Re: [Phys-l] teaching energy

["R. McDermott" <rmcder@gmail.com>]

No, I didn't.  I get that the thermodynamic properties of a rubber band are 
somewhat unusual.  I'll grant that the temperature drops where one might 
expect it to rise.  However, you ARE still resisting/arresting the motion of 
the band as it relaxes, AND you do not know (intuitively) how the internal 
structure of the band impacts on its internal energy.  For a high school 
student, releasing a rubber band, or a spring, causes another object to 
acquire kinetic energy.  This they have observed previously (almost all of 
them).  Relating energy "stored" in the band to energy transferred to an 
object is therefore quite straightforward for them.  Introducing 
thermodynamics is, to say the least, impractical at this level.  ALL models 
have drawbacks in that they approximate "reality".  Models need to be 
revised as new data is uncovered.  That does not mean that a model isn't 
useful or utilitarian.  My purpose is not to teach grad students the 
intricacies of quantum mechanics, relativistic motion, thermodynamics, etc. 
It is to provide BEGINNING physics students, many of them well below the 
ability levels found in introductory college physics course, let alone 
graduate programs, with a consistent framework for understanding BASIC 
physical principles while setting up a mechanism that allows them to modify 
their understandings based upon new data.  For our (high school physics 
teachers), rubber bands work just fine as an analogy for geometric changes 
that result in either storage or transfer of energy.  When the tiny handful 
of them that get to thermodynamics run into this complication with rubber 
bands, several years from now, I'm pretty sure they'll manage to deal with 
it, and it will almost certainly not come up prior to that time.

----- Original Message ----- 
From: "Bernard Cleyet" <bernardcleyet@redshift.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Cc: "Nancy Seese" <nancyseese@redshift.com>
Sent: Saturday, September 30, 2006 12:47 PM
Subject: Re: [Phys-l] teaching energy


> Did you try the experiment?
>
> bc, who certainly knows, not.
>
> p.s. allow means at its "natural rate", i.e. limited only the "spring"
> force and its mass.
>
> R. McDermott wrote:
>
> > ----- Original Message ----- 
> > From: "Bernard Cleyet" <bernardcleyet@redshift.com>
> > To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
> > Sent: Friday, September 29, 2006 11:50 AM
> > Subject: 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?
> >
> > I would expect that if there was no other object interacting with it, yes,
> > but since you are forcing this release to proceed slowly (presumeably by
> > holding it with your hand), it would seem reasonable to me that at least
> > some of the energy in the band would be showing up in your hand(s), thus
> > reducing any observable heating of the band itself.  Not to mention that 
> > the
> > time involved in the release allows energy to be transferred to the air as
> > well.  You would also need to consider whether or not the energy is
> > sufficient to register on your lip as heat in the FIRST place.  Is short,
> > I'm not sure this is a convincing refutation of the model.
> >
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