Chapter 26: Electric Charge and Forces
The electric force is one of the fundamental forces of nature.
Basic ovservations of electrical foces can be done by rubbing plastic rods with wool.
26.1 Developing a Charge Model
A common factor in observation of electrical phenomena is that two objects are rubbed
together. Why should rubbing an object cause forces and sparks?
We will first develop a model for understanding elecric phenomena in terms of charges and
Experimenting with Charges
The following experiments will obtain the results...
1. Two plastic rods--one suspended, one held. When in close proximity, no activity will be
2. Two plastic rods--one suspended, one held. Both rods have been rubbed with wool. In close
proximity, the suspended rod will attempt to move away from the held rod.
3. Suspended plastic rod rubbed with wool, held glass rod rubbed with silk. In proximity, the
plastic rod will be attracted to the glass rod.
4. Further tests conclude that more vigorous rubbing results in more powerful forces, and the
strength of the forces is greater with less distance.
No forces were observed in Exp. 1, so we will say that the original objects are neutral.
Rubbing the rods causes forces to be exerted between them--we shall call the rubbing process
charging and say that a rubbed rod is charged.
There is a repulsive force between two identical objects that have been charged in the same
way, as shown in exp 2. This force is affected by distance, as shown in exp 4.
Additional experiments are needed to explain the results of Exp. 3
5. A charged rod is held over small pieces of paper on the table. They are attracted to the rod,
and jump up to stick to it. This occurs for both a plastic and glass charged rod, but a neutral rod
has no effect on the paper.
6. Either object, charged, is attracted to a held neutral object.
7. Hanging plastic rod rubbed with wool is attracted to the wool it was rubbed with. The plastic
rod is repelled by the silk used to charge a glass rod.
8. Further experiments show that other objects, after being rubbed, attract one of the hanging
charged rods and repel the other. These objects always pick up small pieces of paper. There
appear to be no objects that, after being rubbed, pick up pieces of paper and attract botht he
charged plastic and glass rods. We can tell if an object is charged because it will pick
up small pieces of paper.
Charge model, part I Using what we know now, we can build an elementary
1. Frictional forces, such as charge model. See sidebar Charge model pt I.
We can tell the lack of a third kind of charge because
rubbing, add charge to an object.
The process itself is called every object tested du