1. The problem statement, all variables and given/known data A 2.40 -mm-diameter glass sphere has a charge of + 1.70 nC. What speed does an electron need to orbit the sphere 1.40 mm above the surface? 2. Relevant equations Electric Field of Sphere: E=k*Q/(r^2) q*E=M*v^2/r 3. The attempt at a solution It seems pretty straight forward but I must be doing something wrong with the r values. Could someone tell me what the radius is considered. For the electric field is it the radius of sphere + the distance away or what and same for the other equation. Ive tried pretty much all the combos I can think of and still am getting it wrong so not sure where Im going wrong.