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


----- Original Message ----- From: "Rauber, Joel" <Joel.Rauber@SDSTATE.EDU>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, October 03, 2006 2:04 PM
Subject: Re: [Phys-l] teaching energy



This is perhaps getting to close to arguing semantics, but here it goes
none-the-less

| The idea of putting energy into a system, is certainly valid,
| but this is very abstract and is not understood well by
| beginning students.

Actually I find and found as a student, and witness my students finding
the field concept to be quite abstract, more so than a system of
interacting particles model.

My students tend to think of a fields as operating "like springs", which makes for a very concrete model. The fact that the "springs" cannot be seen is beside the point and seems not to cause them any problem.

|The field does not have to be
| mathematically defined, but can be invoked as the connection
| between objects.

If its not defined, than it is going to be exceedingly abstract and one
might as well talk about an interaction between particles.

Mathematics tends to be abstract in itself. For the purposes of visualization of energy transfers/storage (qualitative) the lack of math is an advantage and removes abstraction.

|It becomes the "container" in which the
| energy resides. Students have not trouble with understanding
| that energy is in a spring or rubber band.
|

They have no trouble with springs, as they are actual objects. Though
one would more properly refer to a wall and mass interacting through the
mechanism of the spring. The spring by itself in some sense is a poor
container for the energy as well, because it always requires something
else in order for it to store PE (not worrying about gauge invariance
here, i.e. lets agree that a relaxed spring has zero PE). There has to
be two something elses interacting via the spring.

Well, I can think of at least one case where that wouldn't be true... A fairly massive spring being hit by a single moving object could certainly have enough inertia to "absorb" some energy in the collision. All that is really required is for a force to act on the spring. Obviously the usual case would require something to prevent the spring from accelerating so that a force may be applied over time (and increase during that time if the spring obeys Hooke's Law). The "field as spring" model, as all models, must simply be USEFUL, it doesn't have to rigorously correct in all respects, nor does it have to include all the nuances that can be dragged in.

<snip>

The field idea is that we may ignore particle 2, view particle one as
producing a field. No reference to particle 2! If we care about what
happens to particle 2 we may then, later, calculate the effects that the
field has on particle 2.

Yes, this IS a weakness, but one not obvious to beginning students, imo. Nor is it a weakness that is otherwise absent - see below.

I.e. is the field implies no particle system.

And yet there are situations that routinely come up in problems wherein a field is specified without referencing a specific source/particle(s). So we DO refer to fields in that manner at times - Or the text problems do.

Consider three point charges at the corners of a triangle, do you think
a student (or even a professor) will have an easier time calculating the
PE using a field concept or an interaction concept?

One uses the approach which is most convenient when solving ANY problem (assuming multiple valid approaches), no? That a specific case favors one approach over the other does not mean that one approach is inherently "better". And I think we're losing sight of the reason for using field as a "container" for beginning students. It isn't the calculations that are the primary focus, imo. It is the convenience of a mental model that facilitates an understanding of energy transfers. It's hard to get a feeling for energy transfers if you cannot visualize energy LOCATIONS.