Charging by Friction, Conduction, and Induction

Charging by friction happens when two different materials rub together and electrons jump from one surface to the other. Shuffle your socks across carpet and electrons pile up on you, leaving you with a surplus of negative charge. Touch the doorknob, a conductor, and those electrons rush off you through your finger in a spark. That's conduction too: direct contact that lets charge flow until it's shared or neutralized.

Induction works without any contact at all. Bring a negatively charged rod near a neutral metal sphere, and the rod repels the sphere's free electrons toward the far side, leaving the near side positive and the far side negative. The sphere is still neutral overall, just rearranged. Now touch a grounding wire to the far side. Because the ground is a huge reservoir for electrons, the excess negative charge drains away into it, while the rod keeps holding the positive side in place by attraction. Disconnect the ground, then pull the rod away: the electrons that left can't come back, so the sphere is left with a permanent, uniform positive charge, opposite in sign to the rod. That's why induction and conduction give opposite results: touching the rod to the sphere would have left it negative instead.

Insulators can't do any of this redistribution over long distances. Their electrons are bound to individual atoms, so a nearby charge only shifts charge slightly within each molecule, called polarization, without moving charge across the whole object. That limits insulators to attracting nearby charges weakly, setting up the contrast with conductors that carries into how fields and forces behave near each material.