Conductors and Insulators

Suppose that we were to electrically charge two isolated metal spheres: one with a positive charge, and the other with an equal negative charge. We could then perform a number of simple experiments. For instance, we could connect the spheres together using a length of string. In this case, we would find that the charges residing on the two spheres were unaffected. Next, we could connect the spheres using a copper wire. In this case, we would find that there was no charge remaining on either sphere. Further investigation would reveal that charge must have flowed through the wire, from one sphere to the other, such that the positive charge on the first sphere completely canceled the negative charge on the second, leaving zero charge on either sphere. Substances can be classified into two main groups, depending on whether they allow the free flow of electric charge. Conductors allow charge to pass freely through them, whereas insulators do not. Obviously, string is an insulator, and copper is a conductor. As a general rule, substances which are good conductors of heat are also good conductors of electricity. Thus, all metals are conductors, whereas air, (pure) water, plastics, glasses, and ceramics are insulators. Incidentally, the distinction between conductors and insulators was first made by the English scientist Stephen Gray in 1729.

Metals are good conductors (both of heat and electricity) because at least one electron per atom is free: i.e., it is not tied to any particular atom, but is, instead, able to move freely throughout the metal. In good insulators, such as glass, all of the electrons are tightly bound to atoms (which are fixed), and so there are no free electrons.