# Capacitance between two coplanar electrodes

## Homework Statement

I have two coplanar electrodes (180 deg between them) and one of them is under voltage causing circular electric field between them. The target is to calculate the capacitance.

## Homework Equations

Using the capacitance equation C = $$\frac{\epsilon_{0}A}{l}$$
I say that A = bdx and l = $$\pi$$r
b - is out of plane length of the electrodes

## The Attempt at a Solution

I'm integrating the equation from R1 to R2 (coordinates of electrode on x axis)
but I have to substitute dx with something else - x = rCos$$\theta$$ so that
dx = cos($$\theta$$)dr

so the solution is C= $$\frac{\epsilon_{0}bCos\theta}{\pi}ln\left(\frac{R2}{R1}\right)$$

Am I doing it rigth?