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Simple question: measuring free energy

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Haines Brown

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Jul 4, 2008, 6:57:26 AM7/4/08
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How does one measure free energy of a system without observing its
effect as this energy does work? That is, can you observe the free
energy for a given state of a system at a given time?

Put otherwise, if you subtract the entropy of a system from its total
energy, what is left is its free energy, but does this not entail a
comparison of system states?
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Haines Brown, KB1GRM



Neil W Rickert

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Jul 5, 2008, 9:57:45 AM7/5/08
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Haines Brown <bro...@teufel.hartford-hwp.com> writes:

>How does one measure free energy of a system without observing its
>effect as this energy does work?

Maybe these links will help:

http://en.wikipedia.org/wiki/Gibbs_free_energy
http://en.wikipedia.org/wiki/Helmholtz_free_energy

Haines Brown

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Jul 5, 2008, 6:16:29 PM7/5/08
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Neil,

Thanks for the citations. But the conclusion seems to be that you can't
measure free energy in a static system. If the system is not closed,
such as in chemistry, you have to look to changed states. In other
words, the free energy depends on entropy measurement, which in turn
depends on system change.

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Haines Brown, KB1GRM



Neil W Rickert

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Jul 6, 2008, 5:30:26 PM7/6/08
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Haines Brown <bro...@teufel.hartford-hwp.com> writes:

The usual equations give the change in entropy. I am not aware of a
useful definition of a base value such as would allow determination
of entropy as an absolute quantity.

Haines Brown

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Jul 8, 2008, 1:07:09 PM7/8/08
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Yes, thanks. Kinda my point.

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Haines Brown, KB1GRM



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