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The electrostatic potential energy of a point charge in a spherical electric field is e2/(4pe0r); the total over that entire spherical surface is [e2/(4pe0r)] x 4pr2 Þ e2r/e0. This total electrostatic potential energy must be less than or equal to the rest energy of the particle, mc2. Thus, for the electron and proton, e2re/e0 mec2 and e2rp/e0 mpc2 Þ re/me = rp/mp Þ re/rp = me/mp.