In my college physics class, we are working on electrostatics. Our homework problems relate to Coulomb's law, but I don't know how to manipulate it to solve them. Here are some examples: 1)Excess electrons are placed on a small lead sphere of mass 20 g so that its net charge is -2.60x10^-9 C. Find the number of excess electrons on the sphere. 2) How many excess electrons are there per lead atom? The atomic number of lead is 82, and the atomic mass is 207 g/mol. Any help/explanations would be greatly appreciated!