Re: New capacitor problem

["Donald E. Simanek" <dsimanek@eagle.lhup.edu>]

On Sat, 29 Mar 1997, brian whatcott wrote:

> 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?

Most here know me as a critic of all analogies except the mathematical
analogy. However *this* analogy is, in my view, very misleading and ought
never to be used. Electrical capacity is *not* "How much charge a
capacitor can hold". It is a quotient of charge to potential. The ultimate
limit on what a capacitor can hold is *not* determined by its capacitance,
but by the breakdown properties of its dielectric, charge leakage, etc. 

The water analogy to electrical capacitance would be something like (mass
of water in the bucket divided by level of the water in the bucket), at
least for a cylindrical bucket.

> 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.

It seems to me that analogies such as this one impoverish students'
conceptual understanding, and lead to incorrect conclusions.

>   (Just wait 'til I get going on the helpful role that silly mnemonics
> play on students' competence....)
>
> :-)
>
> brian whatcott <inet@intellisys.net>
> Altus OK 

This is the kind of useful and provocative give-and-take I was hoping to
inspire by posting this problem.  

	-- Donald

......................................................................
Dr. Donald E. Simanek                            Office: 717-893-2079
Prof. of Physics                    Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745          CIS: 73147,2166 
Home page: http://www.lhup.edu/~dsimanek            FAX: 717-893-2047
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