Area of a Parallel Plate capacitor. Pi*r^2?

[Specimen]

Hi,

I was just going through a question in my Physics textbook today. In calculating the electric field (E=sigma/epsilon0) ... sigma = charge density = q/Area ... they have used the area as pi*r^2, I can't understand why this was done? shouldn't the area of the parallel plate capacitor be length of one electrode * distance between electrodes?

Hope I'm making sense!

 

[jtbell]

No, the area that we use here is the area of one of the electrodes (circular in this case, apparently). The charge resides on the electrodes, not in the space between them.

 

[Specimen]

Hmm, makes sense. Thanks mate!