1. The problem statement, all variables and given/known data an electron is rotating around a proton with a velocity of 2*10^6 m/s on circular path of diameter 5*10^-11 m. what is the magnetic field (B) ? the centripetal force on the electron and the magnetic force should be equal and mv^2/r = Bqv or B= mv/qr. 2. Relevant equations centripetal force F=mv^2/r magnetic force = Bqv.