Determine the dielectric constant

[jamba88]

Homework Statement

Two flat metal circles of radius r=15 mm are brought a distance d=8.0 mm apart to form a parallel plate capacitor. The surface charge density of one plate is measured to be +1.0*10^-3 microCouloubs/mm^2. The potential difference across the capacitor is measured to be .12 Volts. There is a dielectric between the plates. What is the dielectric constant.

Homework Equations

C=K*(E0)*[A/d]
A=(pi)r^2
Q= CV... maybe

The Attempt at a Solution

I plugged what I had into the equation, but I don't know how to find the capacitance using the surface charge density.
I have
C= K(8.85*10^-12)*[pi*(.015^2)/.008]

 

[rock.freak667]

Find the area of the plate(mm²), then multiply that by the density to give charge.