A solid, insulating sphere of radius $a$ has a uniform charge density $\rho$ and a total charge $Q .$ Concentric with this sphere is an uncharged, conducting hollow sphere whose inner and outer radii are $b$ and $c,$ as shown in Figure $\mathrm{P} 24.57$ . (a) Find the magnitude of the electric field in the regions $r< a, a< r< b, b< r< c,$ and $r>c .$ (b) Deter- mine the induced charge per unit area on the inner and outer surfaces of the hollow sphere.