Sorry, I was being very sloppy. I should have said U = kQq/r and V = U/Q => U = QV.
Thanks for the tip about the charge radius, though. Using that for r, does the rest of the problem look good?
Homework Statement
A proton is accelerated through a voltage ∆V with parallel plates towards another stationary proton that is "far away" outside of the proton gun. To get fusion, we want them to get close enough to touch at the surfaces. Find the voltage across the plates that would...