Capacitance, parallel plates, electric field

Chronos000
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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
 
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Assume the plates are infinite if the plates are very close together. Ignore fringe effects.
 
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 )
 
Doesn't matter. It's just the area of your pillbox. It'll cancel.
 

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