I have two questions. The first concerns a charged disk. Consider the center of the surface of the charged disk, O. Mathematically, the value of E at O comes out to be a non-zero finite value. However i am having difficulty linking this with the physical sense. The disk can be thought to be composed of an infinite number of rings; i think that each ring will set up zero electric field at O since for each charge element on the ring, the diametrically opposite charge element of the charge element cancels its effect. So how come does E come out to be non-zero at O? Please explain the flaw in my reasoning in physical terms, not mathematical. The second question involves two charges. Consider one charge to be fixed, while the other moves past it in a straight line. What fields will be set up? What force, if any, will act on each of the charges? Please help. I'll be highly grateful.