physics: parallel-plate capacitor??

Question

two closely spaced circular disks form a parallel capacitor. Transferring a 1.5 X 10^9 electrons from one disk to the other causes the electric field strength to be 1.0 X 10^5 N/C. What are the diameters of the disks?

Answer 1

For parallel plate capacitor
E = sigma/epsilon_0
wher sigma = q/A (charge/area) is surface charge density
A = q/sigma and sigma = E*epsilon_0, so
A = q/(E*epsilon_0)
A = Pi*(d/2)^2, so
d =2*sqrt( A/Pi) = 2*sqrt(q/(Pi*E*epsilon_0))

Answer 2

E = sigma/E0

sigma = Q/A = 4Q/((pi)d^2)

Solve for d.