Re: first law of thermo

["John S. Denker" <jsd@MONMOUTH.COM>]

"Carl E. Mungan" wrote [off list]:

> ... so this [expression of the first law in terms of energy]
> is a first step. I agree many other steps remain. But
> my approach is to begin with the mechanics side of the energy story
> rather than the thermal side.

OK.

But recognizing that, you should re-examine the logical
structure of the first-law discussion.

If you mean to say (as I think you should!) that the
first law of thermodynamics is conservation of energy,
you should just say that.  Getting rid of any mention
of W+Q will make the discussion vastly shorter, clearer,
more useful, and more logical.

> Maybe it would help to cite a specific example (that involves actual
> numerical calculations not just concepts) that can be done using your
> favorite method x but not using conventional textbook methods. Do you
> have such an example? Please post one to PHYS-L for discussion if you
> do.

What about the Rumford experiment, for example?  If
you want it to be numerical (as well as conceptual),
let a cannon with heat capacity C rotate through an
angle theta while subject to a torque tau;  your
mission, should you decide to accept it, is to
calculate delta T.

-- The concept of Q is inapplicable.
-- Consequently any discussion of W+Q is invalid.
-- The concept of W is maybe or maybe not applicable;
   if you think it is applicable it requires (re)defining
   W in such a way that the work/KE theorem no longer applies.
++ The concept of conservation of energy of course
   applies, and leads immediately to a correct method
   for calculating delta T.

This seems like a very large tree standing at the entrance
to the forest.  I don't know why it is so commonly overlooked.

===================

I revised

http://www.monmouth.com/~jsd/physics/thermo-laws.htm#sec-thermal-energy
to emphasize that ironically, the first and second laws of
thermodynamics are essentially non-thermal.