課件工程熱力學yu chapter.5the second law of thermodynamics

1、Chap.5 The Second Law of ThermodynamicsIntroductionThe 1st Law limits the conversion of energy to work to 100% and the impossibility of the PMM1.The 2nd Law will further tell us that not only one cannot convert energy into 100% work, the conversion efficiency must be less than 100% by using the most

2、 efficient machinery.As all laws of thermodynamics cannot be proven, the Second Law is a statement based on past experiences and cannot be proven as well. Three Examples of Spontaneous process Spontaneous heat transferSpontaneous expansionFalling massFrom the instances, it should be clear that the r

3、everse process would not take place spontaneously, even though energy could be conserved.Direction of ProcessDuring spontaneous process, there is an opportunity for developing work, but it would be wasted if the process is uncontrolled.Opportunities for Developing WorkWhat is the theoretical maximum

4、 value for the work that could be obtained?What are the factors that would preclude the realization of the maximum value?Two questionsFirst Law of ThermodynamicsConservation of EnergySays Nothing About the Exact Direction of Energy Transfer熱力學第一定律 能量守恒與轉換定律能量之間數(shù)量的關系所有滿足能量守恒與轉換定律的過程是否都能自發(fā)進行能不能找出共同的規(guī)律

5、性?能不能找到一個判據(jù)? 自然界過程的進行表現(xiàn)在不同的方面熱力學第二定律Aspects of the Second LawPreferred Direction of Energy TransferDetermines Whether a Process Can Occur揭示過程進行的方向、條件和限制a special kind of closed system that always remain at constant temperature even though energy is added or removed by heat transfer.one is that the r

6、eservoir is so large that any finite interaction will cause very little or no effect on its state. The second way is whatever effects the finite interactions may have on the reservoir, they are completely negated by interactions from other systems that are also interacting with the reservoir. Reserv

7、oir (or thermal reservoir)A reservoir is an idealizationThe atmosphere, large bodies of water (lakes, oceans), a large block of cooper, and so on.Reservoir (or thermal reservoir)Kelvin-Planck Statement It is impossible to construct a device, operating in a cycle, to deliver only work by having heat

8、interaction with a single reservoir.Statements of the Second LawL.W.T. KelvinBritish Physicist (1824-1907)Kevin-Planck Statementit is impossible to construct any machine that will have the thermal efficiency of 100%. It must interacts with at least two reservoirs: one to receive heat from and anothe

9、r to reject heat to.Planck，Max Karl Ernst Ludwig (1858-1947) German PhysicistClausius StatementIt is impossible for any device, operating in a cycle, to cause heat (energy) to transfer from a body at lower temperature to a body at higher temperature unaided (without consuming work)Rudolf Clausius182

10、2 1888German Mathematical PhysicistNotice that both statements are “negative” since they both stated the “impossibility.” The two statements are equivalent. It means that if one statement is true, the other one is also true. If one is false, then the other one is also false.Remarks開爾文普朗克表述不可能從單一熱源取熱

11、，并使之完全轉變?yōu)橛杏霉Χ划a生其它影響。不可能制造一部機器，它在循環(huán)動作中把重物升高而同時使一熱庫冷卻但違反了熱力學第二定律第二類永動機：設想的從單一熱源取熱并使之完全變?yōu)楣Φ臒釞C。這類永動機并不違反熱力 學第一定律第二類永動機是不可能制造成功的環(huán)境是個大熱源如果三峽水電站用降溫法發(fā)電，使水溫降低5C，發(fā)電能力可提高11.7倍。設水位差為180米重力勢能轉化為電能：1800MJ1Mkg水降低5C放熱：21000MJ克勞修斯表述不可能將熱從低溫物體傳至高溫物體而不引起其它變化?？照{,制冷代價：耗功 熱量不可能自發(fā)地、不付代價地從低溫物體傳至高溫物體。Proof of Equivalence o

12、f the Kevin-Planck and Clausius StatementsHot reservoirQcCold reservoirQcQHQCWcycle=QH-QCProof of Equivalence of the Kevin-Planck and Clausius StatementsAs shown in Fig. 5.2, the left part is a violation of Clausius statement, then we could deduce the combined system (closed in the dash line) is a v

13、iolation of Kelvin-Planck statement.開爾文普朗克表述 完全等效!克勞修斯表述：違反一種表述,必違反另一種表述!Natural (or Irreversible) ProcessPhysical Processes That Proceed in One Direction But Not The OtherTends Towards EquilibriumEquilibrium Only At End of Process(courtesy F. Remer)Natural (or Irreversible) ProcessExamplesThermal C

14、onductionHotColddQ(courtesy F. Remer)Heat interaction between systems having finite temperature differenceUnrestricted expansion of a gas or liquid to a lower pressureSpontaneous mixing of matter at different compositions or statesFriction contact frictions, viscous friction, etc.Electrical current

15、flows through a resistanceInelastic deformationFactors render irreversibility1、功和熱的轉換2、不等溫傳熱3、自由膨脹4、混合過程定義：可逆過程；不可逆過程；不可逆因素（溫差、壓差等）；內部不可逆因素；外部不可逆因素過程的不可逆性功量摩擦生熱熱量100%熱量發(fā)電廠功量40%Reversible ProcessReversal in direction returns substance & environment to original states(courtesy F. Remer)Reversible Proc

16、essA conceptual processIdealized version of how things should beNo processes are truly reversible Helps investigate Second Law and Entropy(courtesy F. Remer)reversible one can reverse the process and cause the system and the environment both to return to their original conditionirreversible one can

17、reverse the process and cause the system to return to its original condition, but the environment will have suffered a change from the original conditionDistinction between a reversible and an irreversible process Example 5.1Process 1 (fig.5.1b) is the reverse of the original process( fig.5.1.a), wh

18、ich is about to be proved impossible.Process 2 & 3 are possible processes.The cycle consisting of three processes is denied by the Kelvin-Planck statement, thus conclusion can be deduced that process 1 is impossible. Analytical Form of the Kelvin-Planck Statement“l(fā)ess than” signs of the presence of

19、irreversibility“equal to” signs of the absence of irreversibilityHeat EngineA heat engine is a cyclic device that converts thermal energy into work outputIt is a device that takes heat from a high-T reservoir, converts some of to (useful) work, and transfers the rest to the surroundings (a low-T res

20、ervoir)Examples: steam engines; internal combustion engines (car engines)Heat EngineThermal efficiency (“what you get out/what you put in”):No heat engine operating in a cycle can convert all of its heat input completely to workHeat EngineHeat PumpA heat pump is a (cyclic) device that transfers heat

21、 energy from a low-T reservoir to a high-T reservoirExamples: air conditioner; refrigeratorHeat PumpCoefficient of performance (“what you get out/what you put in”):No heat pump operating in a cycle can transfer thermal energy to a low-T reservoir without doing some workHeat PumpRefrigerator (1)Refri

22、gerator (2)Second Law Corollaries for Power CycleHot reservoirCold reservoirQHQCWcycle=QH-QCThe Carnot Corollaries the thermal efficiency of an irreversible power cycle is always less than the thermal efficiency of an reversible power cycle when each operates between the same two thermal reservoirs.

23、All reversible power cycles operating between the same two thermal reservoirs have the same thermal efficiency.Second Law Limitations on Refrigeration and Heat Pump CycleHot reservoirCold reservoirQHQCWcycle=QH-QCKelvin Temperature ScaleMaximum Efficiency of thermodynamic cyclesCarnot CycleThe four processes of the cycleProcess1 a-b Isothermal expa