Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] teaching energy

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
| <
<> >

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

I have to go to class, now so I can't write more;

Any comments, discussion?

Joel R

________________________ Joel Rauber Department of Physics - SDSU 605-688-4293
Forum for Physics Educators