L13_2Feb_posted

L13_2Feb_posted - --------------------------------1Physics...

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Unformatted text preview: --------------------------------1Physics 6BEntropyandThe Second Law ofThermodynamics--------------------------------2Housekeeping•Sample Exam (from Spring 2009) + solutions posted•Review session for midterm: Sunday 2/6, 7-9pm, Classroom Unit 2•Drop-in office hours Saturday 2/5 2:00-3:30 ISB lobby•Website: http://sites.google.com/site/dewey6b/!username: Physics6B!password: *sixB*--------------------------------3Exam logistics•If you requested DRC accommodations and haven’t received an e-mail from PBSci DRCoffice please see me TODAY.•We have an additional room to reduce crowding on Monday…!if your last name begins with K,LorMplease go to PSB 136for the exam•You mayuse a calculator. You may notuse a cell-phone or computer calculator.•You do not need a Scantron--------------------------------4Auto engines•Otto Cycle-- “four stroke” engine!four strokes -- 6 paths» 2 isobaric, 2 isochoric» 2 adiabatic--------------------------------5Entropy•Macroscopic view: Entropy is defined by the relationship•Microscopic “statistical mechanics” view:!Piis the probability of the “i-th” state!Sincreases as the number ofstates increases -- and as their likelihoodsbecome more equal--------------------------------6Second Law of Thermodynamics•Original statement:•Equivalently:•Also equivalently:Heat does not flow spontaneouslyfrom cold to hot bodiesThe entropy of a closedsystem never decreasesNo (cyclic) process is possible wherethe sole result is the absorption of heatand its complete conversion into work--------------------------------7First and Second LawsThe first law says you can't win.The second law says you can't break even.-- Anonymous--------------------------------8Carnot cycle•An ideal Carnot cycle is the most efficient possible!“working fluid” (gas cylinder) is always in equilibrium with its environment.•But even an ideal Carnot cyclecannot completely convertheat into work.In an idealCarnot Engine the total entropy changeof the cylinder + the outside world is zero.--------------------------------9Heat SourceTh“hot reservoir”CoolingTc“cold reservoir”Work DoneQhQcWnetEngine efficiency--------------------------------10Clicker Question•To increase the efficiency of an ideal heat engine, one must increase which of thefollowing?A.the amount of heat consumed per secondB.the temperature of the cold reservoirC.the temperature of the hot reservoirD.the size of the cold reservoirE.the size of the hot reservoir"--------------------------------11Reversibility -- Elastic Collision 1•Forwards…--------------------------------12Reversibility -- Elastic Collision 2•Backwards…--------------------------------13Irreversibility - Inelastic collision 1•Forwards…--------------------------------14Irreversibility - Inelastic collision 2•Backwards…--------------------------------15Entropy Increase (energy “lost” to heat)Irreversible Process--------------------------------16Entropy as a “state variable”...
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L13_2Feb_posted - --------------------------------1Physics...

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