So, I have an apparent paradox: Consider a particle moving with constant velocity, v, along the x-axis. It now enters a constant magnetic field with is parallel to the y-axis. So there is a force, F=q(vxB) which acts on it, and points in the positive z-direction at first, but then as the charge is accelerated, the force continues to rotate causing the point to moving along a circle in the planes parallel to the yz-plane, but it still moves at a constant speed in the x direction because no force acts on it in either the +x or -x direction. So, the charge has been accelerated (and it now has velocity components moving in SHM (circles) in the y and z directions) meaning that its KE must have increased, since the x-component of velocity is unchanged... what could have done this work to cause the change in KE if the force can do no work since it is always perpendicular to the displacement? We know this because F is perpendicular to v, and v points in the direction of an infinitesimal displacement.... Seems like a paradox to me!