1. The problem statement, all variables and given/known data Protons move in a circle of radius 7.70cm in a 0.536 T magnetic field. What value of electric field could make their paths straight? 2. Relevant equations Unsure 3. The attempt at a solution r = 0.0770 m B = 0.536 T m = 1.6*10^-27 kg q = 1.6*10^-19 C E = F / q I'm not sure how to get force without a velocity.