HyperPhysics. ArXiv.org e-Print archive. All Courses. Academic Earth | Online Courses | Academic Video Lectures. Linguistics and Philosophy | 24.00 Problems of Philosophy, Fall 2005 | Lecture Notes. Find, share & upload documents. Get better grades | wePapers. Relativity of Electric and Magnetic Fields. Previous home next Michael Fowler, University of Virginia A Magnetic Puzzle… Suppose we have an infinitely long straight wire, having a charge density of electrons of coulombs per meter, all moving at speed to the right (recall typical speeds are centimeters per minute) and a neutralizing fixed background of positive charge, also of course coulombs per meter. The current in the wire has magnitude (and actually is flowing to the left, since the moving electrons carry negative charge).
Suppose also that a positive charge is outside the wire, a distance from the axis, and this outside charge is moving at the same exact velocity as the electrons in the wire. What force does the positive charge feel? The wire is electrically neutral, since it contains equal densities of positive and negative charges, both uniformly distributed throughout the wire (the illustration above is of course schematic). However, since is moving, it will feel a magnetic force, so the force on the charge is of magnitude.