1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution I've been reading up on surface integrals for several hours now but I can't get my head around this at all. There are just too many things flying around. Do I project dS onto a single plane and work it out as dA = dS/n.k? (where n is unit normal vector of surface and k is unit vector in z direction). This was my original plan but when I go through the steps I end up with a.dS = 0. Or is it a Stoke's Theorem thing? Or Divergence theorem? I thought I had these things nailed last night, but now I'm quite overwhelmed. Thanks for any pointers.