Re: ENERGY WITH Q

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

Carl gets an A.  (I assume, since he reads only the digest, that
he had not had the opportunity to crib off the answer sheet.)

I noticed that, while he correctly uses the pseudowork concept to
answer parts i and j, Carl is unwilling to use it to answer part
f. I take this as a conscious choice on his part not to confuse
"pseudowork" with "real work", a confusion which, as he has noted,
permeates introductory textbooks and, often, the discussions of
work on this list.

A couple of other comments on his solution:

> > k) Now can you determine the internal energy of the gas?  If so,
> > what is it?
>
> Q = 0
> first law => W = delta(K) + delta(E_int)
> E_int_initial = unknown => please specify amount of gas and initial
> temperature, or initial pressure and cross-sectional area of cylinder

The problem statement said that the gas particles were initially
at rest, but I can imagine the statement being interpreted as
applying only to the cm of the gas.  In any event, Carl's answer

> but delta(E_int) =  F*L/8

is correct.

> However, you cooked up this problem because it's supposed to reveal a
> problem with this form of the first law, so I imagine you're going to
> tell me something's wrong with this "solution."

Well...  Perhaps I didn't read your challenge well enough the
first time through.  I didn't actually intend for the problem to
reveal a weakness with the first law in any specific form. My
primary intent was to frame a problem that would require the
solver to be very careful with the definition (or definitions) of
work and to understand how it is (or they are) related to various
system energy changes.  It is that test which you have passed with
flying colors.

> > l) Now can you determine the thermal energy of the gas?  If so,
> > what is it?
>
> I'm still not sure of the definition, but perhaps E_int?

I don't think any reasonable definition of "thermal energy" would
allow us to equate it with the internal energy of the gas given
the information to this point.

> > m) If your answer to any of these is still "cannot determine"
> > please suggest a piece of information that would remove any
> > remaining ambiguity.
>
> Please specify (N,T0) or (P0,A) so I can calculate E_int_initial.

Fair enough.  But I think you also need more information to answer
part l.

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