Ring charge propelling a sphere

[zyphriss2]

A 13.4 cm radius thin ring carries a uniformly distributed 14.8 microC charge. A small 8.2 g sphere with a charge of 2.4 microC is placed exactly at the center of the ring and given a very small push so it moves along the ring axis (+ x axis).

How fast will the sphere be moving when it is 1.8 m from the center of the ring (ignore gravity)?





I know i can treat the ring like a point charge so e=kq/r
and (1/2)mv^2=e


I just think I am doing everything after the e=kq/r step completely wrong. My last answer was 4248m/s.

 

[chrisk]

Treat the sphere as a point charge then determine the electric field produced by the ring as a function of x.

 

[lanedance]

the symmetry of the ring along the x-axis should greatly simplify things