Halliday, Resnick & Walker — Capacitance: Problem 15

15 Assume that a stationary electron is a point of charge. What is the energy density u of its electric field at radial distances (a) r = 1.00 mm, (b) r = 1.00 μm, (c) r = 1.00 nm, (d) r = 1.00 pm, and (e) r = 1.00 fm? (f) What is u in the limit as r → 0?