SHANDONGUNIVERSITYChap6 Summary-1Whyweneed2ndLaw?Allprocessessatisfy1stLawSatisfying1stdoesnotensuretheprocesscanactuallyoccurIntroductionto 2nd LawAprocesshasdirectionEnergyhasqualityandquantityHeat SinkHeat SourceHeatengineThermalenergyReservoirWact.ouOReceiveheatQfromahightemperature source生MthQHWConvert part Qtowork Wnet.outnetoutOHeatEnginesQTihRejectwaste heatQtoalowtemperature sinkO2ndlawKelvin-PlanckStatement:Itisimpossibleforanydevicethatoperatesonacycletoreceiveheatfromasinglereservoirandproduceanetamountofwork.Noheatenginecanhaven=100%Refrigerators/heatpump:ThedevicesdriveheatQtransferfromT,toTHW.RefrigeratorThework inputtotherefrigerator/heatpumpnet,inwants QLQHeatQabsorbedfromrefrigeratedspaceTHeatpumpQHHeatQrejectedtohightemperature THwants QHRefrigerator,HeatPumpDesired outputuDesired outputQ,AirCOPCOPHCOPW.WreLinRequired inputRequired inputConditioner2nd law,Clausius Statement:Heatdoesnot,of its ownvolition,transferfromacoldmediumtoawarmerone.（热不能自发地、不付代价地从低温物体传到高温物体）
SHANDONG UNIVERSITY Chap6 Summary-1 1 Why we need 2nd Law? All processes satisfy 1st Law; Satisfying 1st does not ensure the process can actually occur Heat Engines Refrigerator, Heat Pump Introduction to 2nd Law Refrigerators/heat pump: The devices drive heat Q transfer from TL to TH, Thermal energy Reservoir Receive heat QH from a high temperature source The work input to the refrigerator/heat pump Heat QL absorbed from refrigerated space TL A process has direction Energy has quality and quantity Heat Source Heat Sink Convert part QH to work Wnet,out Reject waste heat QL to a low temperature sink Heat engine 2nd law, Kelvin-Planck Statement: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. No heat engine can have η=100% Heat QH rejected to high temperature TH Refrigerator wants QL Heat pump wants QH COP 2nd law, Clausius Statement: Heat does not, of its own volition, transfer from a cold medium to a warmer one. (热不能自发地、不付代价地从低温物体传到高温 物体) Air Conditioner
SHANDONG UNIVERSITYChap6 Summary-2SystemAprocesscanbereversedwithoutleavinganytraceonthesurroundingsSurroundingsReversible ProcessesInternal RevExternal RevWhy need RevIrreversible:heattransferThe bestknown reversible cycle; four reversible processesIsothermal expansionAdiabatic expansionIsothermalCompressionAdiabaticcompressionCarnotCycleCarnotheat engineReversedCarnotCycleCarnotrefrigerator/heatpumpCarnot Principle 1:Given Tand Th,Nth.irrev<.Nth,revCarnotPrinciple 2:Given T, and Th,Nth.all rev=Nth,revThe heat engine operates on the reversible Carnot CycleirreversibleheatengineTihrevCarnot HeatEngineTLQLreversible heat engineTth,revThTih.reTHOn二impossibleheatengineThrevThe refrigerator / heat pump operates on a reversible Carnot CycleCarnotRefrigeratorCOPR.ECOPRIEVirreversiblerefrigeratoTH/T,-1Carnot Heat PumpCOPCOPRrevreversiblerefrigeratorCOPHPeCOPR.revimpossiblerefrigerator1-T,/TH2
SHANDONG UNIVERSITY Chap6 Summary-2 2 A process can be reversed without leaving any trace on the surroundings. Carnot Cycle Carnot Refrigerator Carnot Heat Pump Reversible Processes The heat engine operates on the reversible Carnot Cycle The best known reversible cycle; four reversible processes Carnot heat engine Carnot Principle 1: Given TL and TH, ηth,irrev < ηth,rev System Surroundings Internal Rev External Rev Why need Rev Irreversible: heat transfer Isothermal expansion Isothermal Compression Adiabatic compression Reversed Carnot Cycle Carnot refrigerator /heat pump Carnot Principle 2: Given TL and TH, ηth,all rev = ηth,rev Carnot Heat Engine The refrigerator / heat pump operates on a reversible Carnot Cycle Adiabatic expansion
SHANDONGUNIVERSITY6-6 reversible and irreversible processesThe second law of thermodynamics statesthat: no heat engine can have anefficiencyof 100percent??? What is the highest efficiency that aheatengine canhave???Toanswerthisquestion,weneedtodefineanidealizedprocess-Reversibleprocess
SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • The second law of thermodynamics states that: no heat engine can have an efficiency of 100 percent. • ??? What is the highest efficiency that a heat engine can have??? – To answer this question, we need to define an idealized process – Reversible process. 3
SHANDONGUNIVERSITY6-6 reversible and irreversible processesAreversible process（可逆过程）is defined as aprocess that can be reversed without leavinganytrace onthe surroundingsSystemreturnedtoinitial states- Surroundings returned to initial states:This is possible only if the net heat and net workexchange between the system and the surroundings iszero for the combined (original and reverse) processProcesses that are not reversible called irreversibleprocesses
SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • A reversible process (可逆过程) is defined as a process that can be reversed without leaving any trace on the surroundings. – System returned to initial states – Surroundings returned to initial states • This is possible only if the net heat and net work exchange between the system and the surroundings is zero for the combined (original and reverse) process • Processes that are not reversible called irreversible processes. 4
SHANDONG UNIVERSITY6-6 reversible and irreversible processes. It should be pointed out that:- A system can be restored to its initial state following aprocess, regardless of whether the process isreversible or irreversible.- But for reversible processes: this restoration is madewithout leaving any net change on the surroundings- While for irreversible processes: the surroundingsusually do some work on the system and thereforedoes not return to their original state
SHANDONG UNIVERSITY 6-6 reversible and irreversible processes • It should be pointed out that: – A system can be restored to its initial state following a process, regardless of whether the process is reversible or irreversible. – But for reversible processes: this restoration is made without leaving any net change on the surroundings. – While for irreversible processes: the surroundings usually do some work on the system and therefore does not return to their original state. 5