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Re: What Flows?



On Sun, 8 Sep 1996, Leigh Palmer wrote:


2) Can you tell me how it is that the cool object increases in energy with
out saying the words "heat" and "flow"? What is the detailed mechanism?

Say that energy flows. Who cares what the "detailed" mechanism is? By
the way, I don't think you should ever say "internal energy flows".

But isn't the purpose of the "energy" (and "heat") concept to simplify and
interlink a number of different detailed mechanisms? I'm aware that my
own understanding of these concepts was greatly improved by a study of the
detailed mechanisms. It showed me that there was a *reason* for the
overall guiding/organizing concepted called "internal energy," and showed
some of the historical development which created the concept.

So, what are the dirty details of Internal Energy? What does it look
like? I'm not entirely confident of my mental model, but here are some
major features:

"Acoustic vibrations." But how does this differ from sound? I think
it differs mostly in wavelength. When the wavelength of sound in a solid
approaches the lattice spacing, the velocity falls radically. Sound
and "thermal vibrations" also differ in coherence, since the short
wavelength makes individual molecules look big and behave as scatterers.
As a result, I think of Internal Energy as being partly composed of high
frequency, short wavelength, very loud white-noise sound which
propagates slowly and can travel between two objects in contact. I've
seen a bit about this viewpoint in an old physics encyclopedia under the
heading "hypersonics." This viewpoint explains conduction, and also
connects to quantum acoustics (phonons) in a natural way.

"Blackbody radiation." Internal energy can propagate between parts of
a substance as EM energy too. Think of the substance as glowing with
light as a consequence of its non-zero temperature. The emission of the
light doesn't happen only on the surface, it also happens inside.
"Heat" can "flow" as radiation. When a thermal vibration encounters a
molecule in the lattice, the molecule can pass on the vibration to its
neighbors, or it can convert it to an IR/microwave photon which will be
absorbed by it's neighbors.

"Particle velocity." When there are no bonds between the component
parts of a substance, the Internal Energy appears as kinetic energy
of individual particles. This of course applies to gasses, but also
connects with the electron-gas within metals. I suspect it also
applies to liquids, where the phenomena is halfway between that of
moving/colliding particles and that of waves propagating through a
crystal lattice.

"Molecular rotation/vibration" In materials having complex molecules,
the Internal Energy can be momentarily stored in individual molecules
mechanically as various types of spinning and vibrating motions.

"Phase change." This one includes all sorts of nonlinear nasty
stuff, various crystal lattice changes and curie point transistions
and chemical reactions, as well as the usual solid/liquid/gas
transformations. Internal energy is stored as large-scale organized
patterns in particle populations rather than within individual
molecules.

Talking about "heat" or "internal energy" hides all of the above. For me
this was both good and bad. Until I delved into it, I always felt that I
had accepted too much of K-12 thermodynamics on faith, and couldn't see
the reasons for the way things were done.


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