One observation from Section 22.1 still needs an explanation. How do charged objects exert an attractive force on a neutral object? To begin answering this question, FIGURE 22.10 shows a positively charged rod held close to—but not touching—a neutral electroscope. The leaves move apart and stay apart as long as you hold the rod near, but they quickly collapse when it is removed.

FIGURE 22.10 A charged rod held close to an electroscope causes the leaves to repel each other.

The charged rod doesn’t touch the electroscope, so no charge is added or removed. Instead, the metal’s sea of electrons is attracted to the positive rod and shifts slightly to create an excess of negative charge on the side near the rod. The far side of the electroscope now has a deficit of electrons—an excess positive charge. We say that the electroscope has been polarized. Charge polarization is a slight separation of the positive and negative charges in a neutral object. Because there’s no net charge, the electron sea quickly readjusts when the rod is removed.

Why don’t all the electrons rush to the side near the positive charge? Once the electron sea shifts slightly, the stationary positive ions begin to exert a force, a restoring force, pulling the electrons back to the right. The equilibrium position for the sea of electrons shifts just enough that the forces due to the external charge and the positive ions are in balance. In practice, the displacement of the electron sea is usually $lessthan{\mathit{\text{ 10}}}^{\mathit{\text{−}}\mathit{\text{15}}}m!$

Charge polarization is the key to understanding how a charged object exerts an attractive force on a neutral object. FIGURE 22.11 shows a positively charged rod near a neutral piece of metal. Because the electric force decreases with distance, the attractive force on the electrons at the top surface is slightly greater than the repulsive force on the ions at the bottom. The net force toward the charged rod is called a polarization force. The polarization force arises because the charges in the metal are separated, not because the rod and metal are oppositely charged.

FIGURE 22.11 The polarization force on a neutral piece of metal is due to the slight charge separation.

A negatively charged rod would push the electron sea slightly away, polarizing the metal to have a positive upper surface charge and a negative lower surface charge. Once again, these are the conditions for the charge to exert a net attractive force on the metal. Thus our charge model explains how a charged object of either sign attracts neutral pieces of metal.