1、Chapter 24 Entropy and the second law of thermodynamics24-1 Two kinds of processes and entropy1.Irreversible processes（不可逆过程）(a)“All naturally occuring processes proceed in one direction only.They never,of their own accord,proceed in the opposite direction.”Such spontaneous one-way processes are“irr
2、eversible”.(b)Although the“wrong-way”events do not occur,none of them would violate the law of conservation of energy.See动画库力学夹5-08过程的方向性2.Reversible process（可逆过程）In reversible process,we make a small change in a system and its environment;by reversing that change,the system and environment will ret
3、urn to their original conditions.In a truly reversible process,there would be no friction,turbulence(紊乱),or other dissipative effects,which will cause non-compensatory losses of energy.See动画库力学夹5-09可逆过程3.Entropy(S)Entropy is a physical quantity that controls the direction of irreversible processes.I
4、t is a property of the state of a system;like T,P,V,Eint.Entropy principle:“If an irreversible process occurs in a closed system,the entropy of that system always increases;it never decreases.”24-2 Entropy change for reversible processes1.The definition of entropy change for a reversible process:(re
5、versible)(24-1)=fiTdQSHere dQ is the increment of heat energy that is transferred into(or out)of the closed system at temperature T.If the process is isothermal,(24-2),the entropy of that system increases.(if QB and C-D are isothermal processes;B-C and D-A are const.volume processes.24-6 Entropy and
6、 the performanceof refrigerators1.The Carnot refrigeratorFig 24-12 shows the basic elements of a refrigerator.We call it a Carnot refrigerator.Fig 24-12WHTLTLQHQABCD0AVBVCVDVAPBPCPDPHTLTLQHQLLHHTQTQ=are satisfied.LHQQW=We define the“coefficient of performance”(24-13)WQforpayyouwhatwantyouwhatKL=LHQQ
7、W=LHLQQQK=(24-14)LLHHTQTQ=LHLTTTK=(24-15)for Carnot refrigerator2.Search for a“perfect”refrigeratorFig24-13 shows another“inventors dream”,a perfect refrigerator,for which,so that .However,it would violate the second law.0=WKFig 24-13QQHTLT00+=+=HLgasTTTQTQSSSSHL)(HLTT WQKL=(for a cycle)This result
8、leads to a third version of the second law of thermodynamics:“No process is possible whose sole result is the transfer of heat from a reservoir at one temperature to another reservoir at a higher temperature.”-Clausius versionIn short,“there are no perfect refrigerators”.0SKelvin version?Sample prob
9、lem 24-6 A household refrigerator,K=4.5,extracts heat from the food chamber at the rate of.(a)how much work per cycle is required to operate the refrigerator?(b)how much heat per cycle is discharged to the room?cycleJ/250Solution:(a)(b)WQKL=5.4cycleJcycleJW/535.4)/250(=cycleJWQQLH/303=+=*24-7 The ef
10、ficiencies of real engine“No real engine can have an efficiency greater than that of a Carnot engine working between the same two temperatures”.QQ(b)(a)xcwHTLTHTLTHXQHCQLCQLXQ24-8 The second law revisitedSo far we have presented three statements of thesecond law of thermodynamics.1.The entropy of su
11、ch system never decreases,.2.You can not change heat energy into work with100%efficiency.That is,there are no perfect engines.3.You can not transfer heat energy from a low-temperature reservoir to a higher temperature reservoir without doing work.That is,there are no perfect refrigerators.0S-Kelvin
12、version-Clausius versionIf statement 2 were false and we could build a perfect engine,connected with a refrigerator as shown in Fig 24-15a.(a)(b)Fig 24-15LCQHCQHQLCQHTLTHTLTWperfect engineKelvin versionClausius version?LCHCHHCQWQQQQ=QThus,a violation of statement 2 implies a violation of statement 3
13、.Therefore,all the three versions of the second law of thermodynamics are equivalent.24-9 A statistical view of entropy1.Entropy and multiplicity(多重度,微观状态数)WKSln=Eq(24-20)is called Boltzmanns entropy equation(24-20)W is the multiplicity associated with the configuration whose entropy we wish to calc
14、ulate.2.Entropy and disorderSee动画库力学夹5-10热二律的统计解释See动画库力学夹5-12都要说不！*Content of Reexam.(补考):Ch.(124)About Final Examination Time:1/17(Thurs.)8:30-10:30 am Classroom:3-308 Question Time(答疑时间):1/10(Thurs.)2:00-4:00 pm 1/16(Wed.)9:00-11:00 am Question Place(答疑地点):Rm.2421,Guanghua Buil.(East)Content:Ch.17-24
