Let us enter a laboratory where we can make observations of electric phenomena. The major tools in the lab are plastic, glass, and metal rods; pieces of wool and silk; and small metal spheres on wood stands. Let’s see what we can learn with these tools.

No forces were observed in Experiment 1. We will say that the original objects are neutral. Rubbing the rods (Experiments 2 and 3) somehow causes forces to be exerted between them. We will call the rubbing process charging and say that a rubbed rod is charged. For now, these are simply descriptive terms. The terms don’t tell us anything about the process itself.

Experiment 2 shows that there is a long-range repulsive force, requiring no contact, between two identical objects that have been charged in the same way. Furthermore, Experiment 4 shows that the force between two charged objects depends on the distance between them. This is the first long-range force we’ve encountered since gravity was introduced in Chapter 5. It is also the first time we’ve observed a repulsive force, so right away we see that new ideas will be needed to understand electricity.

Experiment 3 is a puzzle. Two rods seem to have been charged in the same way, by rubbing, but these two rods attract each other rather than repel. Why does the outcome of Experiment 3 differ from that of Experiment 2? Back to the lab.

Our first set of experiments found that charged objects exert forces on each other. The forces are sometimes attractive, sometimes repulsive. Experiments 5 and 6 show that there is an attractive force between a charged object and a neutral (uncharged) object. This discovery presents us with a problem: How can we tell if an object is charged or neutral? Because of the attractive force between a charged and a neutral object, simply observing an electric force does not imply that an object is charged.

However, an important characteristic of any charged object appears to be that a charged object picks up small pieces of paper. This behavior provides a straightforward test to answer the question, Is this object charged? An object that passes the test by picking up paper is charged; an object that fails the test is neutral.

These observations let us tentatively advance the first stages of a charge model.

Postulate 2 is based on Experiment 8. If an object is charged (i.e., picks up paper), it always attracts one charged rod and repels the other. That is, it acts either “like plastic” or “like glass.” If there were a third kind of charge, different from the first two, an object with that charge should pick up paper and attract both the charged plastic and glass rods. No such objects have ever been found.

The basis for postulate 5 is the observation in Experiment 7 that a charged plastic rod is attracted to the wool used to rub it but repelled by silk that has rubbed glass. It appears that rubbing glass causes the silk to acquire “plastic charge.” The easiest way to explain this is to hypothesize that the silk starts out with equal amounts of “glass charge” and “plastic charge” and that the rubbing somehow transfers “glass charge” from the silk to the rod. This leaves an excess of “glass charge” on the rod and an excess of “plastic charge” on the silk.

While the charge model is consistent with the observations, it is by no means proved. We still have some large unexplained puzzles, such as why charged objects exert attractive forces on neutral objects.