Charge Polarization

Insulators can’t play the free-flowing electron game conductors do when it comes to charging by induction. Instead, the party happens within the atoms and molecules themselves. Imagine them shifting their internal charge distribution like musical chairs, without actually moving their entire bodies (atoms). This creates a separation of positive and negative charges within the molecule, making it like a tiny magnet with a north and south pole. We call this a “polarized” insulator.

Think of a negatively charged rod approaching the insulator. It pulls on the positive side of the molecule while pushing away the negative side. This rearranges the internal charge, making the insulator act like a bunch of tiny dipoles all lined up, responding to the presence of the charged rod.

An electron buzzing around the atomic nucleus produces an electron cloud. (a) The center of the negative cloud normally coincides with the center of the positive nucleus in an atom. (b) When an external negative charge is brought nearby to the right, as on a charged balloon,
the electron cloud is distorted so that the centers of negative and positive charge no longer coincide. The atom is now electrically polarized.

We can understand why electrically neutral bits of paper are attracted to a charged object—a comb passed through your hair, for example. When the charged comb is brought nearby, molecules in the paper are polarized. The sign of charge closest to the comb is opposite to the comb’s charge. Charges of the same sign are slightly more distant. Closeness wins (inverse-square law), and the bits of paper experience a net attraction. Sometimes they will cling to the comb and then suddenly fly off. This repulsion occurs because the paper bits acquire the same sign of charge as the charged comb when they come in contact. Rub an inflated balloon on your hair, and it becomes charged. Place the balloon against the wall, and it sticks. This is because the charge on the balloon induces an opposite surface charge on the wall. Again, closeness wins because the charge on the balloon is slightly closer to the opposite induced charge than to the charge of the same sign