I need this last answer for my homework, can't figure this one out.
The figure below shows two concentric rings, of radii R and R ' = 2.90R, that lie on the same plane. Point P lies on the central z axis, at distance D = 2.10R from the center of the rings. The smaller ring has uniformly distributed charge +Q. What must be the uniformly distributed charge on the larger ring if the net electric field at point P due to the two rings is to be zero?
answer is [________] x Q
The answer comes from this, but this is using different numbers