Electrostatic induction

When a charged object is brought near a neutral conductor, its electric field exerts forces on the conductor's mobile charges. Electrons move within the conductor: they are attracted toward a nearby positive charge and repelled from a nearby negative charge. This separates charge within the object, leaving regions with an excess of opposite signs, even though the object as a whole remains neutral. This redistribution is electrostatic induction, also described as induced polarization. It does not require electrons to pass from the nearby charged object to the conductor.

In a metal conductor, free electrons can move through the material, so an external electric field can produce a substantial redistribution of charge. In electrostatic equilibrium, the mobile charges have moved until the electric field inside the conducting material is zero. In an insulator, charges are not free to travel throughout the object; instead, the electric field can slightly shift bound positive and negative charges within atoms or molecules, polarizing the material. Both responses can cause a neutral object to be attracted to a nearby charged body, though the microscopic motion differs.

Induction can also be used to give a conductor a lasting net charge, but redistribution alone does not do that. In one common procedure, a charged rod is brought near a neutral conductor without touching it. While the rod remains nearby, the conductor is connected to Earth, allowing electrons to flow between the conductor and the ground. The ground connection is then removed before the rod is taken away. The conductor is left with a net charge opposite in sign to the rod: a nearby positive rod draws electrons from Earth into the conductor, while a nearby negative rod repels electrons from the conductor to Earth. The rod itself need not transfer charge to the conductor.

Another classroom demonstration uses two neutral metal spheres touching one another on insulating supports. Bringing a charged object near one sphere shifts electrons through the connected pair, polarizing the system. If the spheres are separated while the charged object is still nearby, each sphere retains a net charge of opposite sign to the other. The total charge of the pair remains unchanged: induction has separated charge rather than created it. Once the inducing object is removed, the charge on each isolated sphere redistributes over its surface.

Induction explains why a charged comb can attract small neutral pieces of paper: the comb polarizes charges in the paper, making the closer opposite-sign charges exert a stronger attraction than the more distant like-sign charges repel. Charge redistribution in conductors is also central to electrostatic shielding. In a conductor at electrostatic equilibrium, mobile charges rearrange in response to external fields; a conducting enclosure can therefore reduce the electric field within its interior under appropriate conditions.