Two parallel uniform charged circular conducting metal disks contains a charge of +Q and -Q on the inner surfaces, and little amounts of charge +q and -q on the outer surfaces. The detail is shown in the diagram. Each plate has a radius R and thickness t. The separation of the plates is d. Question: How much charge q is on the outside surface of the disk. The answer is in terms of Q and without the approach by Gauss's law. Hint: E-field of a uniformly charged disk of radius R a distance z from the center of the disk is as following equation. Please help to solve this problem. It is strange to me that the outer surface carries certain amount of charges.