My reasoning: The magnetic force on charge q is Fm = qv x B B does not change |v|. Therefore, |Fm| is constant at time t > 0 and Fm is always perpendicular to the direction of movement of charge q. Fm behaves as a centripetal force, and thus the charge moves along the circumference of a circle. Here is my drawing depicting what I think is happening: Now, knowing that an electric charge q either at rest or in motion, experience an electric force Fe in the presence of an electric field E, that is, Fe = qE Then if we have a charge q moving with velocity v in the presence of both an electric field E and a magnetic flux density B, the total force exerted on the charge is therefore F = Fe + Fm = q(E + v x B) which is the Lorentz force equation. I am having trouble using what I have done so far and what I know about the magnetic force and electric force to compute the necessary electric force needed to make charge q move in a straight line. If the charge is moving in a circular path about the xy-plane under the influence of a magnetic field in the positive z direction, then the necessary electric field needed to counteract the circular movement of the charge and make it move in a straight line will be a combination of x and y coordinates, correct?