L13_2Feb_posted

# L13_2Feb_posted - Physics 6B Entropy and The Second Law of...

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-------------------------------- 1 Physics 6B Entropy and The Second Law of Thermodynamics -------------------------------- 2 Housekeeping 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* -------------------------------- 3 Exam logistics If you requested DRC accommodations and haven’t received an e-mail from PBSci DRC office please see me TODAY. We have an additional room to reduce crowding on Monday… ! if your last name begins with K , L or M please go to PSB 136 for the exam You may use a calculator. You may not use a cell-phone or computer calculator. You do not need a Scantron -------------------------------- 4 Auto engines Otto Cycle -- “four stroke” engine ! four strokes -- 6 paths » 2 isobaric, 2 isochoric » 2 adiabatic

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-------------------------------- 5 Entropy Macroscopic view: Entropy is defined by the relationship Microscopic “statistical mechanics” view: ! P i is the probability of the “ i-th ” state ! S increases as the number of states increases -- and as their likelihoods become more equal -------------------------------- 6 Second Law of Thermodynamics Original statement: Equivalently: Also equivalently: Heat does not flow spontaneously from cold to hot bodies The entropy of a closed system never decreases No (cyclic) process is possible where the sole result is the absorption of heat and its complete conversion into work -------------------------------- 7 First and Second Laws The first law says you can't win. The second law says you can't break even. -- Anonymous -------------------------------- 8 Carnot 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 cycle cannot completely convert heat into work. In an ideal Carnot Engine the total entropy change of the cylinder + the outside world is zero.
-------------------------------- 9 Heat Source T h “hot reservoir” Cooling T c “cold reservoir” Work Done Q h Q c W net Engine efficiency -------------------------------- 10 Clicker Question To increase the efficiency of an ideal heat engine, one must increase which of the following? A. the amount of heat consumed per second B. the temperature of the cold reservoir C. the temperature of the hot reservoir D. the size of the cold reservoir E. the size of the hot reservoir " -------------------------------- 11 Reversibility -- Elastic Collision 1 Forwards… -------------------------------- 12 Reversibility -- Elastic Collision 2 Backwards…

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-------------------------------- 13 Irreversibility - Inelastic collision 1 Forwards… -------------------------------- 14 Irreversibility - Inelastic collision 2 Backwards… -------------------------------- 15 Entropy Increase (energy “lost” to heat) Irreversible Process -------------------------------- 16 Entropy as a “state variable” For an ideal gas with a given n , p , V ! you can calculate T ! you can also calculate the entropy S » but it’s a lot more complicated Entropy is a state variable !
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