I'm not sure why the answer to the following question would be zero, so far, I'm thinking because work is force times distance and in a circle, there is no distance after a complete orbit. But I have a strange doubt that it might be something else. A magnetic field of .1T forces a proton beam of 1.5 mA to move in a circle of radius .1 m. The plane of the circle is perpendicular to the magnetic field. Of the following which is the best estimate of the work done by the magnetic field on the protons during one complete orbit of the circle? What I have a real problem with is how to solve the second part: Of the following, which is the best estimate of the speed of a proton in the beam as it moves in the circle? Could someone at least give me a formula or something to start with?