Re: ENERGY WITH Q

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

> Comments will be appreciated, either in
> public (pro publico bono?) or in private. The URL is:
>
>  http://alpha.montclair.edu/~kowalskiL/energy.html

I have looked this page over although not read it in detail yet,
although I promise to do so.

My main concern is that you are exceedingly slippery about what you
mean by heat. A quick scan reveals all of the following "definitions"
of heat:

* it can be modeled as a fluid called "caloric"
* it is a form of energy
* it is thermal energy
* it equals the lost mechanical energy
* it is the result of conduction between objects at different temperatures
* it is the random mechanical energy of molecules and atoms

This is a rather long list. Which one do you take as your basic
definition? I sure couldn't tell!

But even leaving the semantics issue aside, I believe that some of
these definitions are mutually exclusive.

From what I have heard on PHYS-L and elsewhere, I suggest the
following basic definitions of heat:

1. Heat is the energy transferred between two bodies owing to their
difference in temperatures. This energy can be transferred by
conduction, convection, or radiation. The canonical example is a hot
plate warming up a gas, where the system is the gas. The heat
transfer can be either reversible or irreversible. If reversible, it
equals the integral of TdS. If irreversible, it is possible to
construct an equivalent hypothetical reversible path between the same
initial and final states of the system, such that the integral of
dQ/T for that process equals the entropy change. At the risk of
agitating some members of this list, I believe this is the
conventional definition found in most texts of either intro physics
or advanced undergraduate thermo and it is the view I prefer.

2. Heat is internal energy, or possibly just certain forms of
internal energy called thermal energy. Heat therefore does not
necessarily get transferred from a hot body. For example, the
adiabatic compression of an ideal gas produces heat in the gas
because the gas warms up. For more general materials, heat is also
produced during phase changes. The heat of an isolated body is time
dependent in general - for example, a rotating fluid initially
possesses mechanical energy but viscosity slowly transforms that into
heat. Heat is the integral of TdS regardless of whether the process
is reversible or irreversible. Hence, in a free expansion of an ideal
gas, heat is positive since the entropy increases, despite the fact
that the gas temperature is constant and the system is isolated. This
appears to be Ludwik's view in at least some parts of his document.

3. There is no such thing as heat. I know what energy is. I know what
work is. Heat is just a special kind of work applicable to specially
contrived problems. So who needs it? Certain list members seems to
hold this view. But they can and have spoken for themselves.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/