Polarizing a conductor is one thing, but why does a charged rod pick up paper, which is an insulator? Consider what happens when we bring a positive charge near an atom. As FIGURE 22.12 shows, the charge polarizes the atom. The electron cloud doesn’t move far, because the force from the positive nucleus pulls it back, but the center of positive charge and the center of negative charge are now slightly separated.

FIGURE 22.12 A neutral atom is polarized by and attracted toward an external charge.

Two opposite charges with a slight separation between them form what is called an electric dipole. (The actual distortion from a perfect sphere is minuscule, nothing like the distortion shown in the figure.) The attractive force on the dipole’s near end slightly exceeds the repulsive force on its far end because the near end is closer to the external charge. The net force, an attractive force between the charge and the atom, is another example of a polarization force.

An insulator has no sea of electrons to shift if an external charge is brought close. Instead, as FIGURE 22.13 shows, all the individual atoms inside the insulator become polarized. The polarization force acting on each atom produces a net polarization force toward the external charge. This solves the puzzle. A charged rod picks up pieces of paper by

• Polarizing the atoms in the paper,

• Then exerting an attractive polarization force on each atom.

This is important. Make sure you understand all the steps in the reasoning.

FIGURE 22.13 The atoms in an insulator are polarized by an external charge.