Re: macroscopic vs microscopic degrees of freedom (was: why pseudowork(NOT)

[Bob Sciamanda <trebor@VELOCITY.NET>]

You make the right point, Leigh.  Let me expand (perhaps corrupt:) a bit.
In the first law of thermodynamics, dE = dQ + dW,  dW is simply a template
to be filled in "ad hoc" to account for an exchange of non-heat energy
between the system and its environment.  The form of the template is
typically further (but still vaguely) specified as dW=Y*dX.

This is adapted ad hoc to include mechanical, electrical, etc ( psychic?
:) interactions with the environment which were not included in dQ.  How
these energy forms are partitioned (even dQ vs dW) is open to a variety of
approaches, so long as the equation balances.  I.E., as far as dE is
concerned, there is no hard and fast, universal definition of these terms;
each situation is open to an ad hoc model for dQ and the various dW's -
whatever it takes to balance the equation.

Mechanical work is a different animal, admitting of specific definition(s)
for specific purely mechanical W-E theorem(s).

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Leigh Palmer <palmer@SFU.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, October 29, 1999 8:30 PM
Subject: Re: macroscopic vs microscopic degrees of freedom (was: why
pseudowork(NOT)


> > On Fri, 29 Oct 1999, Joel Rauber wrote:
> >
> > > If my 1993 Geo metro is rotating as well as translating (as it has
been
> > > known to do on the icy roads of South Dakota), the energy associated
with
> > > that rotational motion is in what category? internal energy?
> >
> > In the context of our paper, we put it in the category of internal
energy.
> > We thought long and hard about this and decided that it made life, in
that
> > context, easier. We fully recognize that there are other useful ways to
> > proceed.
>
> In what follows I ask only rhetorical questions, so I have supplied
> my answers in parentheses
>
> Consider the example I provided earlier. I paddle my canoe. The water
> is churned on a scale comparable to the size of the paddle, and the
> turbulent motion then evolves to smaller scales over an interval of
> time measured in seconds. What part of the work which was done on the
> water (and I will argue that work was done on the water) goes into
> mechanical energy and what part goes into internal energy? Does that
> fraction vary with time? (yes)  Does the amount of work which *was*
> done on the water also vary with time? (obviously not)
>
> Would work, by any definition, be a useful quantity to employ in a
> calculation having to do with the evolution of this system? (yes, but
> only the thermodynamic kind)
>
> I think that whenever work gets hard to define one should not try to
> use the concept. Such efforts always remind me of the sort of "ad
> hocery* that used to characterize explanations of phenomena which
> employed the caloric model. They were often clever, but there was
> lttle or no general value to them. (Some were wrong, of course.)
>
> Leigh