Following exercise: There is a one dimensional grid of infinite size and it consists of anions and cations (each with a charge of 1e / -1e) like this: acacacac The distance between the anions and cations is 5*10^-10m. Now I am supposed to calculate the electrial potential energy of one cation alone. I know that, if I only have two charges, the energy would come from [tex]W = 1/(4\pi \epsilon_0) * (q_1 * q_2)/r[/tex] But I don't understand how to calculate the energy for a grid with an infinite amount of charges. Can someone point me in the right direction please? I'm not asking for a solution, just for a small hint which will guide me in the correct direction.