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"heat" is a bad word



On Wed, 1 May 2002, Tina Fanetti wrote:

She asked if light and heat were the same thing since they were both =
energy. Light energy would be like a photon and heat is internal ene=
rgy.

When we heat an object, the energy usually ends up in three places:
infra-red photons leaping between atoms, high-ultrasound vibrations
(phonons) leaping between atoms, and excited atoms which will soon emit
either a photon or phonon.

Unfortunately the "heat" concept is not so simple. Heating an object can
cause chemical changes (melting, boiling, or chemical reactions), and in
that case the "heat" seems to disappear. Or, chemical changes can cause
"heat" to suddenly appear from nowhere (freezing, condensation, or
exothermic reactions.)

Since "heat" is not a conserved quantity except in very specific
situations, most people on PHYS-L don't like the noun "heat." It
suggests that some stuff called "heat" exits.

Me, I like to imagine that "heat" exists, but it's only a mental model,
and we have to know well the narrow limits of this model. Look closely
at the model, and it breaks up into high-frequency sound, low-frequency
light, and excited atoms.

In "Clouds in a Glass of Beer", Chris Bohren points out the many
misconceptions which the "heat" concept can cause. For example, a warm
object gives off invisible light. Yet "heat" is partially MADE of
invisible light which is trapped within the warm object. So, when warm
objects radiate, are they simply glowing? Or are they giving off a
mysterious stuff called "heat radiation?" If one believes in "heat
radiation", then one must also believe that infrared LEDs give off "heat"
rather than invisible light. And if a colorblind person cannot see red
light, then for that person the red light is also a form of "heat." And
if you close your eyes and warm your hands in the sunlight, you are
receiving "heat" from the sun instead of light. Don't open your eyes
though!


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William J. Beaty SCIENCE HOBBYIST website
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