# Less irreversibility = less change in entropy

1. Sep 14, 2014

### pyroknife

I have a conceptual question that I am trying to clear up.

A reversible process means that the net change in entropy is zero.

If we have two scenarios, where case 1 has a ΔS= 0.1 J/K and case 2 has a ΔS= 0.5 J/K.
Is it a correct statement to say that case 1 is "less irreversible (i.e., more reversible)" than case 2?

Or do I have to think of this in a more binary(black & white) kind of way, where as long as ΔS≠0, the degree of irreversibility is the same?

2. Sep 15, 2014

### duri

It doesn't matter how you call it. Higher entropy change means higher loss and low efficiency process.

3. Sep 15, 2014

### Staff: Mentor

When you say that the net change in entropy is zero for a reversible process, you are referring to the entropy change of the combination of system plus surroundings. The entropy change for either the system or the surroundings does not have to be zero as long as their sum is zero.

Entropy is an intensive property, meaning that it depends on the amount of material involved. You can have a very large system that operates very nearly reversibly, and a smaller system that operates very irreversibly, and, in both cases the change in entropy can be about the same. So entropy change (of combined system and surroundings) is not a good indicator of irreversibility. A better indicator of irreversibility is to compare the entropy change of the system with the sum of Q/T at the interface between the system and surroundings. The greater the difference in these quantities, the more irreversible the process is.

Chet

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