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Re: ENERGY WITH Q

At 12:57 AM 10/21/01 -0400, Ludwik Kowalski wrote:
What I would like to find is a sequence
of energy related topics, AND THE ASSOCIATED
VOCABULARY, which is not wrong.

OK.

1) It is not wrong to introduce "work done by a force" as a
dot product of F and s.

OK, but remember s denotes a _path_ from A to B; work is the integral of F dot ds, integrated along a particular path. And remember all the other caveats and provisos: we can only connect the force to the energy if we know the momentum-versus-energy relationship, which is not always the case; this implies that we are asking for trouble unless we restrict consideration to a point-particle, or an object with no internal degrees of freedom.

E=KE+PEgrv+PEspr, is a useful idealization.

Suggestion: Write
Emech=KE+PEgrv+PEspr
because calling this simply "E" will drive people crazy.

.... one must be aware
that frictional forces are always associated with increases
of temperature.

Ooops, that's not the correct physics. Suppose I have a pot of ice-water. I dump energy into it via friction à la Rumford. I can dump in quite a lot of energy with _no_ increase in temperature; it all goes into the latent heat of melting.

although Q is decreasing the sum of E+Q remains constant.

Eeek! I think Ludwik understands the physics here, but this notation will drive students and colleagues crazy. It would be much better to write
the sum of Emech + Q remains constant, or (equivalently)
the some of Emech + Ethermal remains constant.
while the symbol "E" should be reserved for the total energy, including Emech and Q and everything else.

Referring to a sliding box I would invent a new name for
the above sum, for example, mhenergy.

Technically, this is OK. However, I have some questions about the pedagogical issues.

The pedagogical rationale for this approach has not been given. Perhaps there is a good rationale that hasn't been presented yet. Here is a counterargument and/or a list of issues that the rationale would (I hope) deal with:

I see here a step-by-step approach:
1) we learn Emech is conserved
2) then we are told Emech + Ethermal is conserved
3) et cetera
4) eventually it turns out Etotal is conserved.

This reminds me of high-school geometry, where things were proved step-by-step starting from a handful of axioms. But if Kuhn et al. are to be believed, physical theories don't work this way. A theory may be proved false, but it can never be proved true. So the logical complexity and notational complexity of the step-by-step approach is all for naught; it doesn't prove anything.

One shouldn't criticize something unless one can offer something better, and explain why it is better. In this case I suggest that it would be better to just start from day one with the assertion that E (meaning Etotal) is conserved. We are not going to prove it. But we are going to explore various situations where the energy-conservation theory sticks its neck out and makes very specific predictions, and we shall see that it passes the test each time. Every time it is tested and NOT falsified, we gain confidence in it. Not proof, but confidence. We immediately recognize the following corollaries:
1) Emech is conserved, if mechanical degrees of freedom are the only ones involved.
2) Emech+Ethermal is conserved, if mechanical and thermal degrees of freedom are the only ones involved.

My rationale is that the conceptual burden of the "big picture" is no greater than the conceptual burden of the little pictures enumerated above. Plain old local conservation of energy is grander _and simpler_ and more correct than any of the specialized, restricted sub-cases enumerated above.

Note that the terms "work" and "heat" are now defined.
Work done by a force is a dot product,

OK.

heat generated in a process is thermal energy.

I don't see the value of the words "generated in a process".
Why not simply "heat is thermal energy"????

In other words, unlike heat, work is not a form of energy.

That's for sure.

As a corollary, writing an equation of the form
delta E = W + Q (equation X)
just makes no sense at all. The RHS is apples and oranges.

Also note that we defined W in a way that makes sense only in the case of objects that _don't_ have internal degrees of freedom, while we understand Q only for objects that _do_ have internal degrees of freedom, which is another reason for being horrified by equation (X). Suggestion: forget equation (X). See
http://www.monmouth.com/~jsd/physics/thermo-laws.htm

Note that in Model 2 deltaQ is always positive (heat is generated).

Oooops, wrong physics. Cooling by adiabatic expansion decreases the thermal energy.

The model is not
sufficient to deal with processes in which deltaQ can be
either positive or negative. Is the traditional Model 2, as
described above, acceptable?

No, for the reason just mentioned. This defect in Model 2 is easily remedied; I see no logical reason why Model 2 requires or implies positive delta Q.

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

This Model 1 and Model 2 introduced all sorts of terminology (caloric, mhEnergy, et cetera) that were merely stepping stones along the way to a more general understanding. But to my astonishment, among all this terminology I didn't see the term "entropy". Trying to explain (much less "derive") thermodynamics without entropy is like trying to sell dehydrated water.