andthevolumeofthewateroccupiesabout3/4ofthevolumeoftheinnertube.Useabalancetoweighoutthetotalmassofthecylinderandwaterm,thenmwater=m-mcylinder.Recordinthetable.
2.Puttheinnertubeofthecalorimeterintotheoutertube,puttheresistancewireandthestirrerintothewater,coverthecoverplate,andconnectthecircuit.Readtheinitialtemperaturet1androomtemperaturet0ofthecalorimetersystem,andrecordtheminthetable.
3.Turntheselectionknobofthestudent'spowersupplyvoltageoutputto10voltsor12volts,closetheswitchandstartthestopwatchatthesametime,andquicklyadjusttherheostattomakethecurrentaround1.5-2amperes.Observetheammeterandadjusttherheostatatanytimeinthefuturetokeepthecurrentvaluestable.Duringtheenergizationprocess,thewateriscontinuouslystirredslightlytoaccelerateheattransfer.Readthereadingsoftheammeterandvoltmeterandrecordtheminthetable.
4.Whenthethermometershows10-15℃higherthantheroomtemperature,turnofftheswitchandstoptimingatthesametime.Continuetostirthewaterandobservethethermometerreading.Whenthereadingisthehighest,readthetemperaturet2.Recordthepower-ontimetandthefinaltemperaturet2inthetable.
5.Useequations(1),(2),(3)andexperimentaldatatofindthethermalworkequivalent(checkthespecificheattableforspecificheat),compareitwiththeacceptedvalue,andfindthepercentageerror.
Measuringprinciple
Setthetotalmassoftheinnertubeofthecalorimeterandthestirrertomtube(madeofthesamematerial),andtheinnermassItisaliquidofMliquid,andtheinitialtemperatureist1.Whentheresistancewireisenergizedfortseconds,thefinalliquidtemperatureist2.AssumingthatthecurrentmeterandthevoltageindicatingnumbersareIandUwhenpowerison,theworkdonebythecurrentduringpoweronis
W=IUt
(1)Calorimeterinnercylinder(includingAgitator)andtheheatabsorptionoftheliquidisQ=(Ctubemtube+CliquidMliquid)(T2－t1)
(2)I,U,t,mtube,MLiquid,t1,t2canbemeasuredbyexperiment,thethermalpowerequivalent
J=W/Q