Re: What Flows?

["W. Barlow Newbolt" <wnewbolt@liberty.uc.wlu.edu>]

There are times when you almost have to use thermal energy.  Consider the 
following example,  borrowed from a book somewhere:

	A heavy sealed cylinder with a spark plug fitted into the side is 
filled with a mixture of air and gasoline vapor.  The cylinder is well 
insulated.  When the spark plug is flashed the mixture gets very hot and 
the pressure inside the cylinder rises as does the temperature, but the 
cylinder does not fail.  So,
			delta Q = 0   (insulated)
			delta W = 0   (cylinder does not fail)
			delta U = 0   (the above, and first law)
If the energy of the spark is negligible this is a completely closed 
system, but clearly something has happened.  It has gotten hot, the 
pressure of the gas has gone way up, and the chemistry of the gas inside 
has been altered.  Are you going to tell me that it is not helpful from 
the point of view of understanding to point out that although the 
internal energy is the same, chemical energy has been converted to 
thermal energy?  Unfortunately, I can't talk about it any other way.
					Barlow Newbolt 

 On Wed, 11 Sep 1996 jmclean@chem.ucsd.edu wrote:

> Leigh Palmer says:
> > It seems to me that "thermal energy" must be connected somehow to heat.
> > To illustrate the problems with this arbitrary partition of energy let me
> > propose some questions and ask some more questions about the questions.
> >
> > 1. Take a quantity of gas in an insulated cylinder and allow it to expand
> > a factor of two. It will cool off, of course. By how much has its thermal
> > energy changed in this process?
>
> What if instead you consider "thermal energy" to be connected somehow to
> temperature?  That actually seems to me to be the more direct connection;
> 'heat' only gets into the question by way of the colloquial usage where
> heat=temperature.
>
> Now does it seem strange that the expanding gas looses "thermal energy"
> even with no heat tansfer?
>
> Note: Not that I prefer "thermal energy" over "internal energy"
> necessarily, but I have to admit that it seems more descriptive.
>
> -- 
> --James McLean
> jmclean@chem.ucsd.edu
> post doc
> UCSD