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Re: New capacitor problem



At 11:16 AM 3/29/97 -0500, Donald Simanek wrote:




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.

But, capacitance _is_ basically determined by geometry, so the bucket
analogy may still have some validity. Where it breaks down is that for the
bucket "capacity" depends on the geometric volume of the bucket, while
capacitance is a parametric length times the permittivity.


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.


I think a closer analogy for a bucket would be the gravitational potential
energy of the water divided by the mass of water. This gives a
"gravitational capacitance" which looks like a parametric length times the
local acceleration of gravity.

George Spagna **********************************************
Department of Physics * *
Randolph-Macon College * "It requires a very unusual mind *
P.O. Box 5005 * to undertake the analysis *
Ashland, VA 23005-5505 * of the obvious." *
* *
phone: (804) 752-7344 * - Alfred North Whitehead *
FAX: (804) 752-4724 * *
e-mail: gspagna@rmc.edu **********************************************
URL: www.rmc.edu/~gspagna/gspagna.html