Re: New capacitor problem

[inet@intellisys.net (brian whatcott)]

At 09:32 AM 3/29/97, Donald E. Simanek wrote:
> ....
> 1. A capacitor is charged with 6 microcoulomb, and the potental of the
> capacitor is measured and found to be 3 volt. What is its capacitance?
>
> 2. This same capacitor is now carefully discharged completely. What is its
> capacitance now?
>
> You'd be surprised how many students will answer "zero" to part 2. What is
> the flaw in their thinking? What is incomplete or incorrect about their
> conceptual understanding of capacitance. 
>
> 	-- Donald

It is  quite declasse, I'm sure, to offer water analogies in the didactic 
environment for teaching physics or electronics.

 But try this:

1) A bucket is charged with 8 pounds of water, and the height of
the water level is found to be half way up the bucket.
 What is its water capacity?

2) The bucket is now emptied.
What is its water capacity now?


If people can answer the water problem but not the capacitor question,
it seems to me their teaching has been impoverished by the absense of 
helpful analogy.
  (Just wait 'til I get going on the helpful role that silly mnemonics
play on students' competence....)

:-)

brian whatcott <inet@intellisys.net>
Altus OK