Re: heat is "real", but...

[William Beaty <billb@ESKIMO.COM>]

On Wed, 31 Jan 2001, Larry Woolf wrote:

> > But who insists that "heat" is a substance?  Perhaps words are being put
> > into the mouths of the "heat believers" to make their viewpoint look
> > foolish.  Heat is only "substance-like"  in that it is a conserved
> > quantity in some circumstances.
>
> If heat is a conserved quantity, define heat and the conservation law that
> it obeys.

If the "internal energy" of an object is thermal photons and thermal
phonons and electron band energy and chemical bond energy and nuclear bond
energy and everything else, then as I understand it, the usual meaning of
"heat" is "thermal photon energy plus thermal phonon energy."  Heat is
blackbody radiation within an object, plus the hypersonic white-noise of
thermal vibrations of the lattice.  (Uh, do we need to add the energy
contained in the electrons of hot metals?)  As long as chemical or nuclear
reactions or phase changes aren't adding or subtracting thermal
photons/phonons within the object, then "heat" is conserved because energy
is conserved.

When I say "in some circumstances", maybe I could better say "within
limits."  Inject a joule of electrical energy into a resistor, and this
resistor now contains a joule of "heat."  But do the same with a lead/acid
battery, and the "heat" is not conserved;  it vanishes, and the chemical
bonds of the materials are altered.


> Bohren's point is that almost all situations can be better
> described by using temperature instead of heat.

Yes.  I have no meter which can tell me how much heat is in the above
resistor.

> Temperature is well defined.  Heat is not.

Apply 1v across the terminals of a 1-ohm resistor for one second, and the
resistor will contain one joule of "heat."  That's well defined.  Little
fuzzy about it.  But we have a hard time measuring this "heat" directly.
And if the "heat" should flow into another object in contact with the
above resistor, we have no good way to measure the "heat flow."  The
"heat" may be perfectly real, but if we have no way to count the number of
photons and phonons banging around in a crystal lattice, and if unexpected
phase changes or chemical reactions can mess everything up at any time,
then "Quantity of Heat" certainly loses most of its utility.

I see that the question "Is Heat Real?" is a distraction.  The goal is to
find a reason to avoid teaching misleading and non-useful concepts.  There
might be a joule of genuine "heat" within a container, but if we have no
way to perceive it (with instruments), and if students typically pick up
many misconceptions from such discussions, then what good is it?  "Heat"
and "heat flow" is a good way to understand why the temperatures of a hot
metal block and a pot of cold water will change when the first is dipped
into the second.  Is this worth all the negative consequences?

I personally was somewhat befuddled by "heat" until I saw a demonstration
described in AJP.  Somebody set up a long chain of sleds on an airtrack,
and connected them with springs.  When the end sled was driven slowly,
"sound waves" propagated along the chain.  But at very high frequencies,
individual sleds would resonate, and rather than propagating sound waves,
a very slow "infectious wiggling" would spread along the chain.  This is a
visual model of heat conduction.  It only describes the acoustic component
of "heat flow."  Real heat is not JUST hypersonic sound which spreads from
molecule to molecule; it also includes EM radiation leaping larger gaps.
And then there's all that weird QM stuff with electrons in hot metals. But
the "wiggling sleds" demo put me on the path to understanding "heat", and
to clearly seeing why it is not so appropriate for the intro level (if at
all.)

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