Re: Capacitance of a coil

John Couture wrote:

>   Is the capacitance the same for a coil with the same R and L whether it
> has one turn or 1000 turns? I am having difficulty with this. My research
> and limited tests indicates the Medhurst equation and equations that do not
> include the number of turns or length of wire should be used with caution.

Remember that this capacitance is a first-order approximation for a
transmission line effect, and so is valid at DC too. Only the general
shape of the conductive surface is significant.
The same formula probably works for a hollow thin metallic cylinder 
with the lower end close to ground, or for a cylinder made of a wire
mesh. A coil with a reasonable number of turns is an approximation of a
continuous conductive surface with this form. Of course, with one
turn only the formula would give a large error.

(While making the comment above I was imagining what would be the
effect of the distance to ground in the capacitance of a hollow
cylinder with constant voltage to ground. If the idea is correct, 
it must reach a limit when the cylinder touches the ground, close
to the value given by Medhurst's formula, and not go to infinity, 
as it may appear that is what would happen.
Would Terry's simulator be able to verify this?)

Antonio Carlos M. de Queiroz