1. The problem statement, all variables and given/known data A beam of protons with v=1.8x10^8 are deflected 90 degrees by a constant magnetic field. The trajectory of the beam has a circular arc with a radius of 4m. I am asked to find the magnitude of the constant magnetic field. 2. Relevant equations I know that r=mv/eB. When solving for B the equation is B=mv/er. Right? m=1.67x10-27 v=1.8x108 e=1.6x10-19 r=4 3. The attempt at a solution This gives me 7.515 for the magnitude of the magnetic field. The correct answer is 0.47T. Did I start with the wrong equation?