The force of magnetism arises due to moving charges. The equation that determines the force upon a particle of charge q and velocity v is F = qv x B. Note that v and B (magnetic field) are vectors and the equation shows a vector product. If an electron is travelling horizontally in the positive x-direction and the magnetic field B is pointing downwards in the negative y-direction, the force will be perpendicular to both v and B, and point into the z-direction.