Here is a quote from one of the MIT notes on magnetism : "Note that F(magnetic force) is perpendicular to v and B , and cannot change the particle’s speed v (and thus the kinetic energy). In other words, magnetic force cannot speed up or slow down a charged particle. " How is this possible, it kind of conflicts with the idea of acceleration and "change" in velocity. lets say the velocity of an electron is v in the direction of increasing x axis, while the direction of the B field is along the positive y axis. A force will act on the electron directed towards the negative z axis. The initial velocity was in the x-y plane only but after the force acts on the charge, the "z" component of its velocity changes. Would this not change the total Kinetic Energy possessed by the electron? Clearly the velocity is not the same as before ! Thus the magnetic force HAS changed the velocity. Can anyone help clarify this ?