Capacitance, parallel plates, electric field

[Chronos000]

Homework Statement

To find the capacitance of parallel plates, I am first finding the E field using gauss. However I don't know what the gaussian surface will be. I don't think it will be a gaussian pillbox as the plates aren't infinite, but will it be a sphere??

On a side note, if I have two concentric spheres with radii a<b with charges +Q and -Q, what are the limits of the potential: My thoughts were: b is final, a is initial

 

[Berko]

Assume the plates are infinite if the plates are very close together. Ignore fringe effects.

 

[Chronos000]

in that case, do you know if the area quoted in the capacitance for parallel plates is the area of a circle, and not that of a square/rectangle?

( C= A*epsilon/d )

 

[Berko]

Doesn't matter. It's just the area of your pillbox. It'll cancel.