Energy conservation law

Energydefinition

Energyisameasureoftheconversionofmaterialmotion,referredtoas"energy".Everythingintheworldisconstantlyinmotion.Amongalltheattributesofmatter,motionisthemostbasicattribute,andotherattributesareconcretemanifestationsofmotion.Energyisameasureoftheabilityofaphysicalsystemtodowork.

Energyisoneofthebasicphysicalpropertiesofmatter,anditisaunifiedmeasureofthemovementofmatter.

Theunitofenergyisthesameastheunitofwork,whichisJoule(J)intheInternationalSystemofUnits.Inatomicphysics,nuclearphysics,particlephysicsandotherfields,electronvolts(eV)arecommonlyusedastheunit,1electronvolt=1.602,18×10^-19focus.Inthefieldofphysics,erg(erg)isalsousedasanenergyunit,and1erg=10^-7joules.

Energyexistsinmanydifferentforms;classifiedaccordingtothedifferentformsofmovementofmatter,energycanbedividedintomechanicalenergy,chemicalenergy,internalenergy(thermalenergy),electricalenergy,radiantenergy,andnuclearenergy.Thesedifferentformsofenergycanbetransformedintoeachotherthroughphysicaleffectsorchemicalreactions.Variousfieldsalsohaveenergy.

TheEnglishword"energy"forenergyisderivedfromtheGreek:ἐνέργεια,whichfirstappearedintheworksofAristotleinthe4thcenturyBC.Theideaof​​"energy"hasappearedintheGalileoera,buttheterm"energy"hasnotyetbeenavailable.TheconceptofenergyisderivedfromLeibniz'sideaof​​"vitality"inthe17thcentury.Itisdefinedastheproductofthemassofanobjectandthesquareofitsvelocity,whichisequivalenttotwicethekineticenergyoftoday.Inordertoexplaintheslowdownofspeedduetofriction,Leibniz’stheorybelievesthatthermalenergyiscomposedoftherandommovementoftheconstituentsubstancesintheobject,thatis,theinternalenergyoftheobject’smolecules,andthisideaisconsistentwithNewton,althoughthisconceptIttookacenturybeforeitwasgenerallyaccepted.

ThewordEnergywasintroducedbyT.Yangin1807whenhelecturedonnaturalphilosophyatKing’sCollegeLondon,andproposedtheuseof“energy”inresponsetotheviewpointsof“vitality”or“liftingforce”atthattime.Thiswordexpressesandisrelatedtotheworkdonebytheobject,butithasnotattractedmuchattention.Peoplestillthinkthatdifferentforcesarecontainedindifferentmovements.In1831,theFrenchscholarCoriolisintroducedtheconceptofworkdonebyforce,andaddeda1/2coefficientbefore"vitality",whichiscalledkineticenergy.Therelationshipbetweenworkandkineticenergyisgiventhroughintegration.Theterm"potentialenergy"appearedin1853,andtheterm"kineticenergy"appearedin1856.Onlyafterthelawofconservationofenergywasconfirmed,didpeoplerealizetheimportanceandpracticalvalueoftheconceptofenergy.

Spaceattributeisthemanifestationoftheextensionofthemovementofmatter;timeattributeisthecontinuousmanifestationofthemovementofmatter;theattributeofgravityisthemanifestationoftheinteractioncausedbytheunevenmassdistributionofmatterintheprocessofmovement;theelectromagneticpropertyisTheexternalperformanceofchargedparticlesintheprocessofmovementandchange,andsoon.Therearevariousformsofmovementofmatter,andeachspecificformofmattermovementhasacorrespondingenergyform.

Theenergyformcorrespondingtothemechanicalmotionofmacroscopicobjectsiskineticenergy;theenergyformcorrespondingtomolecularmotionisinternalenergy(thermalenergy);theenergyformcorrespondingtoatomicmotionischemicalenergy;theenergycorrespondingtothedirectedmotionofchargedparticlesTheformiselectricenergy;theenergyformcorrespondingtothemotionofphotons(electromagneticfield)islightenergy(electromagneticwaveenergy),andsoon.Inadditiontothese,therearewindenergy,tidalenergy,etc.Whentheformofmotionisthesame,themotioncharacteristicsoftheobjectcanbedescribedbycertainphysicalorchemicalquantities.Themechanicalmotionofanobjectcanbedescribedbyphysicalquantitiessuchasspeed,acceleration,andmomentum;currentcanbedescribedbyphysicalquantitiessuchascurrentintensity,voltage,andpower.However,iftheformsofmovementaredifferent,theonlyphysicalquantitythatcanbedescribedandcomparedwitheachotherisenergy,andenergyisthecommonpropertyofallmovingmatter.

Differentformsofenergycanbetransformedintoeachotherthroughphysicaleffectsorchemicalreactions.Correspondingtothevariousformsofmovementofmatter,energyhasvariousforms.Inmechanicalmotion,itisexpressedasthemechanicalenergyoftheobjectorthesystemasawhole,suchaskineticenergy,potentialenergy,etc.Inthermalphenomena,itisexpressedastheinternalenergyofthesystem,whichisthekineticenergyoftherandommovementofthemoleculesinthesystem,thepotentialenergyoftheinteractionbetweenmolecules,thesumoftheenergyintheatomsandthenucleus,butdoesnotincludethemechanicalenergyoftheoverallmovementofthesystem.Fortheinternalenergyofthermalmotion(previouslycalledthermalenergy),peopleknowitthroughthemutualconversionbetweenitandmechanicalenergy(seethefirstlawofthermodynamics).Variousfieldsalsohaveenergy.

Mechanicalenergy,chemicalenergy,internalenergy(thermalenergy),electrical(magnetic)energy,radiationenergy,nuclearenergyandotherdifferenttypesofenergycanbeconvertedintovariousways.Forexample,themostcommonelectricalenergy(alternatingcurrentandbatteries)canbeconvertedfrommanyotherformsofenergy,suchasmechanicalenergy-electricalenergyconversion(hydropower),nuclearenergy-internalenergy(thermalenergy)-mechanicalenergy-electricalenergyconversion(nuclearpowergeneration)),chemicalenergy-transformationofelectricalenergy(battery),etc.

Expressingtheessence

Commonexpressions:energyisneitherproducedoutofthinairnordisappearsoutofthinair,itwillonlytransferfromoneobjecttoanother,orfromoneformtoAnotherform,andintheprocessoftransformationortransfer,thetotalamountofenergyremainsunchanged.

Thefirstlawofthermodynamics:Thespecificexpressionoftheuniversallawofconservationandtransformationinallprocessesinvolvingmacroscopicthermalphenomena.Thefirstlawofthermodynamicsconfirmsthatinanyprocess,theamountofheatabsorbedbythesystemfromthesurroundingmedium,theworkdonetothemedium,andtheincreaseinenergywithinthesystemarequantitativelyconserved.

Thefirstlawofthermodynamicsisthelawofconservationofenergy.Itisasummaryofhumanexperienceandcanalsobederivedbyanalogywiththelawofconservationofmatter.Theenergyofathermodynamicsystemisexpressedasinternalenergy,heatandwork.Thefirstlawofthermodynamicsisanexpressionofenergyconservation.Theconclusionsderivedfromithavenotbeenfoundtocontradictthefacts.Accordingtothefirstlawofthermodynamics,itcanbeenvisagedtobuildamachinethatneitherreliesonexternalenergysupplynordoesitreduceenergyitself,butcontinuouslydoesexternalworkwithoutconsumingenergy.Peoplecallthisimaginarymachinethefirsttypeofperpetualmotionmachine.Becausedoingworkontheoutsideworldmustconsumeenergy,andwithoutconsumingenergy,youcannotdoworkontheoutsideworld.Therefore,thefirstlawcanalsobeexpressedas"PerpetualmotionmachineofthefirsttypeItisimpossibletocause>".Conversely,thefirsttypeofperpetualmotionmachinecanneverbecaused,whichprovesthatthefirstlawiscorrect.

Afterthethermodynamicsystemgoesthroughaprocessfromstate1tostate2,theinternalenergyofthesystemgenerallychanges.Accordingtothelawofconservationofenergy:

ΔU=Q－W(1)

whereΔU=U₂－U₁istheinternalenergyincrementofthesystem;Qistheheatabsorbedbythesystemfromtheenvironmentduringthisprocess;Wiswhatthesystemdoestotheenvironmentduringthisprocessachievement.Equation(1)isthemathematicalexpressionofthefirstlawofthermodynamics.

Informula(1),Uisthestatefunction,namelyΔUThevalueofonlydependsontheinitialstateandfinalstateofthesystem,andhasnothingtodowiththespecificprocessthatthesystemgoesthroughfromtheinitialstatetothefinalstate,andQandWarerelatedtotheprocess.PayattentiontothesignsofQandWwhenapplyingformula(1)Itis:thesystemabsorbsheatQ>0,thesystemreleasesheatQ<0;thesystemdoesworkontheenvironmentW>0,theenvironmentdoesworkonthesystemW<0.

Ifthereisaslightchangeinthestateofthesystem,thefirstlawofthermodynamicsiswrittenas:

dU=δQ－δW(2)

whereδQandδWarethetinyheatandtinyworkoftheprocess,respectively.Theyarenotfullydifferential,soweuse"δ"insteadof"d”,whichisdifferentfromtotaldifferential.

Thefirstlawofthermodynamicscanalsobestatedasthefirsttypeofperpetualmotionmachine(amachinethatcancontinuouslyperformworkautomaticallywithoutconsuminganyfuelorenergy)isimpossible.

Whenthesystemisopen,therearenotonlythermalandmechanicalinteractionsbetweenitandthemedium,butalsomaterialexchange.Intheexpressionofthefirstlawofthermodynamics,thereshouldbeanadditionalitemcausedbymaterialexchange.Theincreaseordecreaseofenergy.

Mechanicalenergyistheenergyformofanobjectinmechanicalphenomena,includingkineticenergyandpotentialenergy(potentialenergy),thatis,mechanicalenergy=kineticenergy+potentialenergy.

Inaclosedmechanicalsystem(conservativemechanicalsystem),whenonlyconservativeforcesdowork,andthereisnomutualconversionbetweenmechanicalenergyandotherformsofenergy,mechanicalenergyisconservedandsystemenergyappearsasmechanicalenergy.Conservationofenergyisspecificallyexpressedasthelawofconservationofmechanicalenergy.Thelawofconservationofmechanicalenergyisaspecialcaseofthelawofconservationofenergy.

Thelawofconservationofenergystatesthatenergycanonlychangefromoneformtoanotherandcannotbeproducedoreliminatedoutofthinair.Conservationofenergyisamathematicalconclusiondrawnfromtime'stranslationalsymmetry(translationalinvariance)(seeNoord'stheorem).

Accordingtothelawofconservationofenergy,theenergyflowinginisequaltotheenergyflowingoutplusthechangeininternalenergy.

Thislawisafairlybasiccriterioninphysics.Accordingtothetranslationalsymmetryoftime(translationinvariance),thelawsofphysics(theorems)holdatalltimes.

Inthespecialtheoryofrelativity,thelawofconservationofenergyappearsasthelawofconservationofmassandenergy.Thelawofconservationofmassandenergyisaspecialformofthelawofconservationofenergy.Mass-energyformulaE=mc²Describesthecorrespondingrelationshipbetweenmassandenergy.Inclassicalmechanics,massandenergyareindependentofeachother,butinrelativisticmechanics,energyandmassarethesamerepresentationoftwoaspectsofthemechanicalpropertiesofanobject.Inthetheoryofrelativity,massisextendedtomass-energy.Originally,theindependentconservationofmassandenergyinclassicalmechanicswascombinedintoaunifiedLawofConservationofMassandEnergy,whichfullyreflectstheunityofmatterandmotion.

Therelativisticenergyofasingle-massparticleincludesitsrestmassanditskineticenergy.Ifthekineticenergyofamassparticleiszero(orinarelativelystationaryreferenceframe),orasystemwithkineticenergyisinthemomentumcentersystem,itstotalenergy(includingthekineticenergyinsidethesystem)isrelatedtoitsstationaryorinvariantmass,ItsrelationalexpressionisthefamousE=mc².

Therefore,aslongastheobserver’sframeofreferencedoesnotchange,theconservationofenergywithrespecttotimeinthespecialtheoryofrelativitystillholdstrue,andtheenergyoftheentiresystemremainsunchanged.ObserversindifferentframesofreferencewillmeasureTheenergyisdifferent,buttheenergyvaluemeasuredbyeachobserverwillnotchangewithtime.Theinvariantmassisdefinedbytheenergy-momentumrelationship,whichistheminimumvalueofsystemmassandenergythatallobserverscanobserve.Invariantmassisalsoconserved,andthevalues​​measuredbyallobserversarethesame.

Peoplehaveconfirmedthelawofconservationofenergybasedonalargenumberofexperiments,thatis,whendifferentformsofenergyareconvertedtoeachother,theirvalueisconserved.TheJoulethermalpowerequivalentexperimentisawell-knownexperimentthatconfirmedthelawofconservationofenergyintheearlystage,andthenestablishedthefirstlawofthermodynamicsofenergyconversionandconservationinthemacroscopicfield.Comptoneffectconfirmsthatthelawofconservationofenergyisstillcorrectinthemicroscopicworld,andthengraduallyrealizedthatthelawofconservationofenergyisdeterminedbytheinvarianceoftimetranslation,thusmakingitauniversallawinphysics(seesymmetryandconservationlaws).

Itshouldbenotedthattheconceptofenergyhasitsscopeofapplication.Accordingtothegeneraltheoryofrelativity,themeasurementofenergycannolongerbeusedundercertainconditions.

Relatedexplanations

Theideaof​​thefirstlawofthermodynamicswasfirstproposedbytheGermanphysicistJ.Meyerin1842onthebasisofexperiments.Afterthat,theBritishphysicistJ.Jouledidalotofexperiments,usingvariousmethodstofindtheheatequivalent,andtheresultswereconsistent.Inotherwords,thereisacertainconversionrelationshipbetweenheatandwork.Afteraccurateexperimentaldetermination,1calorie=4.184joules.In1847,theGermanscientistH.Helmholtzmadeastrictmathematicaldescriptionofthefirstlawofthermodynamicsandclearlypointedout:"Thelawofconservationofenergyisoneofthebasiclawsuniversallyapplicabletoallnaturalphenomena."By1850,inthescientificcommunityHasbeenrecognized.

Confirmationoftheexistenceofenergyasaconservedquantitybeganattheendofthe17thcentury,whenG.Leibnizobservedtheparticleenergyintheearth’sgravitationalfield(mv²/2+mgh)conservation.Sincethe1840s,Jouleconfirmedthatheatisonlyaformofenergy,whichlaidthefoundationforthefirstlawofthermodynamics.In1905,Einsteinlinkedenergywiththerestmassofmatterandgavethefamousmass-energyrelationship.Inordertoexplainthepartoftheenergythat"disappeared"duringthebetadecayprocess,W.Pauliproposedthattheremustbeanunrecognizedparticle.Later,E.Ferminamedthisparticleaneutrino,andrecoveredthatpartoftheenergythat"disappeared".

Thefirstlawofthermodynamicsconfirmsthatthereisasingle-valuedstatefunctioninanysystem-internalenergy,andtheinternalenergyofanisolatedsystemisconstant.Theinternalenergyofanobjectisthesumofthekineticenergyoftheirregularthermalmotionofthemicroscopicparticlesthatmakeuptheobjectandthepotentialenergyoftheinteractionbetweenthemwhentheobjectisatrest.Theexperimentalbasisforthemacroscopicdefinitionofinternalenergyisthattheadiabaticworkvalues​​ofthesysteminthesameinitialandfinalstatesareallequal,independentofthepath.Itcanbeseenthattheworkdonebytheoutsideworldtothesystemintheadiabaticprocessisonlyrelatedtothechangeofacertainfunctionofthesystembetweentheinitialandfinalstates,andhasnothingtodowiththepath.Thisstatefunctionisinternalenergy.ItcanbedefinedbytheadiabaticworkAsthatthesystemdoestotheoutsideworld:U₂－U₁=－As,thenegativesignintheformulameansthatexternalworkispositivework.TheunitofworkisJoule.Inapureheattransferprocess,theinternalenergyofthesystemcanbechangedtodefinetheheatanditsvalue,namelyQ=U₂－U₁,hereitisdefinedthattheheatabsorptionofthesystemispositive(Qisgreaterthan0).Theunitofheatisalsojoules.

Heatandworkareprocessquantities.Theyonlyappearwhenthesystemstatechanges.Theirvalues​​arenotonlyrelatedtotheinitialandfinalstateoftheprocess,butalsorelatedtothepaththattheprocessgoesthrough.Workandheatarebothmeasuresofinternalenergychange,indicatingthatthereshouldbesomecommensurabilitybetweenthem.Historically,thenumericalexpressionofthiscommensurabilityiscalledthermalworkequivalent.

Thefirstlawofthermodynamicsisanextensionofthelawofconservationofenergytonon-isolatedsystems.Atthistime,energycanbetransferredintooroutofthesystemintheformofworkWorheatQ.

Howtoexplain:

1.Theincreaseintheinternalenergyofanobjectisequaltothesumoftheheatabsorbedbytheobjectandtheworkdoneontheobject.

2.Whenthesystemisinadiabaticstate,theworkonlydependsontheenergyintheinitialstateandtheendstateofthesystem,andhasnothingtodowiththeprocess.

3.Theenergyoftheisolatedsystemisalwaysconserved.

4.Thesystemgoesthroughanadiabaticcycle,andtheworkdonebyitiszero,sothefirsttypeofperpetualmotionmachineisimpossible(thatis,amachinethatdoesnotconsumeenergytodowork).

5.Whentwosystemsinteract,workhasauniquevalue,whichcanbepositive,negativeorzero.

TheoreticalInterpretation

InEinstein'sspecialtheoryofrelativity,energyisacomponentoffour-dimensionalmomentum.Inanyclosedsystem,whenobservinginanyinertialsystem,eachcomponentofthisvector(oneofwhichisenergyandtheotherthreeismomentum)willbeconserved,andwillnotchangewithtime,andthelengthofthisvectorwillalsobeconserved(Minkowskimoduluslength),thevectorlengthistherestmassofasingleparticle,anditisalsotheinvariantmass(ie,invariantenergy)ofthesystemcomposedofmulti-massparticles.

Inquantummechanics,theenergyofaquantumsystemisdescribedbyaself-adjointoperatorcalledtheHamiltonian,whichactsontheHilbertspace(orwavefunctionspace)ofthesystem)middle.IftheHamiltonianisatime-invariantoperator,asthesystemchanges,themeasurementofitsprobabilityofoccurrencedoesnotchangewithtime,sotheexpectedvalueofenergydoesnotchangewithtime.Thelocalenergyconservationunderquantumfieldtheorycanbeobtainedbyusingenergy-momentumtensoroperatorwithNoord'stheorem.Sincethereisnoglobaltimeoperatorinquantumtheory,theuncertaintyrelationshipbetweentimeandenergywillonlybeestablishedundercertainconditions,andtheuncertaintyrelationshipbetweenpositionandmomentumistheessenceofquantummechanics.Different(seeUncertaintyPrinciple).Theenergyateachfixedtimecanbeaccuratelymeasuredandwillnotbeaffectedbytheuncertainrelationshipbetweentimeandenergy.Therefore,eveninquantummechanics,energyconservationisaclearlydefinedconcept.

Thelawofconservationofenergyisacharacteristicofmanylawsofphysics.Fromamathematicalpointofview,theconservationofenergyistheresultofNoord'stheorem.Ifthephysicalsystemsatisfiescontinuoussymmetrywhenshiftingintime,itsenergy(theconjugatephysicalquantityoftime)isconserved.Onthecontrary,ifthephysicalsystemhasnosymmetryintimetranslation,itsenergyisnotconserved.However,ifoneconsidersthatthissystemexchangesenergywithanothersystem,andthecombinedlargersystemdoesnotchangewithtime,theenergyofthislargersystemwillbeConservation.Sinceanytime-varyingsystemcanbeplacedinalargernon-time-varyingsystem,energyconservationcanbeachievedbyappropriatelyredefiningenergy.Forthephysicaltheoriesinflatspace-time,sincequantummechanicsallowsforshort-termnon-conservation(suchaspositive-anti-particlepairs),energyconservationisnotobservedinquantummechanics.

AccordingtoNoord'stheorem,thelawofconservationofenergyexpressesthecorrespondencebetweencontinuoussymmetryandthelawofconservation.Thelawofconservationisthemostbasiclawthatmustbeobservedduringthemovementofmatter.Ithasbecomeoneofthemostuniversalandprofoundconceptsinphysics.Forexample,thelawsofphysicsdonotchangewithtime,whichmeansthattheyhaveacertainsymmetrywithrespecttotime.Noord'stheoremisdeeplyrelatedtoquantummechanics,becauseitcandistinguishphysicalquantities(suchastimeandenergy)relatedtoHeisenberg'suncertaintyprincipleusingonlytheprinciplesofclassicalmechanics.Thewell-knownlawofconservationofenergyisgivenfortheinvarianceoftimetranslation.

Thespace-timeperformanceisuniformandisotropic.Thetranslationoftheoriginofthecoordinatesystemandtherotationofthecoordinateaxisaresymmetricaltransformations.TheyformtheinhomogeneousLorentzgroup,alsoknownasthePoincarégroup.InthePoincarégroup,thephysicalquantitycorrespondingtothetranslationgeneratoristheenergy-momentumvector.Theconservationofenergy,momentum,andangularmomentumaredirectlyrelatedtotheuniformityandisotropyoftimeandspace,anditdoesnotdependonthespecificcontentofmatter.Whetheritismicroscopicormacroscopic,particlesorfields,allmattermovinginuniformandisotropicspace-timeobeysthelawsofconservationofenergy,momentum,andangularmomentum.

Experimentalverification

TheJoulethermalpowerequivalentexperimentisawell-knownexperimenttoconfirmtheconservationofenergyearly.Underthepremiseofkeepingthetotalenergyconstant,theinherentenergy,kineticenergy,andpotentialenergycanbetransformedintoeachother.Themosttypicalexampleisintheprocessofannihilationofpositronsandnegativeelectronsintophotons,alltheinherentenergyofthepositiveandnegativeelectrons(correspondingtotherestmass)isconvertedintophotonenergy,thatis,electromagneticradiationenergy(thecorrespondingmassisthedynamicmassofthephoton).Anotherexampleisthatintheprocessofnuclearfission,partoftheinherentenergyisconvertedintokineticenergy.Foracompositesystemcomposedofmultiplecomponents,theoverallinherentenergy(orstaticmass)isthesumoftheinherentenergy(orstaticmass)ofeachcomponentandtheinteractionpotentialenergy.Forexample,thestaticmassofastablenucleusissmallerthanthesumofthestaticmassesofitsnuclei(protonsandneutrons).Thedifferencebetweenthetwoiscalledmassloss,andthecorrespondingenergyisthebindingenergyofthenucleus(fromtheInteractionpotentialenergy);nuclearenergyisthebindingenergyofthenucleusreleasedduringthenuclearreaction,anditisthedirectevidenceofthemass-energyrelationship.

Themosttypicalexampleofthesuccessfulapplicationofthelawofconservationofenergyandthelawofconservationofmomentum(angularmomentum)isthediscoveryofneutrinosinelementaryparticleexperiments.Aneutrinoisanelementaryparticlewithasmallstaticmass,nocharge,andextremelyweakinteractionwithmatter.Inthelate1920sandearly1930s,studiesonthebetadecayenergyspectrumofatomicnucleifoundthattheelectrons(ie,betarays)emittedafterdecaycarrylessenergythantheyshouldtakeawayaccordingtothelawofconservationofenergy(itseemstobelost)Partoftheenergy),andthespinofthenucleusandelectronsdonotconformtotheangularmomentumsynthesisrulesinquantummechanics.Inordertoexplainthisphenomenon,eitherabandonthelawsofconservationofenergyandangularmomentum,orassumethatthereisanunobservedelementaryparticle,namelyneutrinos,inordertomaintaintheseconservationlaws.Thephysicistfinallychosethelatter,andusedotherelementaryparticleexperimentstoconfirmtheexistenceofneutrinos(andantineutrinos).Thelawofconservationofenergyandthelawofconservationofmomentum(angularmomentum)arestillvalidintheseprocesses.

Theabove-mentionedspecialrelativityconceptsanddefinitionsofenergy,mass,andmomentum,aswellasthelawofconservationofenergyandthelawofconservationofmomentum(angularmomentum),orthemoregenerallawofconservationofenergy-momentum(conservationofangularmomentumisincludedinit),Notonlyapplicabletomechanicalphenomena,butalsoapplicabletophysicsintheentireflatspace-time.

DiscoveringHistory

BriefOverview

Theproposalof"EnergyConversionandConservationLaw"mustbebasedonthreefoundations:①TheessenceofheatiscorrectUnderstanding;②Discoveryofthetransformationbetweenvariousformsofmaterialmovement;③Correspondingscientificthinking.Bythe19thcentury,allthreeconditionsweremet.

Thelawofconservationofenergydiscoveredinthemiddleofthe19thcenturyisaveryimportantlawinnaturalsciences.Itsdiscoveryistheinevitableresultofthegradualaccumulationofhumanunderstandingofthelawsofnaturalsciencetoacertainextent.ThelawofconservationofenergyisrelatedtomechanicalenergyandThelawofinternalenergy(thermalenergy).

Fromtheendofthe18thcenturytothemiddleofthe19thcentury,mankindestablishedthefirstlawofthermodynamicsonthebasisofaccumulatedexperience,alargenumberofproductionpractices,andscientificexperiments.Intheprocess,theGermandoctorJ.MeyerandtheBritishphysicistJ.Joulemadeimportantcontributions,andtheyeachreachedthesameconclusionthroughindependentresearch.In1842,Meyerproposedtheprincipleofmutualconversionbetweenmechanicalenergyandheatinthearticle"OntheForceoftheInorganicWorld",andcalculatedthevalueofthermalworkequivalentfromthedifferencebetweenthespecificheatcapacityofairataconstantpressureandthespecificheatcapacityofaconstantvolume.Thebook"OntheMovementandMetabolismofOrganisms",publishedin1845,described25situationsinwhichtheformsofmovementweretransformed.Since1840,Joulehasdonealotofexperimentsonthethermaleffectofelectriccurrentandthermalpowerequivalent(seeJoulethermalpowerequivalentexperiment).From1840to1845,hesuccessivelypublished"OntheHeatGeneratedbyVoltaBatteries","TheHeatReleasedinMetalConductorsandBatteryPacksDuringElectrolysis","OntheThermalEffectofMagnetoelectricityandtheMechanicalActionofHeat"and"OntheCauses""Thetemperaturechangecausedbytheexpansionandcontractionofair"andotherarticles.Throughvariouspreciseexperiments,hedirectlyobtainedthenumericalvalueofthethermalpowerequivalent,andtheconsistencyoftheresultslaidasolidexperimentalfoundationforthelawofenergyconservationandconversion.InadditiontoMeyerandJoule,manyscientistshavealsocontributedtotheestablishmentofthefirstlawofthermodynamics.Forexample,in1839,M.Seguinmadeanarticleonthelawthattheheatofreactioninthermochemistryhasnothingtodowiththeintermediateprocess;in1843,L.Koldingpublishedtheexperimentalresultsofmeasuringthethermalequivalent;in1847,H.HelmholtzpublishedUnderthehypothesisofpsychicforce,accordingtothelawsofmechanics,thelawofmutualconversionandconservationofthe"force"ofmechanicalmotion,thermalmotion,andelectromagneticmotionisdiscussedinacomprehensivemanner.Duringthishistoricalperiod,scientistsfromvariouscountrieswereabletoindependentlydiscoverthelawsofconservationandconversionofenergy,whichwasdeterminedbytheproductionconditionsatthattime.Fromthebeginningofthe18thcenturytothesecondhalfofthe18thcentury,themanufactureandimprovementofsteamenginesandtheirwidespreaduseintheBritishironmakingandtextileindustries,aswellastheresearchontheefficiencyofheatenginesandtheproblemsoffrictionandheatgenerationinmachines,greatlypromotedpeople’senergyUnderstandingofthelawofconversion.

DiscoveryAfter

In1798,C.RenfordsubmittedtotheRoyalSocietyanexperimentalreportonthethermalmotiontheoryderivedfromthebarrelexperiment.In1800,D.DavidsupportedRenford'sreportbyrubbingicecubesinavacuumtomeltthem.In1801,T.Yangstatedinhis"OntheTheoryofLightandColor"thatlightandheathavethesamepropertiesandemphasizedthatheatisamovement.Fromthenon,thetheoryofthermalmotionbegantograduallyreplacethetheoryofthermalmass.

Attheturnofthe18thand19thcenturies,themutualtransformationbetweenvariousnaturalphenomenawasdiscoveredoneafteranother:aftertheconversionofheattoworkandthediscoveryofthechemicaleffectoflight,thethermaleffectofinfraredrays(resonanceeffect)wasdiscoveredin1800.).Assoonasthebatterywasinvented,theheatingeffectofcurrentandthephenomenonofelectrolysiswerediscovered.In1820,themagneticeffectofelectriccurrentwasdiscovered,andin1831,thephenomenonofelectromagneticinductionwasdiscovered.Thethermoelectricphenomenonwasdiscoveredin1821,itsinversephenomenonwasdiscoveredin1834,andsoon.

Attheturnofthecentury,thinkingofnatureas"vitality"wasthemainpointofGerman"naturalphilosophy".Thisphilosophytreatstheentireuniverseasaproductofhistoricaldevelopmentcausedbyacertainsourceofforce.Atthattime,thisphilosophicalthoughtwasdominantinGermanyandsomecountriesinWesternEurope.

Carnotfirstproposedtheconversionofheatwork.Hebelieved:"Heatisnothingmorethanakindofpower,orsimplyaformofmovement.Heatisakindofmovement.Forasmallpartofanobject,Iftheannihilationofpoweroccurs,thenatthesametime,itwillinevitablygenerateheatthatisstrictlyproportionaltotheamountofpowertobeeliminated.Onthecontrary,wheretheheatiseliminated,powermustbegenerated.Therefore,thepropositioncanbeestablished:theamountofpowerisItisconstantinnature.Moreprecisely,theamountofpowercanneitherbeproducednordestroyed."Atthesametime,aroughvalueofthethermalequivalentisgiven.

Kano’sthoughtwasonlytakenseriouslyin187846yearsafterhisdeath.In1842,MeyerofGermanyfirststartedfrom"naturalphilosophy",andinaspeculativeway,25formsoftransformationofforcewerereleasedfromthecausalchainof"causeequalseffect".In1845,healsousedthedifferencebetweenconstantpressurespecificheatcapacityandconstantvolumespecificheatcapacity:C_p－C_v=R,thecalculatedthermalpowerequivalentvalueis1cal=365g·m.

In1843,Joule,aBritishexperimentalphysicist,didmoreworkanddeterminedamoreaccurateequivalentvalue.In1850,theresultwaspublished:"Toproduceapoundofwater(weighedinavacuum,withatemperaturebetween55°and60°)andincreasetheheatby1°Fahrenheit,ittakes772poundstofallonefoot.Mechanicalwork."Joule'sworklaidasolidexperimentalfoundationfortheprincipleof"conservationofforce".

GermanscientistHelmholtzpublishedhisbook"TheConservationofPower"in1847.Itisproposedthatallnaturalphenomenashouldbeexplainedbythemovementoftheparticlesinteractingwiththecentralforce.Thisprovesthatthesumofvitalityandtensionconservesthecentralforce.Thenitdiscussestherelationshipbetweenthermal,electrical,chemicalandmechanicalforces,andpointsoutthepossibilityofapplyingtheprincipleof"conservationofforce"tolivingorganisms.BecauseHelmholtz’sdiscoursehasphysicalcharacteristics,itsinfluenceisgreaterthanthatofMeyerandJoule.

Thediscoverersofthelawstillcallenergy"force";andtheexpressionofthelawisnotaccurateenough,butthelawofenergyconversionandconservationhasactuallybeendiscovered.Comparingthetwoexpressions,itcanbeseenthatthe"conservationofforce"ismuchdeeperthanthe"perpetualmotionmachinecannotbecaused"."Conservationofforce"involvesalltheformsofmovementofmatterthatshouldberecognized;atthesametime,itisguidedbycertainphilosophicalthoughts(Meyer),onthebasisofexperiments(Joules),usingaxiomaticstructures(HelmholHereby)establishedtheory.

Althoughtheprincipleof"conservationofforce"hastherelationshipbetweenJoule'sthermalpowerequivalentandelectricthermalequivalent,aswellasvariousrelationshipsintroducedbyHelmholtz,theyareallindependentandhavenotyetbeenused.Aunifiedanalyticalformulatoexpress.

Analyticalexpression

Fortheanalyticalexpressionofthelaw,onlyaccuratedefinitionsoftheconceptsof"heat","work","energy"and"internalenergy"willwork.Inthe18thcentury,theconceptof"calories"wasthequantityofheat.In1829,J.Ponceletclearlydefinedworkastheproductofforceanddistancewhilestudyingthesteamengine.Theconceptof"energy"wasadoptedbyJ.Bernoulliwhendiscussingvirtualdisplacementin1717.In1805,T.Youngreferredtoforceasenergy,thusdefiningYoung'smodulus.Butitsdefinitionhasnotbeenacceptedbypeople.Agroupofpeopleofinsightrealizedthegreatsignificanceofthelawanddidfruitfulworktoperfectthelaw.ThemostfamousoftheseareW.ThomsonoftheUnitedKingdomandR.ClausiusofGermany.Itistheywhoputforwardthefirstandsecondlawsofthermodynamicsonthebasisoftheirpredecessors,andestablishedtheedificeofthetheoreticalsystemofthermodynamics.

In1850,Clausiuspublishedapaperentitled"OntheDynamicsofHeatandtheLawsThatCanBeDerivedfromItonHeat".PointoutthatCarnot'stheoremiscorrect,usethermalmotiontoexplainandaddproof.Itisbelievedthatthesingleprincipleis“Inallcaseswhereworkisproducedbyheat,aheatproportionaltotheworkproducedisconsumed.Onthecontrary,thisamountofheatcanbeproducedbyconsumingthesameamountofwork.”Aprincipleisadded."Withoutanypowerconsumptionorotherchanges,movinganyamountofheatfromacoldbodytoahotbodyisincontradictionwiththebehavioroftheheatelement."Todemonstrate.Thinkofheatasastatequantity.

Clausiusfinallyobtainedtheanalyticalformulaofthefirstlawofthermodynamics:

dQ=dU-dW

Atthistime,theentropyofthelawofenergyconversionandconservationandthesecondlawofthermodynamicsTheexpressionstogetherformthebasisofthetheoreticalsystemofthermodynamics.

In1853,Thomsonreintroducedthedefinitionofenergy:"Weexpresstheenergyofamaterialsysteminagivenstateas:whenittransitionsfromthisgivenstatetoanyfixedstateinanywayInthezerostateofthesystem,thesumofvariouseffectsmeasuredbymechanicalworkunitsgeneratedoutsidethesystem."ThestatefunctionUiscalledinternalenergy.PeoplebegantodistinguishNewton's"force"fromthe"energy"thatcharacterizesthemovementofmatterandusedthemwidely.Onthisbasis,ScottishphysicistW.Rankinchangedtheprincipleof"conservationofforce"totheprincipleof"conservationofenergy".

Since1854,Clausiushasdonealotofwork,tryingtofindamethodofproofthatiseasyforpeopletoexplainthisprinciple.In1860,theprincipleofconservationofenergywaswidelyrecognized.

Processofformation

Energycanneitherbecreatednordestroyed.Conservationofenergyisoneoftheuniversallawsofmattermovement.Therearevariousformsofmaterialmovement,andtheycanbetransformedintoeachother.Beforeandafterthetransformation,thesumoftheenergyasameasureofthemovementofmatterremainsunchanged.Theconceptofenergyconservationhaslongbeenprovedbyphysicistsinthefieldofmechanics.However,theextensionofthisconservedconcepttointernalenergy(heatenergy)tooktwotothreehundredyears.Therehavebeenvariousmisconceptionsaboutheatenergyinhistory.Fromthe18thcenturytothemiddleofthe19thcentury,thenaturalscienceshavelongbeendominatedbythermalmasstheory.Thisone-sidedtheorybelievesthatthereisakindoffluidinmatter,calledthermalmass.Theheattransfercausedbythetemperaturedifferenceisregardedastheflowofheatmassfromahigh-temperatureobjecttoalow-temperatureobject;andfrictionalheatgenerationisregardedastheresultofthereleaseofheatmass.Thistheorycontradictsmanyexperimentalfacts.In1798,Longforddevelopedabarrel.Itwasobservedthattheamountofheatgeneratedwasnotproportionaltotheamountofmetalshavingswornoffbythedrill.Moreover,ifabluntdrillwasusedtocontinuedrillingandgrinding,theamountofheatreleasedwouldbealmostunlimited,indicatingthattheheatqualitywasnotItmaybeasubstance.Later,throughtheworkofH.David,J.Meyer,H.Helmholtz,etc.,especiallythethermalpowerequivalentexperimentconductedbyJ.Joulebetween1840and1848,peoplegraduallyrealizedthatheatmassdoesnotexist.Thetransferorconversionofheat,likethetransferorconversionofmechanicalworkandelectricalwork,isalsoatransferorconversionofenergy,andduringthetransferorconversion,thetotalenergyisconstant.Inthisway,theconservationofenergyisconfirmedonauniversalbasis.

In1860,thelawofconservationofenergy"soonbecamethecornerstoneofallnaturalsciences.Especiallyinphysics,everynewtheorymustfirstbetestedwhetheritconformstotheprincipleofconservationofenergy."ButThediscoverersoftheprincipleonlyfocusedontheconservationofquantitytogeneralizethelaw,withoutemphasizingthetransformationofmotion.

Untilthebeginningofthetwentiethcentury,animportantbasicconceptinthermodynamics-thedefinitionofheatstillusedinthe18thcentury,thisdefinitionisbasedonthetheoryofthermalmass,inthecornerstoneofthethermodynamicbuildingOnepieceisnotstrong.Therefore,in1909,C·Kararedefinedtheinternalenergy:"AnyobjectorobjectsystemhasastatefunctionUinitsequilibriumstate,whichiscalleditsInternalenergy,whentheobjectgoesthroughanadiabaticprocessfromthefirststatetothesecondstate,theincreaseinitsinternalenergyisequaltotheworkdonebytheoutsideworldtoitintheprocessWb>."

U₂－U₁=W

Theinternalenergydefinedinthiswayhasnothingtodowithheat,onlymechanicalenergyandelectromagneticenergy.Atthistime,thefirstlawofthermodynamics,thesecondlawofthermodynamicsandtheentiretheoryofthermodynamicsabandonthetheoryofthermalmass.

Historicalinfluence

Negationofperpetualmotionmachine

ItissaidthattheconceptofperpetualmotionmachineoriginatedinIndiaandwasintroducedtoEuropeinthe12thcentury.Accordingtorecords,oneoftheearliestandmostfamousperpetualmotiondesignsinEuropewasproposedbyFrenchV·Hennecointhe13thcentury.Subsequently,peoplewhoresearchandinventperpetualmotionmachinescontinuetoemerge,althoughmanyscholarshavepointedoutthatperpetualmotionmachinesareimpossible.

TheItalianscholarLeonardoDaVinciduringtheRenaissancespentalotofenergystudyingperpetualmotionmachines,andfinallycametotheconclusionthatperpetualmotionmachinescannotbemanufactured.ThecontemporaryJ.Cardan(famousforgivingtherootsforsolvingcubicequations)alsobelievedthatperpetualmotionmachineswereimpossible.Thefirsttypeofperpetualmotionmachineviolatesthelawofconservationofenergy,whilethesecondtypeofperpetualmotionmachineviolatesthesecondlawofthermodynamics.

Withtherecognitionoftheimpossibilityofperpetualmotionmachines,thepatentofficesofsomecountrieshavedecidednottoacceptpatentapplicationsforperpetualmotionmachines.

Empiricalexpression

Inthe13thcentury,peoplebegantosproutthedesiretocreateperpetualmotionmachines.Inthe15thcentury,DaVinci,agreatartist,scientistandengineer,devotedhimselftotheresearchofperpetualmotionmachines.In1475,DaVinciseriouslysummarizedthelessonsoffailureinhistoryandcametoanimportantconclusion:"Perpetualmotionisimpossible."Healsorealizedthatthereasonwhythemachinecan'tmoveforeverisrelatedtofriction.Soin-depthandeffectiveresearchonfriction.However,DaVincihasnevermadeascientificexplanationforfrictionhinderingthemovementofthemachine,andhasnotyetbeenabletounderstandtheessentialconnectionbetweenfriction(mechanicalmovement)andthermalphenomena.

Sincethen,somescholarshavecometotheconclusionthat"perpetualmotionmotionisimpossible"anduseditasanimportantprincipleinscientificresearch.S.Stevin,amathematicalmechanicintheNetherlands,usedthisprincipletoanalyzethe"Stevinchain"in1586andwasthefirsttodeveloptheparallelogramruleofforce.Galileoalsoappliedthisprinciplewhendemonstratingthelawofinertia.

In1673,C.HuygensreflectedthisviewinhisbookPendulumClock.ApplyingGalileo'sresearchresultsoninclinedplanemotiontocurvedmotion,itisconcludedthatwhenanobjectrotatesaroundahorizontalaxisundertheactionofgravity,itscenterofmasswillnotriseabovetheheightatwhichitfell.Therefore,itisconcludedthatitisimpossibletomakeaperpetualmotionmachinebymechanicalmethods.

ItistheFrenchyoungscientistCarnotwhohasmadebrilliantachievementsinscientificresearchusingtheprincipleof"perpetualmotionmachineisimpossibletomake"inhistory.

In1824,Carnotintroducedthe"CarnotTheorem".Theprinciplecanonlybeusedinmechanicalmotionandtheflowof"thermalmass".Itisnotthelawofenergytransformationandconservationinthemodernsense,butonlytheenergyinmechanicalmotion.Theexperiencesummaryofconservationistheoriginalformofthelaw.

"Thefirsttypeofperpetualmotionmachineisimpossibletocreate"isanotherwayofexpressingthefirstlawofthermodynamics.Beforetheestablishmentofthefirstlaw,manypeopledreamedofmakingamachinethatdoesnotconsumeenergybutcandowork,calledthefirsttypeofperpetualmotionmachine.Thecompletefailureoftheefforttocreatesuchaperpetualmotionmachinehaspromotedtheestablishmentofthelawofenergyconservationandtransformationfromthenegativeside.

Thermalmachinery

In1798,AmericanC·Longforddiscoveredthatthemetalblankbecamehotwhenthebronzeblankofthebarrelwasdrilledwithaboringtool.Longfordnoticedthataslongastheboringdoesnotstop,themetalkeepsheatingup.Theconclusionisthatthemechanicalmovementoftheboringtoolisconvertedintoheat,soheatisaformofmovement,notasubstancepreviouslythought.Longfordtriedtocalculatetheheatgeneratedbyacertainamountofmechanicalenergy,andforthefirsttimegavearoughvaluefortheequivalentofthermalpower.Halfacenturylater,Jouleprovidedthecorrectvalue.

In1712,theBritishT.Newcomeninventedtheatmosphericsteamengine.Thiskindofmachinehasacylinderandapiston.Whenworking,steamisintroducedintothecylinderfirst.Atthistime,thecylinderstopssupplyingsteamandwaterentersthecylinder.Thesteamcondensesintowater,causingtheairpressureinthecylindertodroprapidlyandthewatercanbesuckedup.Thenthesteamisintroducedintothecylinderandentersthenextcycle.Theoriginalsteamenginewentbackandforthabouttentimesperminuteandcouldworkautomatically,whichgreatlyfacilitatedthepumpingworkinthemine.

JWattimprovedthesteamengineinthesecondhalfofthe18thcentury.Therearetwomostimportantimprovements.Oneistheinventionofthecondensertoimprovetheefficiencyofthesteamengine,andtheotheristheinventionofthecentrifugalgovernorsothatthespeedofthesteamenginecanbecontrolledfreely.AfterWattimprovedthesteamengine,itwaswidelyusedinindustry.

Theinventionofthethermometer

Theprecisetheoryofheatshouldbeginwiththemanufactureofthermometers.Inthe17thcentury,G.Galileoandothersbegantomakethermometers.Becauseoftheinconvenientuseoftheadoptedtemperaturescale,latergenerationsrarelyuseit.

In1714,thepracticaltemperaturescalewasthattheGermanphysicistD.Warrenheimbegantousemercuryasathermometerandcontinuedtoimproveit.In1717,theFahrenheittemperaturescalewasdetermined.ScientistsofficiallysettheFahrenheittemperaturescaleas:takingtheboilingpointofwateras212degrees,andsetting32degreesasthefreezingpointofwater.Thisstipulationistotrytoavoidthenormaltemperaturefromtakinganegativevalue.

From1742to1743,theSwedishastronomerA.CelsiusinventedtheCelsiustemperaturescale.Inthestandardstate,thefreezingtemperatureofwateriszeroandtheboilingpointofwateris100degrees.In1948,theCelsiustemperaturescalewassetasaninternationalstandardbytheInternationalConferenceonWeightsandMeasures.

Experimentalfindings

Thermalpowerequivalentexperiment

J·JoulemetProfessorDaltonoftheUniversityofManchesterin1835.Joule’sknowledgeofmathematicsislimited,andresearchmainlydependsonmeasurement.In1840,afterseveralmeasurementsofenergizedconductors,itwasfoundthatelectricalenergycanbeconvertedintointernalenergy(heatenergy),andalawwasderived:theheatgeneratedbyanelectricalconductorisproportionaltothesquareofthecurrentintensity,andtheresistanceoftheconductorisproportionaltothetimepassed.

Joulecontinuestodiscusstheenergyconservationandtransformationrelationshipbetweenvariousformsofmotion.JouledeclaredattheBritishAcademicConference:"Theenergyinnaturecannotbedestroyed.Mechanicalenergyisconsumedthere,anditcanalwaysgetaconsiderableamountofenergy.Heat,heatisonlyaformofenergy."

Joulecontinuouslyimprovesthemeasurementmethodandmeasurementaccuracy,andfinallyobtainsthephysicalconstantof"thermalpowerequivalent".ThemeasuredvalueofJouleis423.9kgm/kcal.Theexactvalueofthisconstantis418.4kgm/kcal.IntheInternationalSystemofUnits,joulesareusedastheunitofheat,and1calorie=4.184joules.

Thediscoveryofheat

Inthe1850s,BritishscientistJ.Blackmixed32°Ficecubeswithanequalweightof172°FwaterandfoundthattheaverageThetemperatureisnot102°F,but32°F,andtheeffectisthatalltheicecubesmeltintowater.Blackconcludedfromthis:Whenicemelts,itneedstoabsorbalotofheat,whichturnstheiceintowater,butdoesnotcausethetemperaturetorise.Heguessedthattheheatabsorbedwhentheicemeltsiscertain.AlargenumberoffurtherexperimentsledBlacktodiscoverthatvarioussubstanceshavethiseffectwhentheirstatechanges(melting,solidification,vaporization,andcondensation).

Blackusesasimpleandintuitivemethodtodeterminetheheatrequiredforwatervaporization.Blackmeasuredthattheheatrequiredtomeltacertainamountoficeisequaltotheheatrequiredtoheatthesameweightofwaterto140°F(equivalenttotheheatrequiredtoheat77.8°C).Thecorrectvalueis143°F(equivalentto80°C).

Basedonexperimentalfacts,Blackbegantorealizethatheatandtemperaturearetwodifferentconcepts,andintroducedtheconceptof"latentheat"(heat).

In1780,FrenchscientistA.LavoisierandP.Laplacejointlyproposedamethodtocorrectlymeasuretheheatcapacityofmatter.Becauseoftheprecisemeasurementofheat,in1822,FrenchscholarJ.Fourierpublishedaconclusivework"AnalyticTheoryofHeat".

Pioneer

Thestrugglebetweenvitalityandvitality

In1644R.Descartesdiscussedcollisionin"PrinciplesofPhilosophy"Inthequestion,theconceptofmomentumwasintroducedtomeasuremovement.In1687,Newtonusedthechangeofmomentumtomeasurepowerin"TheMathematicalPrinciplesofNaturalPhilosophy".ThedifferenceisthatG.LeibnizcriticizedDescartesinapaperin1686,advocatingthatthemassmultipliedbythesquareofthespeedshouldbeusedtomeasuremotion,whichLeibnizcalledvitality.TheforcemeasuredbyNewton'smomentumiscalleddeadforce.Leibniz’spropositionisconsistentwithHuygens’conclusionsonthecollisionproblem,whichstatedthat“whentwoobjectscollidewitheachother,thesumoftheirmassandthesquaredproductofvelocityremainsunchangedbeforeandafterthecollision.”

FromthetimeLeibnizprovokedtheargument,therewasadisputebetweenDescartesandLeibniz.Thiscontroversylastedfornearlyhalfacentury.Manyscholarshaveparticipatedinthecontroversy,andeachhasexperimentalevidence.In1743,FrenchscholarJ.D'Alembertsaidin"OnDynamics":"Formeasuringaforce,useittogivevitalitytoanobjectthatisaffectedbyitandpassacertaindistance,oruseittogiveacertainThemomentumoftheobjectoftimeisalsoreasonable.”D'Alembertrevealedthatvitalityisameasureofforcebasedondistance,andmomentumisameasureofforcebasedontime.Thecontroversyfinallysettled.Vigorisgenerallyacceptedasaformalmechanicsterm.

Althoughtheconceptofvitalityisaccepted,therelationshipbetweenvitalityandpowerisnotclear.In1807,theBritishscholarT.Youngintroducedtheconceptofenergy,andin1831,theFrenchscholarG.Coriolisintroducedtheconceptofforcework,whichmeansthatforceworkistransformedintothekineticenergyofobjects,thatis,theconservationofmechanicalenergyinnature.

Mayer'sdiscovery

J.Mayer(1814-1878)wasaGermanphysicist.DuringthevoyagetoJavain1840,hebecameinterestedinphysicsduetotheconsiderationofanimalbodytemperature.Whenhebleedsasicksailor(apopulartreatmentatthetime),hefindsthatthebloodintheveinsisbrighter.Hethoughtthatthebloodisbrightredinthetropics,andthebodydoesnotneedmoreoxygentoburntomaintainbodytemperaturelikeinthetemperatezone.ThisphenomenonpromptedMeyertothinkaboutthefactthatfoodinthebodyisconvertedintoheatandthebodycandowork.Thusitisconcludedthatheatandworkcanbetransformedintoeachother.

Henoticedthatmanypeople'sexperimentswithperpetualmotionmachinesatthattimeendedinfailure,makinghimguessthat"mechanicalworksimplycannotarisefromnothing."

InhislettertohisfriendsonSeptember12,1841,hefirstmentionedthethermalequivalent:"Themostimportantthingisstilltosolvethefollowingproblem:acertainheavyobject(forexample,100pounds)mustbeliftedtothegroundHowhighisittomaketheamountofexercisecorrespondingtothisheightandtheamountofexerciseobtainedbyputtingtheweightdownexactlyequaltotheheatnecessarytoconvertapoundof0°Ciceinto0°Cwater."

In1840,Meyerbegantothinkaboutwheredoestheheatcomefrom?Themovementoftheheartcannotgeneratesomuchheatandcannotmaintainaperson'sbodytemperature.Bodytemperatureismaintainedbythefleshandbloodofthewholebody,whichcomesfromfoodandultimatelyfromplants.Plantsgrowbyabsorbingtheheatandlightofthesun.Itboilsdowntohowenergyistransformed(transferred)?

Meyerwrotean"OntheForceoftheInorganicWorld",andmeasuredthethermalpowerequivalentof365kgm/kcal.Submittedthepapertothe"PhysicsYearbook",butfailedtogetpublished.Notonlywashenotunderstoodacademically,buthealsoexperiencedmajorblowsinlife.In1858,Mayrwasrediscoveredintheworld,andhewasawardedanhonorarydoctoratebytheBaselAcademyofNaturalSciencesinSwitzerland.ReceivedtheCopleyMedaloftheRoyalSociety,HonoraryDoctorofPhilosophyfromtheUniversityofTübingen,andAcademicianoftheAcademyofSciencesofBavariaandTurin,Italy.

Meyerwasthefirstscholartoconductthermalpowerequivalentexperiments,althoughhisexperimentswererougherthanJoule’sexperiments.Hefirstexpressedthelawofconservationofenergy:"Itistheoppositeproofthatshowstheabsolutetruthofmylaw:ascientificallyrecognizedtheorem:thedesignofaperpetualmotionmachineistheoreticallyabsolutelyimpossible."

Meyerarguedthatthesunistheultimatesourceofallanimateandnon-livingenergiesontheearth.

Later,HelmholtzandJoule’spaperswerepublishedoneafteranother.PeopleattributedtheinventorofthelawofconservationofenergytoHelmholtzandJoule,butdidnotrecognizeMayr.

In1858,HelmholtzreadMeyer’s1852paperandadmittedthatMeyer’sideaspredatedhisinfluentialpaper.ClausiusalsobelievedthatMeyerwasthediscovererofthelawofconservation.In1862,TyndallintroducedMeyer'sworksystematicallyattheRoyalSocietyofLondon,andhisachievementswerefinallyrecognizedbythesociety.

TheDiscoveryofHelmholtz

OnJuly23,1847,H.Helmholtz(1821-1894)presentedtotheInstituteofPhysicsHewroteareportentitled"ConservationofPower"andhandedthearticletotheeditorofTheChroniclesofPhysics.Unexpectedly,hesufferedthesamefateasMeyer'smanuscriptin1841.Theeditorrefusedtopublishitbecausetherewasnoexperimentalfact.Hepublishedthepaperasapamphletinawell-knownpublishinghouse.TheconclusionofthearticleiscompletelyconsistentwithJoule'sexperimentin1843,anditwasquicklycalled"thehighestandmostimportantprincipleinnature".Duetothepublicationofafamouspublishinghouse,HelmholtzandMeyer'sfatearecompletelydifferent.BritishscholarKelvinadoptedtheconceptofenergyproposedbyT.Young,using"potentialenergy"insteadof"elasticity",and"kineticenergy"insteadof"vitality".Theambiguityoftheconceptthathaslastedfornearly200yearsinmechanicshasbeenchanged.

Thelawofconservationofenergyisauniversalbasiclawinnatureandapowerfulweaponforpeopletounderstandandusenature.