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Re: Oh No! - Another question on heating



Hi all-
It is important to distinguish whether a question is about physics
or about language. One way to do this is to use operational definitions
of words, as Dick Hake and I emphasize in the Discussion Labs that he
invented. It goes without saying, of course, (so I'll say it) that
without agreed definitions there is no possibility of communication.
I define "heating of a substance" to mean that the temperature or
entropy of the substance is increased. The answer to Carl's question,
therefore, is that he has supplied insufficient information. If the
internal structure of the "system" was changed by tthe work done, then the
system was heated by the work.
Regards,
Jack


On Fri, 12 Dec 2003, Carl E. Mungan wrote:

Bob LaMontagne queried:

I know this subject has been talked to death, but I'd like to ask the
'heat is a verb' crowd what they would consider an acceptable student
answer to the following question:

'Work is done on a system without changing its temperature. Is the
system heated, cooled or not necessarily either? Justify your answer.'

Not necessarily either.

Example 1: I exert a constant force F horizontally on a rigid block
on a frictionless surface as it moves a distance d. I did Fd work on
the block, its temperature didn't change, and no heat flowed into or
out of the block.

Example 2: I infinitesmally slowly push down a piston in a cylinder
containing an ideal gas in contact with a heat bath. I did work
-integral(PdV) = nRT*ln(V_i/V_f) and the temperature of the gas
stayed constant because heat nRT*ln(V_i/V_f) flowed out of the gas
and into the heat bath.

In example 2, I would be cautious about saying the gas was "cooled"
since its temperature didn't change. Nevertheless, heat *did* flow
out of the gas, so in that sense it was cooled. There's a collision
between two common meanings of "cool": (1) a drop in temperature and
(2) a loss in heat.

Carl

ps: There is a general schema for distinguishing examples 1 and 2.
Pseudowork was done in the first example, but not in the second. The
reason is that in the second example, a second force is doing an
exactly canceling amount of pseuodwork on the gas that I did, namely
the normal force exerted on the gas by the face opposite the piston I
push on. (This is what keeps the gas from gaining the bulk
translation KE that the block gained in case 1.) Happy trails!
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/


--
"Don't push the river, it flows by itself"
Frederick Perls