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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, e²r_e/e₀ £ m_ec² and e²r_p/e₀ £ m_pc² Þ r_e/m_e = r_p/m_p Þ r_e/r_p = m_e/m_p.