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# Re: [Phys-l] teaching energy

• From: Jack Uretsky <jlu@hep.anl.gov>
• Date: Sat, 30 Sep 2006 13:00:10 -0500 (CDT)

Hi all-
Joel sez (in part, and taken slightly out of context):
and helps to eliminate
the common misconception that energy comes in different flavors; ie, that KE is somehow different from PE

I've been doing this for a while, and I like to think of energy coming in different varieties ("flavors" is a term having a different precise meaning). I think that KE is very different from PE, and the difference looks stark to me when I write a Lagrangian (see hep-ph/0609015 in the ArXiV). I suggest that it is dangerously and arrogantly presumptious to try to decide what kind of imagery might be appropriate for a student. My take is to stick to the mathematical descriptions and physical demonstrations and let each student construct his/her own images.

Regards,
Jack
On Sat, 30 Sep 2006, R. McDermott wrote:

----- Original Message -----
From: "Rauber, Joel" <Joel.Rauber@SDSTATE.EDU>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, September 29, 2006 9:59 AM
Subject: Re: [Phys-l] teaching energy

Later they claim:

"If we claim that energy is stored in the gravitational field, and if a
change in energy content is indicated by some change in the thing
that stores it, then what changes in the field when it gains
or loses energy?

R. McDermott wrote in part:
| The central theme is that the energy must reside somewhere.
| Taken in that
| context, the only "place" for it to be is in the field itself.
|

I don't understand the *must*; why not say it resides in the
configuration of the system.

Bear in mind that we are (mostly) talking about a presentation to high
school students. While what you say is certainly correct, it conveys very
little information to the beginning student that they can use to help them
internalize their learning. By getting them to associate energy with a
location, it is easier for them to deal with TRANSFERS of energy, and with
the concept of working changing the amount of energy, and helps to eliminate
the common misconception that energy comes in different flavors; ie, that KE
is somehow different from PE and has to be "converted" by some unknown and
unspecified process. Beginners (most of them anyway) need a framework that
is concrete before they can proceed to abstract.

The change in the thing is . . . The
"thing" would be the two body system, the change in the thing would be
its configuration. For point-like objects that would be its separation
distance. Much like a mass-spring system.

That is why my personal approach is to combine the idea of field lines with
energy so that changes in energy produce changes in the field geometry.
It's basically the same argument you're making, but is (imo) more visually
concrete. My personal opinion, nothing more.

They appear to me to be coming dangerously close to re-enforcing a fluid
& container model of energy; i.e. reifying the idea to a possibly
dangerous degree. Though I must admit to only a cursory glance at the
document.

Since I haven't heard the argument before, does trying to associate energy
with a location constitute this fluid and container model, and if so, what
is the danger involved?

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