# Two circular discs spaced apart from a parallel plate capacitor ?

## Homework Statement

Two circular disks spaced 0.50 mm apart from a parallel-plate capacitor. Transferring 1.90 x 10^9 electrons from one disk to the other causes the electric field strength to be 1.90 x 10^5 N/C. What are the diameters of the disks (in cm)?

## Homework Equations

E = Q / (e_o * A)
A = [pie]*r^2

## The Attempt at a Solution

I though I went through the right process for this, but maybe I'm not using the right equations somehow? Here's what I did:

E = Q / (e_o * A) ==> Q = (8.85*10^-12 C^2 / N*m^2) * A * (1.9*10^5 N/C) = 3.04 * 10^-1

A = [Pie]*r^2 ==> 1.81 * 10^-4 = [pie]*r^2 ==> r = 0.00759 and thusly D = 0.1517m which is equal to 15.2 cm. This turned out to be wrong.

Where did I mess up?

gneill
Mentor
Your derivation is a bit confusing; you seem to be mixing in pre-calculated values without showing how you got them. For example, I assume that the value "3.04 * 10^-1" is supposed to be the charge due to the moved electrons. The mantissa looks okay, but the power of ten is way off. Perhaps your value for A is off because of that.