Electric Field at a Point on the Axis of a Charged Disk?

[smballer23]

Homework Statement

A flat, circular annulus of radius 7 m with a hole in the center of radius 2 m is charged on one surface such that the surface charge density varies with the radial distance from the center of the disk by the equation sigma= (r2 + 961)1/2 x 10-6, where is in C/m2 and r is in meters.

Find the magnitude of the electric field at a point on the axis of the disk, a distance 31 m away from the center of the disk.

Homework Equations

The Attempt at a Solution

Do I combine the equations and integrate from 2 to 7? I am lost...

 

[gneill]

You write: "sigma= (r2 + 961)1/2 x 10-6". Please clarify. Is that r2 or r²? Is 961 in units of meters or meters²? Is that (r2 + 961) x 1/(2 x 10^-6), or is it (r2 + 961) x (1/2) x 2 x 10 - 6? Or something else?

 

[smballer23]

r2 is r squared. 961 is meters.
It is (sqrt(r squared + 961))e-6.