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Re: heat is still a noun



Since I brought Craig Bohren's book "Atmosperic Thermodynamics" into this
conversation, I thought that his thoughts on this matter would be of
interest to the group. I asked Craig to respond and he was kind enough (as
usual) to do so.

Regards
Larry Woolf

Here are Craig Bohren's responses to the commments of a recent post,
containing a large fraction of John Denker's points.


Subject: Re: heat is still a noun


Aaaiiieeee! Is this is yet another attempt to re-define "heat"????

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

1) Once upon a time, everybody used "heat" to mean "thermal energy".

Zemansky published an article in the Physics Teacher (Vol. 8, pp. 295-300,
1970))in which he said some very harsh things about "thermal energy".

2) Then there was an attempt to re-define it to mean "energy transfer
due to a difference in temperature".

This is a good way to define the PROCESS of heating (as opposed the
SUBSTANCE heat).

3) Now apparently there is an attempt to re-define it to mean "change
in
enthalpy"????

No such attempt was made. But I see that I am going to be tarred with this
brush no matter what I say.

I think this is a step in the wrong direction.

So do I, but no such step was taken.

This works nicely for constant-pressure
processes when no "other" work (besides compression/expansion work)
is being done. By and large, I think they've made the right
decision, given their audience and the scope of their book.

We merely pointed out that in certain kinds of processes for certain kinds
of systems the rate of change of enthalpy just happens to be the heating
rate. This in no way implies that we attempted to redefine heat as an
enthalpy change.

I haven't read the book. But judging from the reports on this list,
they've paid a terrible price for little (if any) gain.

Here is someone who hasn't read our book but knows in advance that we are
wicked. And, by the way, there have been other wicked people who have made
similar comments about heat.

There are >>just too many processes where Q is not equal to Delta-H, so
we
need separate names for the two quantities.

I'll state again that no attempt was made to define Q as a delta H.

Maybe we ought to listen
and find some way to improve the language for generations to come. I've
tried to re-write some of my labs with this injunction in mind, and it is
_hard_. It is one thing to object to current language usage, and quite
another to propose acceptable alternatives. I need help with the latter.

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? 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). Thermodynamic internal energy (internal
energy or energy for short) is the sum of these two kinds. So why do we need
a term "thermal energy"? And how in the name of all that is holy can anyone
"create" thermal energy? I thought that energy could be neither created nor
destroyed. This is what students are told on day one. Doesn't it seem a bit
contradictory, if not confusing, to then talk about the creation of energy?
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. 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? 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. Why not just say "transformed"? The word transformation is
well suited: it means a change of form.

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

Some partisan neologists claim that anyone who uses "heat" as a noun must
adhere to the caloric theory that Rumford discredited. It is tiresome to
hear such a preposterous accusation repeated so often.

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."

My goodness I really am a wild-eyed, bomb-throwing heretic. First I trash
the notion of heat as a substance and then I dethrone Rumford, one of the
saints of American physics.

I just got back from a three-day symposium in honor of Richardson, Reppy,
and Lee. It was, among other things, a reunion of most of the people who
had ever worked in the low-temperature physics lab at Cornell. These are
people who teach undergrads in the morning and do thermodynamics research
in the afternoon. They build refrigerators that will get down to
milliKelvin or microKelvin temperatures.

I assure you that these people use "heat" as a noun. They talk about heat
pulses, heat capacity, heat leaks, heat shields, heat switches, and heat
exchangers. If you tried to tell them that "heat" is not a noun they
would
think you were from the far side of Mars.

See previous comment about what experts can do that students should not.
Also, I also note that in all these examples "heat" is used essentially as
an adjective.

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

I quite agree.

(Energy transfer due to temperature differences is only a minor part of
the
larger picture.)

As far as the larger picture is concerned, I agree, but like it or not
energy transfer due to temperature differences is what is of most relevance
to our everyday lives. I think that people should first learn about the
world around them before tackling low temperature physics.

Some of my heretical ideas about heat were presented last year at a Gordon
Conference on the teaching of thermodynamics. I shall send you the relevant
sections as an attachment. Anyone who wants the full text can write to me,
but it is not for the faint of heart.

My parting shot is to note the following. In the first law of
thermodynamics work and heat appear symmetrically, as equals. 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).
Why isn't work talked about in the same way that heat is? Why is it that
physicists don't seem to have any difficulty accepting working as a process,
as a way of doing something? My guess is that the reason for this is that
the rate of working can be expressed as the product of fairly concrete
quantities (Force and speed) whereas no such tidy expression for heating
exists. Heat is inherently more mysterious.

Craig