How Do You Calculate the Diameter of Plates in a Parallel Plate Capacitor?

[tomfrom77]

Homework Statement

A parallel plate capacitor is made of two circular disks. The distance between the disks is very small compared to their diameter. Initially, the disks are electrically neutral. After transferring 3.9x10^9 electrons from one disk to the other, the electric field strength in the space between the plates is 1.0x10^5 N/C. What is the diameter of the plates?

e = 1.6x10^-19C

Homework Equations

The Attempt at a Solution

I was able to calculate the charge on the plates by multiplying the number of electrons by their fundamental unit charge:

3.9x10^9 * 1.6^10-19C = 6.24x10^-10C

However I'm really lost as to what to do next. Any guidance would be much appreciated.

 

[collinsmark]

Can you find or derive an equation that gives the electric field of a uniformly charged plate or plane? An equation relating to a capacitor might help.

If you can't find an existing equation, Gauss' law can help you derive it (given that the problem states "distance between the disks is very small compared to their diameter").