Chapter 6, Problem 86AP

Two non-conducting spheres of radii $R_1$ and $R_2$ are uniformly charged with charge densities $p_1$ and $p_2$ , respectively. They are separated at center-to-center distance a (see below). Find the electric field at point P located

at a distance r from the center of sphere 1 and is in the direction $\theta$ from the line joining the two spheres assuming their charge densities are not affected by the presence of the other sphere. (Hint: Work one sphere at a time and use the superposition principle.)