1. The problem statement, all variables and given/known data An electron orbits a 3.0 cm diameter metal ball 5.0 mm above the surface. The orbital period of the electron is l2.0 micro seconds. What is the charge on the metal ball? r=1.5cm or .015m R=.02m (.015+.005) Orbit=12*10^-6s 2. Relevant equations E=(kq/r)R which gives electric field k=9*10^9 Maybe F=qE? Is this the only equation? 3. The attempt at a solution (9*10^9*1.6*10^-19/.015)*.02 which gives E. not sure what to do next! I fell like the time is involved with the problem.. but do not know where it takes part and cannot find another formula in my book.