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2ndLaw-Spring2010-8

2ndLaw-Spring2010-8 - 2nd Law of Thermodynamics 1st Law...

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2nd Law of Thermodynamics 1st Law: energy is conserved But is that enough ? – Object drops converting KE to heat but never see the opposite – H 2 and O 2 react to form H 2 O when ignited at room temperature but not the reverse 1st Law would permit the reverse but 2nd Law does not.
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Irreversibility A flywheel is spinning in a fluid in an isolated box. Eventually flywheel (and gas) slow down and stop; the fluid is now hotter. • KE flywheel has been converted to U fluid Organized motion has become random Reverse does not happen by itself
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Entropy Mechanical energy [organized] generally more useful than heat [random]; i.e., it’s more valuable form of energy (e.g., flywheel can drive a generator directly). Entropy (S) is a measure of the disorder/ randomness of a system. Systems naturally tend towards disorder.
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Properties of the Entropy Entropy increases with increases disorder, reduced “useful” energy. Entropy can only be created not destroyed. (2nd Law) Production: P s = Δ S Isolated system P s = Δ S 0
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Isolated System Can imagine 3 types of processes: • P s < 0 impossible (2nd Law) • P s = 0
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