Magnetic Field and Magnetic Forces
Everybody uses magnetic forces. Without them there would be no electric motors, TV picture tubes, microwave ovens, loudspeakers, computer printers, or disk drives. The most familiar aspects of magnetism are those associated with permanent magnets, which attract un magnetized iron objects and can also attract or repel other magnets. A compass needle aligning itself with the earth’s magnetism is an example of this interaction. But the fundamental nature of magnetism is the interaction of moving electric charges. Unlike electric forces, which act on electric charges whether they are moving or not, magnetic forces act only on moving charges.
As we did for the electric force, we will describe magnetic forces using the concept of a field. A magnetic field is established by a permanent magnet, by an electric current in a conductor, or by other moving charges. This magnetic field, in turn, exerts forces on other moving charges and current-carrying conductors. In this chapter we study the magnetic forces and torques exerted on moving charges and currents by magnetic fields. In Chapter 29 we’ll see how to calculate the magnetic fields produced by particular configurations of moving charges andcurrents.