"An alpha particle is accelerated in the +x direction through a voltage of 1000V. The particle then moves in an undeflected path between two oppositely charged parallel plates in a uniform magnetic field of 50 mT in the +y direction. (a) If the plates are parallel to the xy plane, what is the magnitude of the electric field between them?" Since the particle's path is undeflected, the electric force and the magnetic force on the particle must be equal in magnitude. So Ee = qE = Fm = qvB E = vB However, I don't have v, the velocity. How do I solve this?