Let`s say you have a "magnetic field" (call it B) and let`s say it has a z direction, all over the space. Now let's kick an electron into the scene perpendicular to z. It will feel a force F=e v B to the axis z. Given a velocity v = v0 this won't change. We have an electron running in a circle for eternity and the energy of the particle is conserved! But we didn't applied the full set of Maxwell equations. There is an interaction, otherwise the electron would follow a straight line. The electron will radiate and get the energy from the magnetic field. Is this correct?