Hi, a charged particle moves across a constant magnetic field. the magnetic force on this particle? What would you agree on? a) changes the particles speed b) causes the particle to accelerate c) is in the direction of the particles motion or both a & b. I chose C but my test was marked as incorrect why?? Now, the force exerted on a charged particle by a magnetic field is always perpendicular to its instantaneous direction of motion. Does this mean that the field causes the particle to execute a circular orbit? Suppose that a positive particle of charge and mass moves in a plane perpendicular to a uniform magnetic field . the field points into the plane of the paper. Suppose that the particle moves, in an anti-clockwise manner, with constant speed (remember that the magnetic field cannot do work on the particle, so it cannot affect its speed) in an circular orbit of radius . The magnetic force acting on the particle is of magnitude and, according to the right-hand rule, this force is always directed towards the centre of the orbit.