Why do particles spiral around magnetic field lines?
A charged particle moving through a magnetic field experiences a magnetic force (the “Lorentz force”) which is perpendicular to its velocity. This force acts much like the force provided by a string in the case of a ball going around and around on a string: the force changes the direction of the velocity of the charged particle, but not the speed of the particle. The result is that the particle goes around and around in a circle, with the magnetic force acting as a centripetal (towards-the-center) force. If the particle has a component of velocity parallel to the magnetic field line, it will move along the field line, too. The net result is that charged particles spiral around and around magnetic field lines.
