I'm not quite getting the following problem: A thin disk of dielectric material, having a total charge +Q uniformly distributed over its surface, and having radius a, rotates n times per second about an axis through the center of the disk and perpendicular to the disk. Show that the magnetic field produced at the center of the disk is unQ/a. I know that B at the center of a wire ring is uI/2r. If I start here then dB = udI/(2dr). I = dq/dt. In this case I = nQ/sec, so can the equation become db = undQ/(2dr)? I'm sure this isn't right but here goes. Q/(pi)a^2 = dq/2(pi)rdr so dq = 2Qrdr/a^2 ? If I plug this into my equation the dr's cancel, so I must be doing something wrong? Thanks in advance for the help.