Re: discharging a capacitor

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

On Sat, 6 Mar 1999, John S. Denker wrote:

> At 02:31 PM 3/6/99 -0500, Donald E. Simanek wrote:

[asking for detailed analysis of why a capacitor can be discharged]

Excellent and detailed answer, John! I'd give it an A+

This shows why one seldom gets the answer in an introductory physics
textbook and course. Yet this is a very basic and reasonable question for
a student to ask. Many other such questions, relating to things the
student is expected to use (even the practical things), are never raised
and never answered in such a course. Maybe they ought to be.

> > Students actually ask, and are troubled by this question. Few books even
> > address it. Most physics teachers (and I have tested this) are speechless
> > when asked this question "cold". Or they mumble some totally insufficient
> > "answer".
>
> That's understandable.  The analysis seems to require rescinding two
> beloved simplifying assumptions.  That's hard for people to do.

Again, you've put your finger on it precisely. And maybe we ought more
often to question those assumptions, not to make a huge deal about it, but
just to remind ourselves, and students, that such assumptions, if taken
seriously, could lead to absurd results, such as, in this case, to
conclude that you can't discharge a capacitor.

Other such assumptions:

Frictionless surfaces and bearings.

Rigid (incompressible) bodies. Leads to absurdities when you examine a
collision in detail.

Thin lenses, and their equation.

Perhaps one way to deal with some of these in class is to consider an
example where the absurd conclusions arise.

There are other questions which stump some teachers:

Why can the transmission of light (intensity) be greater through
oil-soaked paper than through the same paper dry? Pioneer settlers in the
midwest were said to use oil or lard-soaked paper sheets as windows, they
shed rain, blocked the wind, but also transmitted light.

Why does warm air rise, rather than hotter molecules simply diffusing
outward and, through collisions, transmitting energy to other molecules
outward from a hot object in all directions equally? I.e., why do more
of the faster molecules choose to move upward, and more of the slower ones
choose to move downward. A kinetic model answer is wanted. To simply say
"Warm air is less dense and therefore rises because of Archimedes
principle" is evading the question. Details, please.

If you ask enough questions around here, you often get very good answers,
as John's answer above shows. If you answer any of these, please change
the subject header.

        -- Donald

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