2. Electrostatic energy: A ball of radius R carries a radially stratied charge density p(r) = k r (where k is a given constant). Calculate the system's electrostatic energy using three different methods: (a) (1 point) Calculate the work required to gradually assemble the charge distribution in innitesimal spherical symmetric layers. (b) (1 point) Use the formula U = f (11' p V, where V is the electrostatic potential. (c) (1 point) Integrate the energy density stored in the electric eld over all space