Re: Dissectable Capacitor

["Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU>]

On Thu, 4 Mar 1999, John S. Denker wrote:

> At 01:26 PM 3/4/99 -0500, Sam wrote:
> > I'm not sure what you just accomplished with this experiment.
>
> Don't ask me what is accomplished with this experiment!
> I didn't create this device.
> I didn't buy this device.
> I didn't post the questions about this device.
> I did express my doubts about the pedagogic value of this device.

I'm usually a critic of certain demos and activities in physics class, and
question them, asking "What do they really show? Is it something important
enough to justify the time?" etc.

Any demo's value can be compromised if the teacher doesn't have a clear
unerstanding of it, or desn't fully understand it, or tosses it off
without sufficient analysis.

And I must admit sometimes we continue to do something because *we* like
it, or the students seem to like it, without asking these questions about
whether anyone is *learning any physics* from it.

In this demo, on the negative side one may say:

1. The usual presentation makes it seem more like a challenge "trick",
almost like a deception. (But if one takes enough time for discussion this
becomes unimportant.)

2. The sequence of steps is contrived to make the result (the spark after
dissambly) to *seem* unexpected.

3. This specialized capacitor, and its use in this demo, are of no
practical value in technology.

On the positive side:

1. The demo is an old classic, used for many decades, and appears in many
demo books. Surely some people think it must have some value. :-)

2. It challenges students to observe carefully, and to think, in physical
terms, about what they see. (Do they notice when you use an insulating rod
to carry the inner can, ask why you did that, and correctly infer that if
you did it by hand you'd get a nasty shock?)

3. It tests student understanding of capacitors, electric fields, and the
processes of charge transfer.

4. Students can be encouraged to suggest further tests of their
understanding, such as those I noted in my 20+ step description in a
previous post.

5. The demo is great for engaging students in detailed discussion of many
important points of electrostatics, and it motivates students to get
involved in such discussion. (If you do this demo without discussion,
you've wasted it. If you cloud the discussion with talk of dielectricts,
you confuse the students.)

        -- Donald

.....................................................................
Donald E. Simanek
dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek
.....................................................................