Exercise 23.43 shows that, outside a spherical shell with uniform surface charge, the potential is the same as if all the charge were concentrated into a point charge at the center of the sphere.
(a) Use this result to show that for two uniformly charged insulating shells, the force they exert on each other and their mutual electrical energy are the same as if all the charge were concentrated at their centers.
(b) Does this same result hold for solid insulating spheres, with charge distributed uniformly throughout their volume?
(c) Does this same result hold for the force between two charged conducting shells? Between two charged solid conductors? Explain.