22.1.2 Electric Properties of Materials

We still need to clarify how different types of materials respond to charges.

Discovering electricity III

Experiment 9

Charge a plastic rod by rubbing it with wool. Touch a neutral metal sphere with the rubbed area of the rod. The metal sphere then picks up small pieces of paper and repels a charged, hanging plastic rod. The metal sphere appears to have acquired “plastic charge.”

Experiment 10

Charge a plastic rod, then run your finger along it. After you’ve done so, the rod no longer picks up small pieces of paper or repels a charged, hanging plastic rod. Similarly, the metal sphere of Experiment 9 no longer repels the plastic rod after you touch it with your finger.

Experiment 11

Full Alternative Text

Place two metal spheres close together with a plastic rod connecting them. Charge a second plastic rod, by rubbing, and touch it to one of the metal spheres. Afterward, the metal sphere that was touched picks up small pieces of paper and repels a charged, hanging plastic rod. The other metal sphere does neither.

Experiment 12

Full Alternative Text

Repeat Experiment 11 with a metal rod connecting the two metal spheres. Touch one metal sphere with a charged plastic rod. Afterward, both metal spheres pick up small pieces of paper and repel a charged, hanging plastic rod.

Our final set of experiments has shown that

Charge can be transferred from one object to another, but only when the objects touch. Contact is required. Removing charge from an object, which you can do by touching it, is called discharging.
There are two types or classes of materials with very different electric properties. We call these conductors and insulators.

Experiment 12, in which a metal rod is used, is in sharp contrast to Experiment 11. Charge somehow moves through or along a metal rod, from one sphere to the other, but remains fixed in place on a plastic or glass rod. Let us define conductors as those materials through or along which charge easily moves and insulators as those materials on or in which charges are immobile. Glass and plastic are insulators; metal is a conductor.

This information lets us add two more postulates to our charge model:

MODEL 22.1 Charge model, part II

There are two types of materials. Conductors are materials through or along which charge easily moves. Insulators are materials on or in which charges are immobile.
Charge can be transferred from one object to another by contact.

NOTE Both insulators and conductors can be charged. They differ in the mobility of the charge.

We have by no means exhausted the number of experiments and observations we might try. Early scientific investigators were faced with all of these results, plus many others. How should we make sense of it all? The charge model seems promising, but certainly not proven. We have not yet explained how charged objects exert attractive forces on neutral objects, nor have we explained what charge is, how it is transferred, or why it moves through some objects but not others. Nonetheless, we will take advantage of our historical hindsight and continue to pursue this model. Homework problems will let you practice using the model to explain other observations.

EXAMPLE 22.1 Transferring charge

In Experiment 12, touching one metal sphere with a charged plastic rod caused a second metal sphere to become charged with the same type of charge as the rod. Use the postulates of the charge model to explain this.

SOLVE We need the following postulates from the charge model:

7. Charge is transferred upon contact.
6. Metal is a conductor, and charge moves through a conductor
3. Like charges repel.

The plastic rod was charged by rubbing with wool. The charge doesn’t move around on the rod, because it is an insulator, but some of the “plastic charge” is transferred to the metal upon contact. Once in the metal, which is a conductor, the charges are free to move around. Furthermore, because like charges repel, these plastic charges quickly move as far apart as they possibly can. Some move through the connecting metal rod to the second sphere. Consequently, the second sphere acquires “plastic charge.”