1. The problem statement, all variables and given/known data Evaluate ∫∫r.ndS where r=(x,y,z) and n is a normal unit vector to the surface S, which is a sphere of radius a centred on the origin. 2. The attempt at a solution I decided to use polar coordinates. The radius of the sphere is clearly constant, a. So a surface element is dS = a2dθdø. r = a(sinθcosø, sinθsinø, cosθ) n = (sinθcosø, sinθsinø, cosθ) r.n = a therefore, ∫∫r.ndS = ∫∫a3dθdø where θ varies from 0 to 2π, and ø varies from 0 to π. This gives an answer of 2π2a3. Is this correct? I'm not so sure.