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At 09:59 PM 9/17/01 -0600, Jim Green wrote:
Energy is a _property_ of a system; It is not stuff which can be moved,
transformed, transferred, poured, etc. Blue might be a property of a
system -- One would not conceive of saying that "blue"
moves/flows/transfers/etc. Blue _paint_ can flow -- a substance can
flow. But properties don't move independently.
This is a colorful analogy, but we have not been given a proof, or even a
plausibility argument as to the accuracy of the analogy. Why should we
believe that "blue" is like "energy"????
In fact, there is no reason why we should -- and every reason why we should
not.
-- I can take a blue interference fringe and change it into a red
interference fringe with the greatest of ease. I am not hindered by any
conservation law. There doesn't need to be any outward flux of blueness or
any inward flux of redness.
-- In contrast, according to all available theoretical and experimental
evidence, energy behaves completely differently.
Likewise, it is also inaccurate to say that for example mass moves -- a
massive block can move, but mass doesn't move independently. Further, it
would be incorrect to say that relativity equates mass and energy,
therefore energy is stuff. No. No No. Neither energy nor mass is
stuff. They are both properties of some system. It is also incorrect to
picture energy as "propagating" through a surface -- or through anything
else. These comments are valid for all other properties of a system as
well eg charge, roughness, size, etc etc ad nauseam.
Repeating an invalid argument ad nauseam does not make it more valid.
Here is a valid analogy that addresses some of the issues that have been
raised: I cannot put a gallon of liquid water on my desk all by
itself. The water could be in a glass pitcher. It could be in a metal
bucket. It could be in a group of styrofoam cups. But the containers are
just a distraction. The water has a reality quite independent of the
containers.
We can apply this analogy to energy: It is true that the thing we call
energy cannot exist in isolation. It could be embodied as the kinetic
energy of some material object. It could be embodied as gravitational
potential energy. It could be embodied in a photon. But worrying too much
about the embodiments is just a distraction. The energy has a reality
quite independent of its embodiment.
This reality is expressed by a local conservation law. Indeed, energy is
more strongly conserved than water is; you can create water from scratch
if you really want to.
The energy of a system can be increased or decreased -- but not
transferred, The _only_ way to change the level of the energy of a system
is to do work on the system.
This statement has one meaning that is 100% clear. This meaning is alas
100% wrong.
Thermal processes are universally excluded from the definition of
work. Even people who feud about the exact meaning of "heat" and "work"
agree on this exclusion.
Thermal processes can change the energy state of a system. Therefore
asserting that work is the _only_ way to change the energy level is clearly
100% false.
======
This statement also has another meaning, slightly hidden: It suggests we
should speak of change, and avoid speaking of transfer. This meaning is
also wrong, for the following reason:
The laws of physics require that when we change the energy level of system
"X", there is an equal-and-opposite change in the energy level of some
other system "Y". This guarantees that it is harmless to speak of an
energy _transfer_ from X to Y.
You may wish to ignore this law of physics, but you cannot force the rest
of us to ignore it.
Consider the following example:....
Someone in a tall building wants to descend quickly.
Now does energy move? Well yes. The internal energy of the person moves
as the hapless fellow slides. So does the kinetic and potential energy of
the person. The brown of his hair moves. His mass moves. The charge
of his molecules moves.
But none of these properties is "transferred" from one system to
another.
The notion of energy transfer is indispensable to any useful conception of
energy conservation. The key here is that we want to have a *local*
conservation law. The alternative (a nonlocal conservation law) would
permit energy to disappear from "here" and simultaneously appear somewhere
far away without necessarily flowing through the intervening space. Such a
law would express the idea that the total energy of the universe is not
changing, but it would be just a technicality, academic in the worst sense
of the word. It would not be a useful law. It would have no applications,
because any apparent violation could be explained away by assuming that the
energy "must have" mysteriously appeared in some distant unobserved part of
the universe. A law that can be freely violated is no law at all.
A *local* conservation law is incomparably more powerful. It says that
energy cannot disappear from within a given boundary except by flowing
across the boundary. Local conservation implies global conservation, but
not vice versa.
Experimentally it is possible to verify *local* conservation of energy,
because the flow of energy, like energy itself, involves identifiable,
controllable, and measurable physical processes.
Also note that a *local* conservation law can be made consistent with
special relativity, while a nonlocal law cannot.
The *local* conservation of energy that I describe is manifested and
confirmed every day by innumerable physical processes -- motors,
generators, antennas, cannon-boring machines, levers, roller-coasters,
toasters, air conditioners, et cetera.
Yes, the person does work on his environment and changes its energy:
canvas, Earth, etc. And the total energy of the Universe remains
constant.
Stating that the total energy of the universe remains constant is a
nonlocal statement. It is not a satisfactory expression of the laws of
physics, for the reasons given above.
I agree that sometimes information can be transferred more fluidly with
shorthand language. But only with the great danger that the conversants
forget that it is shorthand -- and in complicated discussions commonly
do. In the classroom it is usually a disaster.
There is no evidence to support the allegation of disaster. And much
evidence to the contrary.
A person walks into the local branch of his bank with some green
dollars. He deposits these dollars in his account. He flies to another
city and goes to his bank's branch there. He draws out some of his
dollars -- Well no he doesn't. He draws out somebody else's dollars, The
physical dollars don't move from city to city. Just the numbers associated
with his account do.
Alright alright alright I don't like it either.
Again, the behavior illustrated by this story is utterly non-analogous to
the behavior of real energy.
1) It does not illustrate even non-local conservation of money. The
amount of greenbacks in the world is not constant. Even if we treat money
as merely a representation of something else (like "value" or "wealth")
those other things aren't conserved either.
2) It blatantly does not illustrate local conservation. The idea that I
can put greenbacks in "here" and take greenbacks out "there" without any
detectable flow of greenbacks from here to there -- that is exactly how
energy does *NOT* behave.
You might wish energy would behave that way. But wishing won't make it so.
Telling random stories doesn't help. Stories about blueness don't tell us
anything about energy. Stories about money don't tell us anything about
energy. Stories about about wombats wouldn't help either -- unless we are
given some physics-based explanation (theory and/or experiment) of why
energy behaves the same way.