A nonconducting circular disk, of radius R, carries a uniformly distributed electric charge Q. The plate is set spinning with angular velocity w about an axis perpendicular to the plate through its center (Fig. 26-41). Determine (a) its magnetic dipole moment and (b) the magnetic field at points on its axis a distance x from its center; (c) does Eq. 26-7b apply in this case for x>>R?