6/10/2015 Whatisasimpledefintionofthelawsofthermodynamics?

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Question

Whatisasimpledefintionofthelawsofthermodynamics?

Askedby:JamesBeal

Answer

Thermodynamicsisthestudyoftheinterrelationbetweenheat,workandinternalenergyofasystem.

TheBritishscientistandauthorC.P.Snowhadanexcellentwayofrememberingthethreelaws:

1. Youcannotwin(thatis,youcannotgetsomethingfornothing,becausematterandenergyareconserved).

2. Youcannotbreakeven(youcannotreturntothesameenergystate,becausethereisalwaysanincreaseindisorderentropyalwaysincreases).

3. Youcannotgetoutofthegame(becauseabsolutezeroisunattainable).

Answeredby:DanSummons,PhysicsUndergradStudent,UOS,Souhampton

Insimplestterms,theLawsofThermodynamicsdictatethespecificsforthemovementofheatandwork.Basically,theFirstLawofThermodynamicsisastatementoftheconservationofenergytheSecondLawisastatementaboutthedirectionofthatconservationandtheThirdLawisastatementaboutreachingAbsoluteZero(0K).

However,sincetheirconception,theselawshavebecomesomeofthemostimportantlawsofallscienceandareoftenassociatedwithconceptsfarbeyondwhatisdirectlystatedinthewording.Togiveyouabetterunderstandingonhowtheselawscameaboutandtheirmodernscopeofcoverage,youhavetounderstandwhenandwhytheselawsweregenerated.

Ourstorybeginsbackinthemidseventeenthcentury.Societypriortotheeighteenthcenturyfavored

6/10/2015 Whatisasimpledefintionofthelawsofthermodynamics?

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developmentsinthelifesciences(largelyformedicalresearch)andastronomy(fornavigationandarecordofthepassageoftimealsoasourceforearlymythologyandfolklore).Sciencewasviewedaspurelyaphilosophicendeavor,wherelittleresearchwasconductedbeyondthemostusefulfields.Indeed,philosophyandsciencewereinseparableinseveralemergingdisciplines(thisisalwaystrueofnewfieldswherenofirmbasisofstudyhasyetbeenconducted).

However,Europeansocietywasabouttoexperienceunforeseeablerapidchanges.Priortothemideighteenthcentury,thegeneralEuropeanpopulacerandomlydottedthelandinsmallagriculturalcommunities,industrywasrunoutofcountrycottages,andscientificdevelopmentswerenearlyatastandstill.Suddenly,withoutmuchofatransition,newpocketsofindustryarose,focusingtowardslargescaledmachinesratherthansmallhandtoolslargeindustrialcorporationsoftencrushedsmallagriculturallycenteredcommerceandinmanyareas,cityliferenderedcountryfarmcottagesobsolete.Coincidingwithaneraofvastsocialandpoliticalchanges,thishistoriceventwouldlatercometobecalledtheIndustrialRevolution.

Ifnecessitywerethemotherofallinnovation,thentheIndustrialRevolutionwouldbethemotherofallnecessities.Horriblelivingconditionsintheovercrowdedindustrialcitiesbredaplethoraofdiseasesandviruses.Thisalongwithotherresultsofspontaneousurbanizationdemandedscienceagaintoaddresstheproblemsofaneverchanginghumancivilization.

ScienceoftheIndustrialAgerespondedtosuchneedsbycenteringonmedicaladvancesintheearlystagesoftherevolution.Suchwastheeraofcrucialmedicalbreakthroughs,andageofgreatestphysiologistssuchasMarieCurie(radium),WilhelmRoentgen(xrays),LouisPasteur(pasteurization),EdwardJenner(smallpoxvaccination),JosephLister(bacteriaantiseptic),andCharlesDarwin(evolution).

Oncethemedicalcrisiswasrectified,sciencecouldconcentrateontheheartofanindustrialsocietylargescaledmachinery.Trueofnineteenthcenturymassindustry,thecompanywiththegreatestmachinesproducedmoreproducts,mademoremoney,andwasconsequentlymoresuccessful.Itisnatural,therefore,thatfiercecompetitionarosetofindthemostindustriousmachinerypossible,andhowfarthelimitsofthesemachinescouldbepushedastoachievemaximumproductivity(withoutconsumingmuchenergy).

Again,societywouldfuelscientificadvancement.Nineteenthcenturyscientistswereencouragedtostudythemachine,anditsefficiency.Todothis,physicistsanalyzedtheflowofheatinthesemachines,andthechemicalchangesthattranspirewhentheyperformwork.Thuswastheestablishmentofmodernthermodynamics.Firstontheagendaofthisnewdisciplinewastofindameansconvertheat(asproducedbymachines)intoworkwithfullefficiency.Ifsuchaflawlessconversioncouldbeaccomplished,amachinecouldrunoffitsownheat,producinganeverendingcycleofheattowork,renderingheat,convertingtowork,andsoforthadinfinitum

Theideaofsuchamachinethatcouldruncontinuouslyoffitsownexhaust,or'perpetualmotion'machineasitwasdubbed,excitedtheindustrialcorporations,whocontributedmuchfundingforitsdevelopment.However,astheresearchwascompleted,theresultswereallbutpleasingtothesponsors.Asitturnedout,theverysameresearchorientedtocreateaperpetualmotionmachineprovedthattheveryconceptisnotpossible.Theproofliesintwotheories(nowthree)thatarecurrentlyconsideredthemostimportantlawsinthewholebodyofsciencetheFirstandSecondLawsofThermodynamics.

TheFirstLawofThermodynamicsisreallyapreludetothesecond.Itstatesthatthetotalenergyoutput(asthatproducedbyamachine)isequaltotheamountofheatsupplied.Generally,energycanneitherbecreatednordestroyed,sothesumofmassandenergyisalwaysconserved.AmathematicalapproachtothislawproducedtheequationU=QW(thechangeintheinternalenergyofaclosedsystemequalstheheataddedtothesystemminustheworkdonebythesystem).Byitsnature,thisfindingdidnotrestricttheuseofperpetualmotionmachines.However,thenextlawwoulddealablowtoallbelieversofsuchawondermachine.

Thefirstlaw,abellwetherinthefrontierpasturesofThermodynamics,containedonemajorflawthatrendereditinaccurateasitstood.Thislawisbasedonaconceptualreality,onethatdoesnottakeintoconsiderationlimitsplacedbytransactionsoccurringintherealworld.Inotherwords,thefirstlawfailedtorecognizethatnotall

6/10/2015 Whatisasimpledefintionofthelawsofthermodynamics?

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circumstancesthatconserveenergyactuallyensuenaturally.Astheimpracticalityofthefirstlaw(todescribeallnaturalphenomenon)becameapparent,arevisionbecameessentialifsciencehopedfullytounderstandthermalinteractions,andthuskeeppacewithamachinedrivensociety.

Bornasamodificationtoitsoldersibling,theSecondLawofThermodynamicsmadenoearlypromisesofimportance.Furtherresearchintothenaturaltendenciesofthermalmovementinthelatternineteenthcenturydevelopedacodeofrestrictionsastohowheatconversionisachievedinthenaturalworld.Physicistsattemptingtotransformheatintoworkwithfullefficacyquicklylearnedthatalwayssomeheatwouldescapeintothesurroundingenvironment,eternallydoomedtobewastedenergy(recallthatenergycannotbedestroyed).Beingobsolete,thisenergycanneverbeconvertedintoanythingusefulagain.

OnephysicistnotedforsignificantexperimentsinthisfieldistheFrenchman,SadiCarnot.Hisidealengine,soproperlytitledthe'CarnotEngine,'wouldtheoreticallyhaveaworkoutputequaltothatofitsheatinput(thusnotlosinganyenergyintheprocess).However,hefellintoasimilartrapasinthefirstlaw,andfailedtoconducthisexperimentsaswouldnaturallyoccur.Realizinghiserror,heconcluded(afterfurtherexperimentation)thatnodevicecouldcompletelymakethedesiredconversion,withoutlosingatleastsomeenergytotheenvironment.

Carnotcreatedanequationheemployedtoprovethisstatement,andcurrentlyusedtoshowthethermodynamicefficiencyofaheatmachine:e=1TL/TH(theefficiencyofaheatmachineisequaltooneminusthelowoperatingtemperatureofthemachineindegreesKelvin,dividedbythehighoperatingtemperateofthemachineindegreesKelvin).Foramachinetoattainfullefficiency,temperaturesofabsolutezerowouldhavetobeincorporated.ReachingabsolutezeroislaterprovedimpossiblebytheThirdLawofThermodynamics(whichwouldsurfaceinthelate19thcentury).

Thesefindingsfrustratedthebelieversofaperpetualmotionmachine,andangeredtheindustrialtycoonswhosponsoredthewholeendeavor.Yet,notallwascompletelylost.Carnot'sequationhelpedindustrialengineersdesignenginesthatcouldoperateuptoan80%efficiencylevelanenormousimprovementoverpriordesigns,increasingproductivityexponentially.Moreover,byreversingtheheattoworkprocess,theinventionoftherefrigeratorwasmadepossible!Yet,thegreatestoverallfruitofthisventurewasthedevelopmentoftheSecondThermodynamicLaw,whichwouldlaterachievealegendarystatusasafundamentallawofnaturalscience.

LetusshortlyreturntoCarnotandtheheatengine.Theirrevocablelossofsomeenergytotheenvironmentwasassociatedwithanincreaseofdisorderinthatsystem.Scientistswishingtofurtherpenetratetherealmofchaosneededavariablethatcouldbeusedtocalculatedisorder.Thankstomidnineteenthcenturyphysicist,R.J.E.Clausius,thisPandemoniumcouldbemeasuredintermsofaquantitynamedentropy(thevariableS).Entropyactsasafunctionofthestateofasystemwhereahighamountofentropytranslatestohigherchaoswithinthesystem,andlowentropysignalsahighlyorderedstate.

LikeCarnot,Clausiusworkedoutageneralequation,hisbeingdevotedtothemeasurementofentropychangeoveraperiodoftime:(change)S=Q/T(thechangeinentropyisequaltotheamountofheataddedtothesystem[byaninvertibleprocess]dividedbythetemperatureindegreesKelvin).Thebeautyofthisequationisthatitcanbeusedtocomputetheentropicchangeofanyexchangeinnature,notsolelylimitedtomachines.Thisdevelopmentbroughtthermodynamicsoutoftheindustrialworkplace,andopenedthepossibilityforfurtherstudiesintothetendenciesofnaturalorder(andlackthereforeof),eventuallyextendingtotheuniverseasawhole.

Applyingthisknowledgetonature,physicistsfoundthatthetotalentropychange(changeinS)alwaysincreasesforeverynaturallyoccurringevent(withinaclosedsystem)thatcouldbethenobserved.Thus,theytheorized,disordermustbecontinuallyaugmentingevenlythroughouttheuniverse.Whenyouputiceintoahotcupoftea(aristocratsoftheVictorianerawereconstantlythinkingoftea),heatwillflowfromthehotteatothecoldiceandmelttheiceinthebelovedbeverage.Then,oncetheenergyinthecupisevenlydistributed,thecooledteawouldreachamaximumstateofentropy.Thissituationrepresentsastandardincreaseindisorder,believedtobeperpetuallyoccurringthroughouttheentireuniverse.

Answeredby:AndrewV.Liaugminas,IndependentResearcherChicago,Illinois.

6/10/2015 Whatisasimpledefintionofthelawsofthermodynamics?

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