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Re: [Phys-l] internal/external conservative/nonconservative forces!?!?

A good take on where the energy resides is to say it is in the system. Of
course the same thing is true of gasoline and Oxygen. Gasoline has no
energy by itself because you can't get it if the gas is in a sealed airless
container. But the energy is not in the Oxygen either, so it must be in the
system. All too often the texts erroneously say that the energy is in the
bonds. But of course in the case of an elastic object you can have the
elastic energy in that one object. I don't know if sharing presents a good
picture.

In Modeling the idea of the energy as being in the field is presented to the
students. In both Modeling and the Minds on Physics Text one must pick a
system to define the energy. So there is no gravitational energy unless the
system includes the Earth as well as the object.

My pet peeve is the fact that biology texts say bonds contain energy and
that energy in the Krebs cycle comes from breaking bonds. In reality it
comes from forming hydrogen bonds or hydrolysis, but the texts seem to
ignore that final step.

John M. Clement
Houston, TX


One of my pet peeves. In approach (a), it is not proper to associate
the energy with just one object. The energy is shared between that
object and the Earth. Of course, it's common to use mgh to describe
the energy of just one object, and there isn't much harm done in doing
so, but students should always be aware of the bigger picture.

Bill




On Dec 14, 2010, at 4:03 PM, John Denker wrote:

Here's one more point about whether the energy is "internal"
or "external".

We associate gravitational potential energy (m g h) with an
object of mass m at height h. A similar equation applies to
electrostatic potential energy of a charged object.

The question is, what does it mean to "associate" an energy???

a) There is one school of thought that says the gravitational
potential energy is "internal" to the object.

b) There is another school of thought that says the energy
actually resides in the gravitational field, and is therefore
almost entirely external to the object.

Approach (a) seems to be common at the high-school level,
while approach (b) seems to prevail at the more advanced
levels.

Similar words apply to the electrostatic version, although
approach (b) is even more strongly favored. Poynting had
something to say about this.

=============

The intriguing thing is that for most purposes, it doesn't
actually matter whether the energy is considered internal
or external. To an excellent approximation, the equations
of motion (including the principle of virtual work) don't
care where the energy resides.

It would matter if you ever measured the energy density of
the field (perhaps by setting up a Keplerian orbit around
the field, since the orbital period depends on the amount of
mass i.e. energy inside) ... but that is rarely (if ever!)
done. Feynman has a long discussion of this issue, namely
the puzzle over where the energy resides.

This a sufficiently well-known puzzle that whenever you
see a document such as
http://www.physicsclassroom.com/class/energy/u5l2a.cfm
ranting about the importance of the internal/external
distinction, you know immediately that they haven't
thought about the issue at all.

Also the idea that there could ever be a correspondence
that maps "internal/external" to "conservative/nonconservative"
is dead on arrival --- another proof that they haven't
thought about the issue at all.

It remains a mystery why anybody would go to the trouble of
inventing new and heretical versions of the basic concepts
of physics. What is wrong with the plain old conventional
notions of energy, conservation, et cetera? And wouldn't
you think that somebody who invented a new version would
feel obliged to check whether it was consistent with the
known facts?

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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l