Heat Cannot Be Completely Converted
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Next: Isolated systems become disordered Up: 2nd Law of Thermodynamics Previous: Heat flows from hot Heat cannot be completely converted The 2nd form of the 2nd law heat cannot be completely converted into other forms of energy places some practical restrictions on the efficiency of, for example, internal combustion and steam powered engines. Before discussing this in more detail, however, let us see why this statement is surprising. As with the example of the ice cube melting on a hot day, nothing from energy conservation would prevent work from being completely converted into other forms of energy, and indeed such total conversion can happen for energies other than heat. For example, a ball released from rest which falls to the ground has all gravitational potential energy at the top and all kinetic energy at the bottom the potential energy at the top thus gets completely converted to kinetic energy at the bottom. Or if the ball is connected to a rope and pulley of some sort, its gravitational potential energy can be converted into useful work (churning butter for example). Although in practice some of the potential energy will be lost to friction, there is no reason in principle that all the potential energy cannot be converted into work. If we go back to our analogy between heat flowing from hot to cold and an object running down hill, we can understand that in principle, instead of going into kinetic energy to raise the temperature of the cooler substance heat can be harnessed to do useful work. In this context the second law makes the very surprising statement that some of the heat energy must always be lost, so that the conversion from heat to work is never 100% efficient. Note, however, that this form of the 2nd law places no restriction on converting other forms of energy into heat it is the conversion of heat into other forms of energy that turns out never to be 100% efficient, even in principle. An machine which converts heat into other forms of energy is called a heat engine; the generic operation, in accordance with the 2nd law, is pictured below. Figure 8.8: A generic heat engine
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Transcript of Heat Cannot Be Completely Converted
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7/14/2015 Heatcannotbecompletelyconverted
http://theory.uwinnipeg.ca/mod_tech/node80.html 1/3
Next:IsolatedsystemsbecomedisorderedUp:2ndLawofThermodynamicsPrevious:Heatflowsfromhot
Heatcannotbecompletelyconverted
The2ndformofthe2ndlawheatcannotbecompletelyconvertedintootherformsofenergyplacessomepracticalrestrictionsontheefficiencyof,forexample,internalcombustionandsteampoweredengines.
Beforediscussingthisinmoredetail,however,letusseewhythisstatementissurprising.Aswiththeexampleoftheicecubemeltingonahotday,nothingfromenergyconservationwouldpreventworkfrombeingcompletelyconvertedintootherformsofenergy,andindeedsuchtotalconversioncanhappenforenergiesotherthanheat.Forexample,aballreleasedfromrestwhichfallstothegroundhasallgravitationalpotentialenergyatthetopandallkineticenergyatthebottomthepotentialenergyatthetopthusgetscompletelyconvertedtokineticenergyatthebottom.Oriftheballisconnectedtoaropeandpulleyofsomesort,itsgravitationalpotentialenergycanbeconvertedintousefulwork(churningbutterforexample).Althoughinpracticesomeofthepotentialenergywillbelosttofriction,thereisnoreasoninprinciplethatallthepotentialenergycannotbeconvertedintowork.Ifwegobacktoouranalogybetweenheatflowingfromhottocoldandanobjectrunningdownhill,wecanunderstandthatinprinciple,insteadofgoingintokineticenergytoraisethetemperatureofthecoolersubstanceheatcanbeharnessedtodousefulwork.Inthiscontextthesecondlawmakestheverysurprisingstatementthatsomeoftheheatenergymustalwaysbelost,sothattheconversionfromheattoworkisnever100%efficient.Note,however,thatthisformofthe2ndlawplacesnorestrictiononconvertingotherformsofenergyintoheatitistheconversionofheatintootherformsofenergythatturnsoutnevertobe100%efficient,eveninprinciple.
Anmachinewhichconvertsheatintootherformsofenergyiscalledaheatenginethegenericoperation,inaccordancewiththe2ndlaw,ispicturedbelow.
Figure8.8:Agenericheatengine
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7/14/2015 Heatcannotbecompletelyconverted
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Theimportantaspecthereisthatsome``wasteheat''isalwaysexpelledintothecoolerreservoirnoheatenginecouldoperatewithoutsuchexpulsion.Thisiswhy,forexample,onenoticesinthewinternearasteampoweredelectricalgeneratingplantthatnearbyiceonariverismeltedthiscomesfromthewasteheatoftheplantbeingexpelledintotheriver.
Onecanshowthatthereisanidealmaximumefficiencypresentfortheconversionofheatintoexternalwork:thisis
wherethetemperaturesareexpressedintheKelvinscale.Itisimportanttonotethatthisisanidealefficiencyrealenginesalsolosesomeefficiencyduetofriction,etc.,butthisisabovethistheoreticallimit.Thus,aheatenginewouldoperatewith100%efficiencyinconvertingheatintousefulworkonlyifthecoolreservoirwasat0K(273o),whichisnotpossible.
Forexample,asteampoweredelectricalgeneratingplantwhichoperatesbetween500Kand300K(roomtemperature)hasamaximumefficiencyof40%.Similarconsiderationsholdforaninternalcombustionengine,thebasicoperationofwhichisillustratedbelow.
Figure8.9:Asimplifiedinternalcombustionengine
Inthissegmentofthecycle,thefuelmixtureexplodes,eitherfromasparkplugforagasengineorfromthehighpressureforadieselengine.Thisdrivesthepistondownwards,whichsubsequentlyturnsthecrankshaftandeventuallythewheelsthisisthepartwhichconvertstheenergyofheatintousefulwork.Thepistonthenrises,expellingtheexhaustgaseswhichcarryawaythewasteheat.The
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7/14/2015 Heatcannotbecompletelyconverted
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cyclethengoesontodrawinmorefuelmixturetorepeatthecycle.Themajorpointhereisthattheexhaustgasescarrywiththemexcessheatwhichcouldnotbeconvertedintousefulwork.
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