Heat Engines, Entropy, & the Second Law## Entropy

Entropy, S, is another "state variable" -- along with P, V, and T. We define the

change in entropybydS is the change in entropy. dQ

_{r}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

totalentropy of the system and its surroundings will be unchanged. For an irreversible process, thetotalentropy of the system and its surroundings willincrease.In general, we will need to add up or integrate these small entropy changes,

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