Re: Thermodynamics essay

["RAUBER, JOEL" <JOEL_RAUBER@SDSTATE.EDU>]

> > E.g. 3 units of energy are transfered from system A in
> thermal form and
> > arrive at B in nonthermal form.
> >
> > Then Delta E = +3 and W = 0 and Q =+3, the structure allowed it.
>
> Nice try.  This indicates an understanding of what repairs
> are necessary.
> But this attempt to repair the W+Q equation violates the
> usual definition
> of Q, namely energy that was thermally transferred from a
> warmer body to a
> cooler body.

I thought you meant when you said that energy is transfered from system A in
thermal form, that you meant "energy was thermally transferred"?  Otherwise,
what did you mean?

Actually my quibble is independent of the answer above (though I would like
to here the answer

My quibble is that JD states that the structure of the usual form of the
first law can not handle all five cases.  I hope to show that it can handle
all five cases by explicit construction.  (usual structure = Delta E = W +Q
)

For the sake of definiteness, lets stipulate 10 units of energy is
transfered from A to B.

1) Energy could leave A in nonthermal form and arrive at B in nonthermal
form. This is the normal case studied in elementary mechanics courses.

Delta E = 10
Q = 0
W = 10

2)Energy could leave A in thermal form and arrive at B in thermal form.

Delta E = 10
Q = 10
W = 0

3)Energy could leave A in thermal form and arrive at B in nonthermal form.

Delta E = 10
Q = 10
W = 0

4)Energy could leave A in nonthermal form and arrive at B in thermal form.
See section 5.2 for an example.

Delta E = 10
Q=0
W=10

5)Combinations of the above. (lets say three units leave in nonthermal form
and seven leave in thermal form, it doesn't matter what bins you put the
energy into for system B)
Delta E=10
Q=7
W=7

By explicit construction the structure of the usual statement has
accomodated all five cases.

What I have *not* stated or done with the above construction.

> It is bad practice to have Q mean one thing on Monday and
> another thing on
> Tuesday.

I have not done that! I also haven't claimed that this is the usual manner
of proceeding nor have I claimed that this is terribly convenient.  All I
claim is that the structure allows for a consistent partitioning.  Nor do I
claim that in actual situations is this partitioning easy to determine
(which I think is JD's main arguement with the usual 1st law statement
combined with the loss of information on the LHS, and I don't disagree).


> I don't see how to have a consistent definition of
> Q that covers
> all the cases.  Also note that I've added a fifth bullet,
> "combinations of
> the above", that will be even harder to handle.

I hope its clear that the following is at least consistent.

"Energy could leave A in nonthermal form"  --> that is W
"Energy could leave A in thermal form"     --> that is Q

And we don't bother to distinguish the form of arrival in B as we lump it
all together and call it Delta E.