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Re: [Phys-L] let's define energy



I meant "lost" in the sense that it is no longer useful to do something. Why does the car stop? The energy is "lost" to heat, etc. Students can understand conservation but they need something concrete to be able to say why the car stopped. Thus I use "lost" in quotes to indicate the energy is not useful. They know it is not lost as such but the quotes has come to have meaning to them when they explain what is happening.

On Sep 29, 2015, at 3:39 PM, John Denker wrote:

On 09/29/2015 11:50 AM, Marty Weiss wrote:

I would argue that the "meaning" we should concentrate on with our
students is in the conservation of the energy in the system, the
"loss" of the energy from useful to wasted energy, or concepts like
those, rather than what the word itself means.

This is a discussion well worth having.

As I see it, there are *TWO* concepts here, not one.
The physics energy is strictly conserved, never "lost",
never "degraded". Talking about the "loss" of energy,
even when "loss" is in scare quotes, is confusing to
students. For that matter, I can't make sense of it
myself.

Here's how I explain it: In a steam engine, there are
three things you need to keep track of: the H2O, the
energy, and the entropy. It makes no sense to say that
the H2O is "degraded", and by the same token it makes
no sense to say that the energy is "degraded". At the
end of each cycle, all the H2O and all of the energy
is still there.
-- The Carnot efficiency formula, even though it is
expressed in terms of energy and temperature, is
understandable in terms of entropy and not otherwise.
-- The irreversibility of a chemical reaction is
understandable in terms of entropy and not otherwise.
-- Contrary to what it says in the NGSS, if you see a
ball to roll to the bottom of the bowl /and stay there/
(rather than endlessly oscillating or orbiting) you
cannot explain it in terms of the physics energy; it
is understandable in terms of entropy and not otherwise.

Explaining the concept of entropy is not easy, but it
is doable and worthwhile. Explaining "degradation" of
energy is AFAICT not doable. Students hate it. Teachers
hate it.

Entropy is especially worthwhile because its applications
reach far beyond classical thermo to include communications,
crypto, learning theory, etc. etc. etc.

Trying to use one word (energy) to cover two disparate
concepts (energy and entropy) results in an endless,
unwinnable game of whack-a-mole: Maybe you can fix
some sub-part of the problem, but only by making some
other part worse.
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