1. The problem statement, all variables and given/known data Of the charge Q initially on a tiny sphere, a portion q is to be transferred to a second, nearby sphere. Both spheres can be treated as particles. For what value of q/Q will the electrostatic force between the two spheres be maximized? 2. Relevant equations (k)(q1q2)/r^2 3. The attempt at a solution I posted a similar problem concerning an electric field some time ago, and am attempting to use the same solution, that is, taking the derivative of the equation I end up with, but the derivative ends up being zero for me. I end up with: k(Q-q)q / r^2 And then after factoring out k/r^2, I'm left with just (Q-q)q. I tried doing a derivation using the product rule, but I just end up with zero. When I try to derive this, I get: 1 * (1-1) + (Q-q) which just leaves me with Q - q, which is not the answer. What am I doing wrong?