Induced charge

When a charge is placed near a conductor, the mobile electrons inside the conductor rearrange themselves. Negative charge accumulates on the side facing a nearby positive source; positive charge appears on the opposite face. These induced surface charges are not new charges — the total charge of the conductor is unchanged — but they redistribute so that the field inside the conductor's bulk is zero.

Induced charge is the mechanism behind electrostatic attraction of neutral objects. A charged balloon picks up neutral paper scraps because the balloon's field polarises each scrap: positive charge migrates one way, negative the other, and the end closer to the balloon carries the opposite sign and is therefore attracted more strongly than the farther end is repelled. The net force is attractive. The same effect is why dust sticks to TV screens, why rubbed amber picks up straw (the two-thousand-year-old observation that launched electrostatics), and why humidity makes static cling worse — water molecules polarise easily and redistribute surface charges rapidly.

In problems with conductors the induced charge is exactly what makes calculations tractable. You cannot choose where the induced charges sit; they go wherever they need to so as to make the conductor's interior field vanish. The beautiful consequence is that for many geometries — a point charge near a flat conductor, or near a grounded sphere — the induced distribution is equivalent to replacing the conductor with a single fictitious "image charge" at a mathematically convenient location. That is the method of images.