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Re: [Phys-l] operational definition on heat

On 09/11/09 16:40, wrote:

Does anyone have a good operational definition on heat?

I've got several of them; see below.

It is not unusual for a word to have several definitions.
That last time I looked up "mark" it had a couple dozen

This only becomes a problem if/when the multiplicity
is not recognized. Then we get some kind of misbegotten
chimera. Alas, non-experts think heat is one idea, when
in fact it is two or three ideas trying go by one name.

Just as the notion of phlogiston was replaced by two
more-modern ideas (energy and oxygen), the notion of
heat needs to be replaced by two or three more-modern
ideas (energy and entropy, and possibly temperature).

My advice: If you are confused about how to quantify
heat, forget about heat. Quantify energy and entropy
instead. That's how thermal physics is really done,

By the way, some students have difficulty understanding "heat is a verb".

Good for them. Anybody who believes the claims of the
"heat is a verb" faction is not paying attention.

In technical use:
1) Sometimes it is a verb: Please heat some soup ....
2) Sometimes it is a noun:
2a) Sometimes the noun is an extensive scalar with
dimensions of energy: heat exchanger, heat capacity,
heat of reaction ....
2b) Sometimes it is an intensive scalar with dimensions
of temperature: uncomfortable heat wave.
2c) Sometimes the noun is a vector with the same
dimensions as the gradient of energy: T dS.
*) Not to mention innumerable nontechnical and
metaphorical meanings.

Meaning (2c) is the one that is most often mutilated.
Students aren't normally confused by it, although
all-too-often the textbook authors are. I would have
thought that the distinction between "vector" and "verb"
would be straightforward, but evidently some people
garble this distinction and then insist that their
"definition" is the only possible definition. So
they're wrong twice over.

Let's be clear:
-- T dS is not a verb. It's a vector.
This is not a tricky concept.
-- T dS is not the only definition of "heat". Never
was and never will be.

And in case anybody is wondering, I take my definitions
from the way thoughtful experts use the terms.

I remark in passing that although T dS has the same
dimensions as dE, there is more to physics than
dimensional analysis. T dS is not equal to dE nor
to dQ nor to d(anything). In general there cannot
possibly be any potential Q such that T dS = dQ.
This is not something you want to explain to naive
students on the first day, but it is a goal to keep
firmly in mind. This is the dividing line between
those who have some clue about what T dS is and
those who don't.

On the first day you don't need to say anything
about heat. Talk about energy and entropy instead.

The operational definitions of heat exchanger and heat
capacity are rather well known. I can discuss them if
anybody is interested. As always, what matters is the
energy and the entropy. If you know the energy and the
entropy, you don't need to worry about heat.

The so-called heat of reaction is a misnomer; it really
really should be called the enthalpy of reaction. The
operational procedures for measuring enthalpy are rather
well known.

The most common operational procedure for measuring T dS
starts from the identity dE = T dS - P dV. Solve for
T dS. Measure and/or control dE and P dV.