I'm looking for the appropriate term to use for what gets "used up" as potential energy is converted to heat and work. For example, some of the the energy in solar radiation is converted by photosynthesis to chemical energy; much of the rest is lost as waste heat. The bio-chemical energy is used to power various biological functions until it has all been lost to waste heat. At the various steps along this path what term would you use for what there is less and less of? It can't be "energy" since that's conserved. It's potential energy, but is that really a standard term for what's lost when potential energy is converted to waste heat?

-
Are you looking for entropy? en.wikipedia.org/wiki/Entropy – Bernhard Feb 5 '12 at 21:03
@Bernhard: perhaps you could expand that into an answer? – David Z Feb 5 '12 at 21:23
@Bernhard Sort of, I guess. But whether some energy is waste heat or not depends on the temperature of the environment. Entropy is more absolute than that isn't it? I guess one can talk about the entropy change. – RussAbbott Feb 6 '12 at 1:47
Also, I'm confused since potential energy is (or can be) used to do work as it moves from potential to waste heat. Is it really accurate to say that an entropy increase can be used to do work? – RussAbbott Feb 6 '12 at 1:55

The term closest to what you're asking for is "free energy". Free energy is the amount of energy that can be used to do work. It always decreases in spontaneous processes. See Work content, Thermodynamic free energy.

It usually includes an entropy term, but we cannot say that it is entropy. Then again, you may have been looking for entropy after all. Entropy is a measure of randomness, and it is always "used up" (By used up I mean that it increases). But to relate it with work and energy, one has to use free energy anyways.

-
Yes, that's it. Thanks. – RussAbbott Feb 6 '12 at 4:45