In a shocking and remorseless affront to solidarity, Leigh has
impugned my attempt to distinguish the bulk translational kinetic
energy of a system from its internal energy. And I thought we were
friends ...
Nevertheless, if only to demonstrate that I can (occasionally) rise
above petty squabbles, I won't quarrel with the notion that classical
thermodynamics is perfectly capable of--and even, perhaps, demands--
dealing with bulk translational energy. However, I think that, as a
*practical* matter, we usually define internal energy to be pretty
much whatever we want as long as it includes all forms of energy that
"participate" in the process of interest and as long as we use
definitions for work and heat which properly account for all changes
in our chosen internal energy. For instance, we might neglect the
chemical or nuclear potential energy in a gas (even though they are
surely a property "of" the system) because they don't change in the
process of interest.
As another example, consider a cylinder of oxygen on an intergalactic
chemical delivery cargo ship. Do we really want to complicate our
lives by saying that the internal energy of the gas depends on the
speed of the spaceship? Do we want to say that the internal energy
continuously changes as the spaceship uniformly accelerates? If so,
does the internal energy of the same cylinder continuously change
when it is at rest on the surface of the earth? And if not, what
happened to the principle of equivalence? (Oops; there I go again!)
Since it is easy to define work in such a way that it excludes
changes in bulk translational energy (pdV, for instance), I don't
really see any harm in doing so.
Heretically yours,
John
-----------------------------------------------------------------
A. John Mallinckrodt http://www.intranet.csupomona.edu/~ajm
Professor of Physics mailto:ajmallinckro@csupomona.edu
Physics Department voice:909-869-4054
Cal Poly Pomona fax:909-869-5090
Pomona, CA 91768-4031 office:Building 8, Room 223