We began our investigation of electricity in Chapter 22 by looking at the results of simple experiments with charged rods. We’ll do the same with magnetism.
What do these experiments tell us?
First, magnetism is not the same as electricity. Magnetic poles and electric charges share some similar behavior, but they are not the same.
Magnetism is a long-range force. Paper clips leap upward to a magnet.
Magnets have two poles, called north and south poles, and thus are magnetic dipoles. Two like poles exert repulsive forces on each other; two opposite poles attract. The behavior is analogous to electric charges, but, as noted, magnetic poles and electric charges are not the same. Unlike charges, isolated north or south poles do not exist.
The poles of a bar magnet can be identified by using it as a compass. The poles of other magnets can be identified by testing them against a bar magnet. A pole that attracts a known north pole and repels a known south pole must be a south magnetic pole.
Materials that are attracted to a magnet are called magnetic materials. The most common magnetic material is iron. Magnetic materials are attracted to both poles of a magnet. This attraction is analogous to how neutral objects are attracted to both positively and negatively charged rods by the polarization force. The difference is that all neutral objects are attracted to a charged rod whereas only a few materials are attracted to a magnet.
Our goal is to develop a theory of magnetism to explain these observations.