1. The problem statement, all variables and given/known data In a certain particle accelerator, a current of 300μA is carried by a 4.00-MeV proton beam that has a radius of 1.30 mm. The mass of a proton is 1.67 x 10-27kg. If the beam hits a target, how many protons hit the target in 3s? 2. Relevant equations The only equation that even remotely comes to mind regarding this problem is I=e2n(tau)AE/me I do not know which variable has the electron volt unit, or an equation involving current and time. 3. The attempt at a solution I don't even know where to begin as I don't know a suitable formula to solve the problem. Perhaps if I knew that, I could get somewhere. --Also, for personal curiosity-- In the formula for current that I listed, what exactly is tau? Also, is n the number of particles, or the number density? And is E the electric field? My book didn't exactly break the formula down as well as I'd like.