Re: Oh No! - Another question on heating

["Carl E. Mungan" <mungan@USNA.EDU>]

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/