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# Re: [Phys-l] teaching energy

"The analogy to the rubber band is a visual example: The band changes its geometry as you stretch it, and energy resides in that altered geometry. Allowing the band to return to its "natural" geometry releases the energy."

Not a good example except in the "mind's eye"

Try this experiment:

W/ a high quality very large rubber band Stretch it and hold 'till equilibrium. Then using your lips as a thermometer (hold gently against one) allow the band to relax. If you are releasing it's energy it should warm, right? What does happen?

Rubber makes rather interesting thermodynamic engines, vide Scientific American:
--------------------

Some Delightful Engines Driven By the Heating of Rubber Bands
---------------------
by C. L. Stong
April, 1971
---------------

IN THE 19TH CENTURY the British
physicist James Prescott Joule observed
that if stretched rubber is warmed, it
tends to contract. Having described the
effect, Joule turned to other matters.
About five years ago the phenomenon
attracted the interest of Paul B.
Archibald, who is associated with the
Lawrence Radiation Laboratory of the
University of California. He decided to
put the effect to work in the form of a
heat engine. His project illustrates the essential difference between scientific
and engineering experimentation. The scientist experiments to discover and
confirm the facts of nature's behavior; the engineer seeks to put those facts to
use as economically as possible. Archibald describes his apparatus and
experiments as follows:

"My first Joule-effect engine consists essentially of rubber bands stretched
between the rims of a pair of disks [see Figure 1]. The assembly resembles a
squirrel cage, except that the disks are canted with respect to each other at .53
an angle of about eight degrees. Each r: disk turns on its own shaft, but the .......

R. McDermott wrote:

----- Original Message ----- From: "John M Clement" <clement@hal-pc.org>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Thursday, September 28, 2006 4:48 PM
Subject: Re: [Phys-l] teaching energy

One of the important things that needs to be done is that students
need to have a good analogy to be able to make sense of energy. The
basic one which is used in Modeling is the spring or rubber band. So
going from this to make sense of where E_p is located is to say it is
in the connection which acts like a funny rubber band. The system
approach is also used, but the connecting rubber band is a good
analogy, and is a lot less nebulous. In the case of some forces you
can readily see a strengthening of the field which makes one
comfortable with locating the energy in the field. However in the
case of gravitational the "field" decreases with increasing energy.

Actually, the STRENGTH of the field decreases, and the strength of the field does not have to equate with its energy. Taking the Modeling model of energy one step further, and incorporating the model of fiwld lines, I would argue that energy within a field results in an alteration of the field geometry, and it is THERE that the energy "resides". Allowing the geometry to revert to its orignal condition would transfer that energy to some object. The analogy to the rubber band is a visual example: The band changes its geometry as you stretch it, and energy resides in that altered geometry. Allowing the band to return to its "natural" geometry releases the energy.

In either case putting the energy in the field makes a good
visualization of "where" it is located.

And this is the primary focus for energy in Modeling - WHERE is the energy, and what changes occur when there is an alteration in the amount of energy.

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