To calculate the work done by a force it is #F*d#. One way of calculating the dot product is to multiply #(F_(par))(d)# where #F_(par)# is the component of the force that is parallel to the direction the object is moving. For magnetic forces acting on moving charges, the force is ALWAYS perpendicular to the velocity (the direction of motion), so there is NEVER any component of the force parallel to the direction of motion. This makes the work done by the magnetic force on a moving charged particle always zero.