2-15. A spherical charge distribution has a volume charge density which is a function only of r, the distance from the center of the distribution. In other p(r). If p(r) is as given below, determine the electric field as a words, p = function of r. Integrate the result to obtain an expression for the electro- static potential U(r), subject to the restriction that U(0): = 0. (a) p = A/r with A a constant for 02r2 R; p = 0 for r > R. (b) po (i.e., constant) for 0 2 r > R; p = 0 for r> R. 2-15. (a) ) (A/ t0){R - ) for U AR /2eor for (Pu/2ec)(R? R* pc/3eor for r2 R (b) U 3r) for r < R --