This one deals with the motion of charged particles in a magnetic field. The magnetic field is going directly into the plane of paper. The charged particles start with an initial velocity in the x,y plane. According the right-hand-rule for the Lorentz force, they will describe a circular trajectory in the x-y plane. The radius of the circle depends on the mass, initial velocity and charge of the particle. Red charges are positive (going anticlockwise). Black charges are negative (going clockwise). For simplicity’s sake, I have ignored the effects of the electric field generated by the charged particles themselves.