The Second Law of Thermodynamics for Closed and Open Systems

we can see that this statement is consistent with the general definition presented in Section 3.4. Let s consider the most efficient engine possible, the Carnot engine. If qc = 0. Equation (3.4) gives  

In general, there are two ways to apply the second law of thermodynamics  

We may determine whether a process is possible (and estimate how efficient it is). This applies to real, irreversible processes for which the entropy change of the universe is greater than 0  

The second law can also be used to provide an additional constraint to solve a problem that is, it provides us another equation to use. To apply the second law in this way, we must assume the process is reversible. In this case, we apply  

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