Ring of Charge--how to maximize the Electric Field A thin nonconducting rod with a uniform distribution of positive charge Q is bent into a circle of radius R. The central perpendicular axis through the ring is a z-axis, with the origin at the center of the ring. (c) In terms of R, at what positive value of z is that magnitude maximum (of the Electric Field)? After going through the integration, the equation for a ring of charge is k*(Qz / (z2+R2)3/2)) Edit: So, with some help, I realized that to maximize the function, I'd have to differentiate... But what am I differentiating with respect to? z? r? And I know k is a constant; are everything else non-constants?