Re: [Phys-l] teaching energy

["John M Clement" <clement@hal-pc.org>]

I certainly can not defend one point of view over the other as being 
more correct.  But I think that they are trying to get students to 
build a consistent model of energy, so one point of view may be more 
productive than the other.

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.

In either case putting the energy in the field makes a good 
visualization of "where" it is located.  Then when one discusses 
chemical bonds it makes sense that they do not contain energy, unless 
you consider negative energy.  One of the unfortunate things here is 
that the chemists talk about breaking bonds, but a better analogy 
would be to say they lengthen and thin as they absorb energy.  The 
word breaking conjurs up the image of a rubber band breaking with a 
release of kinetic energy.

So when reading some of the Modeling documents it is important to 
understand what they are doing and that the target of the instuction 
is mainly secondary students.  The methods and explanations have come 
from various experiences about how students learn.  But they do not 
have an equivalent of the FCI for energy and momentum, so some of 
these ideas may have to be changed.

After all students first need to build a reasonably good model, even 
if it is not exactly the currently agreed upon one.  Then with more 
experience and further reflection they learn to modify the naive model 
and move towards a more sophisticated on.

John M. Clement
Houston, TX

John M. Clement
Houston, TX

 


> I just made a quick (5 minute) cursory glance at the reference 
> below; it
> appears very interesting, however, I'm bothered a bit by the 
> discussion
> of gravitational PE on page 3.
>
> | 
> | Please read "Making Work Work" at
> | <http://modeling.asu.edu/modeling/MakingWorkWork.pdf
> <http://modeling.asu.edu/modeling/MakingWorkWork.pdf> >
> |
>
> The author makes much of saying saying that we won't say there is a 
> PE
> associated with the system of two masses, since we can't locate the
> energy in either body. Therefore they conclude that it resides in 
> the
> gravitational field.
>
> I fail to see what is wrong with associating PE with the system, the
> distance between the objects being the state variable. They mention 
> this
> interpretation and then reject it.
>
> I suppose they must do the same thing when it comes to discussing PE 
> of
> a system of 2 charges. I.e. the modelers calculate energy in a 
> charge
> configuration by integrating E^2 over a volume, rather than by
> qQ/distance???
>
> I have to go to class, now so I can't write more;
>
> Any comments, discussion?
>
> Joel R
>
>
>
> ________________________ 
> Joel Rauber 
> Department of Physics - SDSU 
>
> Joel.Rauber@sdstate.edu 
> 605-688-4293 
>
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