Figure (a) shows a circular disk that is uniformly charged. The central z axis is perpendicular to the disk face, with the origin at the disk. Figure (b) gives the magnitude of the electric field along that axis in terms of the maximum magnitude Em at the disk surface. The z axis scale is set by z = 21.0 cm. What is the radius of the disk? (a) Number Units Em 0.5Em 0 z (cm) (b)