A hydrogen nucleus has a radius of 1 x 10-15 m and the electron is about 5.4 x 10-11 m from the nucleus. Assume the hydrogen atom is a ball with a radius of about 5.4 x 10-11 m and the nucleus is a ball with a radius of 1 x 10-15 m. How much work (in electron volts) must be performed by an external force to bring in another proton (from very far away) to the "surface" of the nucleus? (Ignore the effects of the electron.) HELP: Find the change in electric potential when proton is moved from very far away to the surface of the nucleus (a distance of 1 x 10-15 m from the proton that is already there). HELP: Now relate this quantity to the work done by the external force. Watch your units and be careful of the sign!