In class, we had demonstrated the calculation of magnetic field at the center of a spherical shell with uniform surface charge density p _ s rotating with an angular velocity W = wz ( hat ) , using the Biot - Savart equation. Now let ' s extend this problem by further finding the following quantities. ( a ) Total current I flowing on the surface of the sphere. * Note: now the line current density K is measured in A / m rather than A / m ^ 2 . ( b ) Total magnetic moment m of the system