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Re: ENERGY WITH Q



On Fri, 26 Oct 2001, Ludwik Kowalski wrote:

1) My recollection is that, in traditional thermodynamics,
S is a state function and that a reference value S=0, can
be assigned to any particular state. Is this still true?

Since most of the important results of thermodynamics can be
derived from various derivatives of or changes in S, its absolute
value, while not arbitrary (as JD points out), is often
inconsequential. Thus one can usually "assign" any arbitrary
value to S at any arbitrary temperature.

This is quite similar to the situation with potential energy. To
the extent that one is only interested in results that derive from
derivatives of or changes in potential energy, one may assign it
any arbitrary value in any arbitrary configuration. However, when
one needs to know *the* total energy of a configuration as one
does in relativistic calculations one must either a) choose the
zero level of potential energy "correctly" (my favored position)
or b) reject "potential energy" altogether as a contributor to
total energy and, instead, integrate the energy locked up in the
the attendant fields.

I confess, however, that I can't think of any specific situations
in which knowing the "correct" absolute value of the entropy is
important. Can anybody suggest one?

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm