For the test next Tuesday, in preparation, focus on these problems:
Chapter 20: 1, 8, 25, 33, 38, 44, 58, 63
Chapter 21: 3, 8, 22, 34, 40, 73
Topics include electricity, conduction, capacitors, excess charge and ratios, dipoles, electric fields, comparison of energies and forces, electric potential, movement of charges.
For #73, the Iron atom is overall neutral. The potential energy between the positive charges (proton and nucleus) will push them away from each other. It’s as if the proton is rolling up a hill as it approaches the nucleus. The problem asks for the situation necessary for the proton to just reach the edge of the nucleus. The mathematics is the same as if the charge of the nucleus was concentrated at the very center of the nucleus. It would take much more energy to actually enter the nucleus.
Let me know if you have questions.