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Re: Formula for self C of a Coil (not Medhurst)
Medhurst is an empirical formula determined by a tabular fit to measured
data, and as such, is entirely composed of fiddle factors.
I note that if you try to analytically calculate the capacitance of a
cylinder above a ground plane, you get results very similar to Medhurst.
There are several ways to look at the integration problem, for instance,
consider it as concentric cylinders where the space between the cylinders
varies as a linear function of distance along the cylinder. However, the
integration gets real evil real fast. Numerical methods will come up with
results similar to Medhurst.
The field around a cylinder is sufficiently different from that of a sphere
that a discrepancy between the two isn't surprising. For instance, if the
cylinder is long and skinny (a sort of limiting case), the field drops off
as 1/log(r) (I think) as you move away. The field drops off as 1/r for a
sphere.
Check the archives.. there was some discussion about this about 2 years
ago.
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> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Formula for self C of a Coil (not Medhurst)
> Date: Tuesday, April 25, 2000 1:25 AM
>
> Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
>
> Hi all,
>
> Does anybody have the formula or a link for the capacitance of an
isolated
> cylinder that is not due to Medhurst?
>
> My suspicion is that the Medhurst formula is incorrect or it contains a
> fiddle
> factor. If you compare the values it generates to the C of spheres it
> appears
> to underestimate C by at least a factor of 2.
>
> I believe the formula actual produces a value that when used with the
> inductance of a coil correctly calculates the resonance frequency but
it
> is not the self C of the coil. Its just an imperical relation to
calculate
> Fr.
> I have assumed the formula in wintesla is an accurate Medhurst formula.
>
> Thanks in advance.
>
> Regards Bob
>
>
>
>
>
>
>