The Electric Field Due to a Charged Disk

[brcole]

Homework Statement

A circular plastic disk with radius R = 1.80 cm has a uniformly distributed charge of Q = + 2.15 106 e on one face. A circular ring of width 30 µm is centered on that face, with the center of the ring at radius r = 0.50 cm. What charge is contained within the width of the ring?

Homework Equations

E=σ/2ε(1-z/√(z^2+R^2 ))

The Attempt at a Solution

e=8.85e-12
o= 2.15e6
z=.5 cm
r= 1.8
Ans=3.15 e10

 

[Delphi51]

I don't see it as an electric field question. The question asks "what charge ...".