[Martin Taylor 931220 1830]
(Bob Clark 931218.1530)
For energy to be useful, there has to be the possibility for entropy
gain, as energy passes from the high to the low level.
Rather, "If energy is useful as it passes from the high to the low
level (that is, from high temperature to low temperature), entropy
will increase." Entropy is a derived concept, not a limiting one.
Derived concepts can still be limiting. And why "high temperature to
low temperature" rather than high shelf to low floor, or high-level
potential to high temperature? The key is whether entropy will increase,
not whether a hot reservoir gives some heat to a cold one.
I don't know how energy is defined nowadays. There's a commission
that does these things.
There has been no change in the definition of "energy." The
commissions (plural) you refer to determine definitions of units of
measurement, characteristics of materials and the like. Not the
definitions of the established terminology.
I looked up a variety of sources, from Van Nostrand to physics dictionaries,
and I still don't know how energy is defined nowadays. One (I think it
might have been Van Nostrand) commented that energy used to be defined as
the product of force and the distance through which a force is applied.
Another (or maybe the same) said that the best definition seemed to be
kind of by default--it is that which changes when useful work is done.
This allows for chemical, nuclear, thermal, potential, kinetic and other
kinds of energy to be kept on the same footing. Another brought it down
to the fundamental fact that increasing the energy in a body increased its
mass according to E=mc^2, but noted that this is not a very useful definition
because so much of the energy is bound up in the rest mass and it is very
hard to get useful work out of that part. Especially if the mass is iron!
So I don't know any more than I did.
When your face gets warm near a dark stove element, you can sense
energy better than force or distance.
You sense "temperature" not "energy." Your skin is well equipped with
temperature sensors. "Energy" is still a derived concept, not
directly sensed.
I think actually you sense energy flow through the skin, from which you
derive a temperature differential between inside and outside. Anyway,
you sense what your nerves signal, and I don't think we have any that
vary their output according to their temperature. But I could be wrong.
They are ALL derived quantities, anyway, as we both pointed out to Bill P.
What we think of as being "derived" depends on what we think of as being
more fundamental and/or tied to direct sensory observation.
Martin