Re: Fw: Why work before energy in texts

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Wed, 24 Oct 2001, John S. Denker wrote:

> It is OK to talk about W when nonthermal energy transfer is the only thing
> going on.
>
> It is OK to talk about Q when purely thermal energy transfer is the only
> thing going on.
>
> Trying to separate the more-general energy transfer into a W piece and a Q
> piece is a really bad idea, especially when dissipation is involved, for
> reasons discussed at
>    http://www.monmouth.com/~jsd/physics/thermo-laws.htm#sec-thermal

I almost agree with all of the above but it makes things more
complicated than they need be. JD mentions what, IMO, is the all
important word--"dissipation".

Trying to separate energy transfer into a W piece and a Q piece is
not so much a "really bad idea" as it is simply fruitless and not
well-defined when dissipation is involved (i.e., when entropy is
increasing) and this *includes* situations in which you might be
tempted to say that "only nonthermal" or "only thermal" energy
transfer is involved.  You can make a decision about which part to
call Q and which part to call W, but the decision will be
idiosynchratic and. more importantly, you won't be able to use the
results of the decision to make any further meaningful statements.

But separating Q from W is not only *not* "a bad idea," it is both
well-defined and fruitful when dissipation is not involved as in a
quasistatic process.  In that case knowing which part is Q allows
you to determine the change in entropy during the process.

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