On Sat, 31 Jul 2004, David VanHorn wrote:

> Ok, here's a puzzler.
>
> How do you calculate capacitance between two rods, or a rod and a plate,
> where the separation distance is relatively large?

While working on a somewhat similar problem to the one you describe, I
came across Chalmers Butler's paper 'Capacitance of a finite-length
conducting cylindrical tube':

http://content.aip.org/JAPIAU/v51/i11/5607_1.html

You'll either have to buy this article or go to a University Library and
look it up there.

My objective was to validate some models I was creating for FastCap
(http://www.fastfieldsolvers.com/  - actually, the original source is
from MIT, but the one here has unix and windows versions available for
free downloading).

Due to the complexity of these types of problems, I wouldn't trust any of
the analytic solutions except for the ones that have very simple
geometries. Or, I'd trust them perhaps if I didn't want to accurate of an
answer. (Or perhaps you could glean some kind of trend from an analytic
solution.) In your case, the distance of the wire/bar from the plate is
relatively large. Simple analytic solutions for these types of geometries
typically assume that the distance is small, or equivalently, the plate
and wire extend indefinitely. When the space between conductors is on the
order of the size of the conductors, then the problem is very difficult to
solve in close form.

I'm not sure what your goal is here, but you may wish to consider a field
solver like FastCap.

Scott

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics