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.

 

[paranormal]

I would suggest using a cylindrical gaussian surface instead of a sphere for the parallel plate problem. This is because the plates are parallel to each other and a cylindrical surface would enclose the electric field lines between the plates, giving a more accurate calculation of the electric field.

For the concentric spheres problem, the limits of the potential would be a as the initial point and b as the final point. This is because potential decreases with distance from a point charge, so the potential at point a would be larger than the potential at point b. It is important to note that the absolute value of potential is not as significant as the potential difference between two points, which is what ultimately determines the behavior of electric charges.