In Fig. 22-41, two curved plastic rods, one of charge +q and the other of charge -q, form a circle of radius R = 3.28 cm in an xy plane. The x axis passes through both of the connecting points, and the charge is distributed uniformly on both rods. If q = 11.8 pC, what is the magnitude of the electric field produced at P, the center of the circle? The Figure is simply an x and y axis, with a circle being drawn on it, the top half of the circle being + and the Lower half being -. The equations that I am given is E = (q x z)/ (4pi x permitivity constant)(z squared + R squared) ^ 3/2 where q is the charge, z is the vertical component height point P is above the circle, and R is the radius of the circle. For this problem, I am unsure how to use this equation. They are asking me to find the charge in the middle of the circle, therefore there is no vertical component (z), which would give me an answer of 0, which is wrong. So I am unsure of how to go about solving this, or if there is another equation that I am missing or something.