Insulators are often charged by rubbing. The charge diagrams of FIGURE 22.5 show that the charges on the rod are on the surface and that charge is conserved. The charge can be transferred to another object upon contact, but it doesn’t move around on the rod.

FIGURE 22.5 An insulating rod is charged by rubbing.

Metals usually cannot be charged by rubbing, but Experiment 9 showed that a metal sphere can be charged by contact with a charged plastic rod. FIGURE 22.6 gives a pictorial explanation. An essential idea is that the electrons in a conductor are free to move. Once charge is transferred to the metal, repulsive forces between the negative charges cause the electrons to move apart from each other.

FIGURE 22.6 A conductor is charged by contact with a charged plastic rod.

Note that the newly added electrons do not themselves need to move to the far corners of the metal. Because of the repulsive forces, the newcomers simply “shove” the entire electron sea a little to the side. The electron sea takes an extremely short time to adjust itself to the presence of the added charge, typically much less than ${10}^{-9}\text{s}$. For all practical purposes, a conductor responds instantaneously to the addition or removal of charge.

Other than this very brief interval during which the electron sea is adjusting, the charges in an isolated conductor are in static equilibrium. That is, the charges are at rest (i.e., static) and there is no net force on any charge. This condition is called electrostatic equilibrium. If there were a net force on one of the charges, it would quickly move to an equilibrium point at which the force was zero.

Electrostatic equilibrium has an important consequence:

In an isolated conductor, any excess charge is located on the surface of the conductor.

To see this, suppose there were an excess electron in the interior of an isolated conductor. The extra electron would upset the electrical neutrality of the interior and exert forces on nearby electrons, causing them to move. But their motion would violate the assumption of static equilibrium, so we’re forced to conclude that there cannot be any excess electrons in the interior. Any excess electrons push each other apart until they’re all on the surface.

EXAMPLE 22.2 Charging an electroscope

Many electricity demonstrations are carried out with the help of an electroscope like the one shown in FIGURE 22.7. Touching the sphere at the top of an electroscope with a charged plastic rod causes the leaves to fly apart and remain hanging at an angle. Use charge diagrams to explain why.

FIGURE 22.7 A charged electroscope.

FIGURE 22.7 Full Alternative Text

MODEL We’ll use the charge model and the model of a conductor as a material through which electrons move.

VISUALIZE FIGURE 22.8 uses a series of charge diagrams to show the charging of an electroscope.

FIGURE 22.8 The process by which an electroscope is charged.

FIGURE 22.8 Full Alternative Text