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ROCHESTER PHY 121 - Lecture 26 Notes - The Second Law of Thermodynamics

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1Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121, April 29, 2008.The Second Law of Thermodynamics.http://www.horizons.uc.edu/MasterJuly1998/oncampus.htmFrank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.April 29, 2008.• Course Information• Topics to be discussed today:• The Second Law of Thermodynamics• Applications of the Second Law - the Carnot Engine• Entropy• The End!Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.April 29, 2008.• Homework set # 10 is now available and is due onWednesday evening, April 30, at 11.30 pm.• Exam # 3 will be returned in workshops.• Any complaints about Exam # 3 must be addressed by me;please write down why you feel the exam was not gradedproperly and hand it to me before next Monday, May 5. Iwill do the re-grading on Tuesday morning, May 6.• I have distributed information about the score you need toobtain on the final exam to get a C-, a B-, and an A- in thiscourse.2Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.Grade Requirements.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterPhysics 121.Final Exam - Details.• The final exam will be held on Thursday May 8 between 4pm and 7 pm in Hubbell. The final exam will cover all thematerial discussed in the course; there will be NO particularfocus on thermodynamics. NOTE: no error analysis.• Extra office hours of the TAs and me will be announced viaemail later this week.• The formula sheet to be attached to the final exam will bedistributed via email by the end of the weekend, and willalso be available on the WEB.• Please use all the resources at your disposal to prepare forthe exam (e.g. the detailed solutions of previous exams, thehomework assignments, the study guide, etc.).• The final exam grades will be distributed via email onMonday May 12.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.• There are several different formsof the second law ofthermodynamics:• It is not possible to completelychange heat into work with noother change taking place.• Heat flows naturally from a hotobject to a cold object; heat willnot flow spontaneously from acold object to a hot object.• Many naturally processes do notviolate conservation of energywhen executed in reverse, butwould violate the second law.3Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat engines.• Most engines rely on atemperature difference to operate.• Let’s understand why:• The steam pushes the piston to theright and does work on the piston:Win = nRTin(1-Vin/Vout)• To remove the steam, the pistonhas to do work on the steam:Wout = nRTout(1-Vout/Vin)• If Tin = Tout: Win + Wout = 0 (nonet work is done).• In order to do work Tin > Tout andwe must thus cool the steambefore compression starts.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat engines.• The efficiency of an engine is defined asthe ratio of the heat extracted from the hotreservoir and the work done:Efficiency = | W | / | QH |• The work done and the heat extracted areusually measured per engine cycle.• Because of the second law, no engine canhave a 100% efficiency!• Note: the cost of operation does not onlydepend on the cost of maintaining thehigh temperature reservoir, but may alsoinclude the cost of maintaining the coldtemperature reservoir.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat pumps.• In many cases (heat engines), theconversion of flow of heat towork is the primary purpose ofthe engine (e.g. the car engine).• In many other applications (heatpumps), work is converted to aflow of heat (e.g. airconditioning).• The performance of a heat pumpis usually specified by providingthe coefficient of performance K:K = | QL | / | W |4Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat pumps.Note: you can not cool your house by opening the door ofyour refrigerator!Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat pumps.Note: You usually pay for the work done but not for the heat extracted from the outside. You can thus get more energythan what you pay for!Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat pumps.• Heat pumps:• The heat capacity increases withincreasing outside temperature.• Additional heaters may berequired in colder climates.• The heat capacity can also beincreased by changing the sourceof heat from the air to the ground.http://irc.nrc-cnrc.gc.ca/cbd/cbd195e.htmlhttp://www.bchydro.com/powersmart/elibrary/elibrary685.html5Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond law of thermodynamics.Heat pumps.• Heat pumps:• Heaters in the winter: take heatfrom the outside to the inside.• Air conditioners in the summer:take heat from the inside to theoutside.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond Law of Thermodynamics.The Carnot Cycle.A “perfect” engineused to determine the limits on efficiency.Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond Law of Thermodynamics.The Carnot Cycle.• Step 1: a to b.• The gas is in contact with a heatbath at temperature TH and weightis removed from the piston.• The gas expands, whilemaintaining a constanttemperature (the change in theinternal energy is thus equal to 0J).• Using the first law ofthermodynamics we see that QH= WH= n R THlnVbVa6Frank L. H. Wolfs Department of Physics and Astronomy, University of RochesterSecond Law of Thermodynamics.The Carnot Cycle.• Step 2: b to c.• The gas is isolated from theenvironment and some moreweight is removed from thepiston.• The gas expands and during theadiabatic expansion, thetemperature of the gas


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ROCHESTER PHY 121 - Lecture 26 Notes - The Second Law of Thermodynamics

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