1. The problem statement, all variables and given/known data beam of protons moving in +x direction at speed v electric field is perpendicular to magnetic field magnetic field in +y direction no deflection What direction/sign has the electric field? 2. Relevant equations not looking for numerical solutions 3. The attempt at a solution There's no deflection, the magnetic and electric force have same magnitude, but opposing directions, correct? The magnetic force is perpendicular to the magnetic field, while the electric force is in the direction of the electric field. How do I then find the direction of the electric field? +x and -x are perpendicular to +y. Let's say the magnetic force has a magnitude of 5, the electric force would be -5. Is the direction of the magnetic force -x, i.e. magnitude of -5 and thus the electric force a direction of +x, i.e. magnitude of 5?