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Dec18-12, 10:47 AM
P: 789
Yes, sorry, I misquoted it. It should be something like "Entropy is a measure of the energy NOT available for useful work". I also see the point that the example I gave is not good. In order for the expansion to be slow, there has to be an opposing force almost equal to the pressure force, and that force has to diminish as the volume increases (and pressure decreases). This opposing force would be the mechanism by which work was done on the environment. Something like a mass in a gravitational field, in which the mass is slowly being reduced by removal.

I found a web site which seems to give an explanation, I will have to look at it closely.

Also the Wikipedia quote says it is "energy per unit temperature" not available for work, which I cannot immediately decipher.