Originally Posted by **maple_tree** Hi arbolis!
Indeed, you are making a mistake by including the z in your expression to be integrated. We are looking at a surface on the xy plane, and so z=0, diving you the double integral of = -2 rdrdphi which eventually gives -2pia^2, do you get it? |

Thanks, it makes sense. By the way, in post #5 I took the wrong V (disregard this post), hence my surprise as 0 as answer.