on a charged particle which is moving with velocity
in a magnetic field
is given by :
Since, the velocity and magnetic fields are perpendicular, the particle experiences a constant force, perpendicular to B and v. There is a constant acceleration in x-y plane always perpendicular to the velocity. Hence the charged particle moves in a circular path of a fixed radius and center.
If the charged particle has a velocity component in z direction also, then it will move in a helical path.