Heat Engines, Entropy, & the Second Law
Entropy, S, is another "state variable" -- along with P, V, and T. We define the change in entropy by
dS is the change in entropy. dQr is the amount of heat that would flow out of the system along a reversible path between the two states and T is the absolute temperature.
Entropy is a description of the disorder of a system. For a reversible process, the total entropy of the system and its surroundings will be unchanged. For an irreversible process, the total entropy of the system and its surroundings will increase.
In general, we will need to add up or integrate these small entropy changes,
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(c) Doug Davis, 2002; all rights reserved