# Homework Help: Ring of radius R and uniform charge

1. May 17, 2012

### arl146

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

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

3. 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 dont 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 im not

2. May 17, 2012

### arl146

never mind .. i got it now!