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Re: heat is still a noun

At 05:04 PM 5/7/01 -0700, Larry Woolf forwarded a note from Craig Bohren:
>>Aaaiiieeee! Is this is yet another attempt to re-define "heat"????

No this is not another attempt to re-define heat.

OK, good.

>>I think this is a step in the wrong direction.

So do I, but no such step was taken.

Again, good.

[snip ....
.... I won't comment on comments on passages I quoted from others.]

>>As I have said before, there are two issues here; a big issue and a small
>>issue. The definition of the word "heat" and other language details are
>> the_small_ issue. The much larger issue is conceptual.
>>
>>The big conceptual blunder is the over-emphasis on "energy transfer due to
>>a difference in temperature". It's really quite secondary to a modern
>>understanding of thermodynamics. Focusing thereon puts you at a
>> tremendous
>>disadvantage when discussing situations where thermal energy is being
>>_created_ not just transferred:

What is this vague stuff called "thermal energy"? Why not just say internal
energy or just energy?

To answer the second question: We don't say "internal energy" when we mean
"thermal energy", because not all internal energy is thermal.

To answer the previous question: Calling it vague doesn't make it
vague. It's not vague to me.

To answer both questions: In particular, in the spirit of a black box:
1) Consider a black cylinder with a piston poking out one end. It is
basically a simple piston with a gas behind it, at moderate pressure, and
that this explains the observed force on the piston.
2) Also consider a second black cylinder with a piston poking out,
externally identical with the previous one. But inside there no gas, just
a long, long spring made of Invar metal, which explains the observed force
on the piston.

Neither cylinder contains any significant thermal insulation.

Suppose we lose track of which cylinder is which. Since they are
superficially identical, we can't identify them just by looking. But we
can do an experiment. We compress each one twice. For each cylinder, we
compare the isentropic compressibility to the isothermal
compressibility. The difference between cylinders will be immediately
apparent.

The difference is that one cylinder stores energy in a way that is (to an
excellent approximation) nonthermal, while the other stores energy in a way
that obviously involves thermal processes.

As far as I have been able to determine there are
only two kinds of (non-relativistic) energy: kinetic (energy of motion) and
potential (energy of position).

If you want to lump things together, you can always lump things
together. Why not go all the way and disparage the distinction between
kinetic and potential energy? Just because they _can_ be lumped together
doesn't mean we can't un-lump them when convenient.

Thermodynamic internal energy (internal
energy or energy for short) is the sum of these two kinds.

That's narrowly true, but it is not the whole story, as illustrated by the
parable of the two black cylinders.

So why do we need a term "thermal energy"? [1] And how in the name of all
that is holy can anyone "create" thermal energy? [2] I thought that energy
could be neither created nor destroyed. This is what students are told on
day one. [3] Doesn't it seem a bit contradictory, if not confusing, to
then talk about the creation of energy? [4] Since the defining property of
energy is that it is a quantity that is conserved, the only thing that can
happen is that energy of one measurable kind can be transformed into
another kind. [5] Example: I drop a rock onto the floor. Just before the
rock hits the floor it has a certain (mass-motion) kinetic energy, which
can be measured. After the rock hits the floor its mass-motion kinetic
energy is zero. But if we believe that energy is conserved, we also must
believe that the mass-motion kinetic energy has been transformed into
another kind that requires different kinds of instruments (e.g.,
thermometers) to measure. Is it wise to say that "thermal energy" has been
created? [6] I think not given the confusion this is likely to cause
because of the frequently repeated mantra that energy can be neither
created nor destroyed. [7] Why not just say "transformed"? The word
transformation is well suited: it means a change of form.

Most people on this list are quite aware of the law of conservation of
energy. It is not necessary to explain it to us 7 times in one paragraph.

Also please, let us not play word games. Abraham Lincoln pointed out "a
specious and fantastic arrangement of words, by which a man can prove a
horse chestnut to be a chestnut horse."

I did not say "energy is being created".
I spoke of a situation where "thermal energy is being created".

Taking the last four words out of context seems amazingly obtuse.

-- Students who want to be obtuse can be obtuse, and there's not much I
can do about it.
-- Those who want to understand will understand, which is the best I can do.

Perhaps I could have written something more nitpick-resistant, perhaps
speaking of situations where "some other form of energy is being
transformed into thermal energy" but some people would find that less
clear, not more clear... and I still think anyone who wants to understand
will understand what it means to say "thermal energy is being created".

In chemistry, it is very common to speak of reactants being "consumed" as
the products are being "created", and only the amazingly obtuse would think
that this implies non-conservation of the atoms involved.


>>To the modern way of thinking:
>> 0) The zeroth law of thermodynamics states that there is such a thing
>> as thermal equilibrium. In equilibrium, objects have the same
>>temperature. This is true and important.
>> 1) The first law of thermodynamics states that energy is conserved. In
>>particular, energy obeys a _local_ conservation law. This is the first
>>law; nothing more, nothing less. This is true and important.
>> 2) The second law of thermodynamics states that entropy obeys a local
>>law of nondecrease. This is true and important.
>> 3) The third law of thermodynamics alleges that the entropy of some
>>things goes to zero as temperature goes to zero. This is true except when
>>it's not true. It's not very important.
>> 4) Entropy is defined in terms of statistics. It is well defined even
>>when the temperature is unknown, irrelevant, or zero. This is true and
>>important.
>>
>>
>>Note that the word "heat" does not appear in the foregoing laws. Getting
>>the big picture does not require worrying about the exact definition of
>>"heat".

>>Moving now from the large issue to the small issue: Once you have some
>>semblance of a modern understanding of thermodynamics, there is no harm in
>>using "heat" as a noun, in just the way it was used by Rumford in 1798 and
>>is still used by respectable experts today.

You are right,

Good. Thanks for saying so.

but students do not have a "semblance of modern understanding
of thermodynamics."

Well, then, my recommendation is to focus on the fundamental conceptual
issues, rather than finagling with the nomenclature.

Experts in a field can say the most outrageous and
illogical things because they know better. They can use sloppy metaphors,
ill-chosen figures of speech, without much harm--when they are talking
amongst themselves.

Sometimes experts say silly things. But sometimes they know what they're
talking about. What's the point here?

Sometimes experts say non-silly things, especially when I ask them to give
a careful, thoughtful explanation of how and why they use "heat" as a noun.

But someone who is learning something for the first time
has no choice but to accept all statements as literal.
Students are not capable of distinguishing between the literal and the
figurative.

I have no idea why this passage is relevant to the present discussion. I
am not recommending anything that I consider outrageous, illogical,
ill-chosen, sloppy, metaphorical, or figurative.

As a matter of fact, Rumford did not discredit the caloric theory. The role
of Rumford in thermodynamics has been greatly exaggerated in textbooks,
especially ones written by Americans. I suggest that you read Fox's
historical treatise The Caloric Theory of Gases, in which he states that the
"history of the theory could be written with scarcely any reference to
Rumford."

I'm not a historian, and I was not making a historical argument.

Let me rephrase my argument as follows: Reading a certain paper
(attributed to Rumford and apparently published in 1798) provides a modern
student with information which is sufficient to make the caloric theory
untenable.

That makes the point I was trying to make. The new phrasing is more
nitpick-resistant than the original version. In any case, the actual
historical trajectory of caloric theory has no direct bearing on my
research or on my teaching.

>>To summarize: My advice is to keep your eye on the ball. Energy is
>>important! Entropy and temperature are important!

I quite agree.

OK.

My parting shot is to note the following. In the first law of
thermodynamics work and heat appear symmetrically, as equals.

The way I write it, they don't appear at all.

I think it is unwise to focus on "work" (a nonthermal transfer of energy)
for exactly the same the reasons that I think it is unwise to focus on
"h&@t" (or whatever one calls a thermal transfer of energy). Any process
involving dissipation exposes the folly of such a focus.

like it or not energy transfer due to temperature differences is what is
of most relevance to our everyday lives.

What is the basis for that assertion?

I'll concede that cannon-boring is not important to most students in their
everyday lives, but what about the energy transformation that occurs in
vehicle brakes? My everyday life is full of dissipative processes.

Maybe some people live in an enchanted world where everything is
thermodynamically reversible. I don't.

It is
impossible to make a measurement on any system to determine how much heat it
contains or how much work it contains. So why don't the folks who insist
that heat is some kind of substance (i.e., a noun) also make the same claims
about work? What is good for the goose (heat) is good for the gander (work).

Again, I agree that elevating work to first-law status is just as silly as
elevating h&@t to first-law status.