1. The problem statement, all variables and given/known data (I'll be using k=1/4pi*permittivity of free space) Find the net force that the southern hemisphere of a uniformly charged sphere exerts on the northern hemisphere. Express the answer in terms of the radius R and the total charge Q. I tried to show how I attempted the problem, but trying to follow that, especially written like this, may be inconvenient, so if someone could lead me in the right direction that'd cool. The way I tried it may not be correct anyway. 2. Relevant equations This equation is from another problem, it looked useful so: the force per unit area on a charged slab thickness a is: integral from 0 to a of rho times the electric field times dx rho is the volume charge density 3. The attempt at a solution What I tried to do is make a cylindrical column from the north pole to the center. I then used the above formula, and got the force per unit area to be 3kQ^2 / 8piR^4. To get this I integrated Q over the volume of a sphere, which is rho, times the formula for the electric field of a uniformly charged sphere inside the sphere, kQr/R^3. Then, I multiplied by a surface area element in spherical coordinates (with phi as azimuthal angle), r^2 sin phi dphi dtheta. Then I multiplied by cos phi, because I want the upward force component. I integrated this over the top half of the sphere (phi goes from 0 to pi/2, and theta from 0 to 2pi). I obtained 3kQ^2/8R^2, while the book says the right answer is half that! It's from Griffith's Intro to Electrodynamics, problem 2.44 if that helps.