Ring of radius R and uniform charge

[arl146]

Homework Statement

At what distance along the central axis of a ring of radius R = 0.200 m and uniform charge is the magnitude of the electric field due to the ring's charge maximum? What is the positive solution for z?

Homework Equations

E = \frac{kqz}{(z^2+R^2)^(3/2)}

The Attempt at a Solution

I know I should differentiate that above equation with respect to z and then set it equal to 0 to get z but i just don't know how to differentiate that ..

E = \frac{kqz}{(z^2+R^2)^(3/2)}

the k and the q are held as constants and can be taken out of the differentiation ..

= kq*[\frac{d}{dz}((z²+R²)^3/2]

isn't \frac{d}{dz}((z²+R²)^3/2 = -3z(z²+R²)^(-5/2) ??

so it would be kq [-3z(z²+R²)^(-5/2)]

but that doesn't work for when you set it = 0. what am i supposed to be doing that I am not

 

[arl146]

never mind .. i got it now!