The Electric Field Due to a Charged Disk

brcole

1. The problem statement, all variables and given/known data
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?


2. Relevant equations
E=σ/2ε(1-z/√(z^2+R^2 ))


3. 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 ...".