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Re: Formula for self C of a Coil (not Medhurst)



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> From: Tesla List <tesla@pupman.com>
> To: tesla@pupman.com
> Subject: Re: Formula for self C of a Coil  (not Medhurst)
> Date: Thursday, April 27, 2000 3:59 AM
> 
> Original Poster: "Robert Jones" <alwynj48@earthlink.net> 
> 
> Hi Jim,
> 
> Thanks for your reply.
> 
> I believe your just the man I need for the formula of the mutual
inductance
> of two helical coils. If so can you send it to me or a link. Thanks in
> advance.
> 
> So I can assume there are  analytically solutions for  cylinder above a
> ground plain.

Nope... no analytical solution that I've seen.  Just an approximation. 
Much like calculating the capacitance between two spheres.  You calculate
the image of one sphere in the other, then the image in the image, etc. 
You repeat this until you've got enough accuracy.


> 
> The Medhurst  formula generates, for a square cylinder with either the
same
> volume or area of sphere, approximately half the C of the sphere. A
cylinder
> with the same diameter of a sphere must have a H/D ration of 2.5 to have
the
> same C as the sphere. I find this  difficult to believe even if the
cylinder
> is a tube. But I know intuition is a poor substitute for field equations
and
> integration.
> 
> I have seen formula for the C of solid cylinders in round and square
> cavities. They may have been derived empirically.

C of a cylinder in a cylinder (i.e. coaxial cable) is fairly easy
analytically, IFF the tubes are infinitely long (i.e. you don't have to
deal with fringing)

C of a cylinder in a square tube might be a bit tougher, but at least it is
essentially a 2D problem, unlike a cylinder over a plane.