Types of Materials

The macroscopic objects that we interact with every day are made up of atoms, and those atoms are made of positive nuclei surrounded by negative electrons, bound together by their electric attraction to each other. (The reason the electrons don't just fall entirely into the nuclei is a subject for quantum mechanics.) Electrons vary in how tightly they're held onto by their nuclei. Electrons which remain closer to the nuclei are very tightly bound, and it's hard to separate them. The outermost electrons, called valence electrons, are the easiest to remove, and in fact the sharing and exchange of valence electrons is what underlies chemical reactions.

In metals, one or more of each atom's electrons are so weakly bonded that they basically break loose, and wander freely throughout the bulk of the metal. That means that metals have a large collection of electrons (sometimes called the "electron sea") sloshing around inside them, free to go anywhere they go. These electrons are called charge carriers.

The presence of charge carriers will affect the way a material will interact with outside charge, as is shown in this figure. When some positive charge is placed on one end of a neutral material without charge carriers, that positive charge will remain where it is placed, and does not spread out. However, when you place the same charge on a neutral metal, the electrons inside rush over to where the charge has been introduced, attracted to the positive charge, and thus making that end of the material less positive. However, the other end of the rod has become more positive, because electrons have abandoned it. The net result is that the entire material becomes positively charged. Charge carriers carry the positive charge from one end of the block to the other.

Materials without charge carriers are called insulators (or electrical insulators to be more specific), and include materials like dry wood, rubber, some plastics, paper, and so forth. Conductors are materials that contain charge carriers, and are so called because they conduct electricity. Metals are the most common example, but not the only one. When sodium chloride (NaCl) is dissolved in water, it breaks apart into separate ions $\mathrm Na^+$ and $\mathrm Cl^-$, which can act as charge carriers. The same is true for other ionic compounds, and so water that contains any type of salt (which is almost all kinds of water you run into on a regular basis) is a conductor. Pure distilled water, however, is an insulator.

For most practical purposes, it doesn't matter whether a material has positive or negative charge carriers; our predictions will work the same either way. Thus in many of our examples we will talk as if our conductors have positive, negative, or both types of charge carriers, whatever is easiest to picture. (One case where the charge carrier sign matters is called the Hall Effect, which will come up later when we discuss magnetism.)

It is possible to have materials which lie somewhere in between conductors and insulators. Our bodies, for example, will conduct electricity, but not nearly as well as a metal wire would. There are certain materials called semiconductors whose conductivity can vary depending on external circumstances, such as temperature, the presence of light, or the direction of an applied voltage.

Is air a conductor or an insulator?

Air is an insulator; if it weren't, then walking next to an electrical socket would be as dangerous as sticking a metal fork into one, and batteries would drain very quickly. However, this may come as a surprise, because you can probably think of a rather dramatic example of charge travelling through air: a bolt of lightning. However, that only occurs when the electric field is strong enough to rip electrons off of the atoms in the air, briefly turning the air into a conductor. This is called electric breakdown, and we'll talk about it in the next section.