Question

Two 10cm diameter metal disks separated by a 0.63mm thick piece of pyrex glass are charged to a potential difference of 1000V. Dialectric constant for pyrex is K=4.7. What is the surface charge density on the disks?

Answer 1

Parallel-plate capacitor has there fore formula is

C=(ϵ0A)/d
putting valuesC=(8.85*10^-12*pi*.05^2)/.00063
=1.1*10^-10F then Q=CV=1.1*10^-10*1000=1.1*10^-7C 
as
η=Q/Atherefore
(1.1*10^-7)/(pi*.05^2)
=1.4*10^-5C/m^ our answer
hope this helps