If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface.
Charging by induction can be illustrated using two insulated metal spheres.
Uncharged insulated metal spheres touching each other, in effect, form a single noncharged conductor.
When a negatively charged rod is held near one sphere, electrons in the metal are repelled by the rod.
Excess negative charge has moved to the other sphere, leaving the first sphere with an excess positive charge.
The charge on the spheres has been redistributed, or induced.
When the spheres are separated and the rod removed, the spheres are charged equally and oppositely.
They have been charged by induction, which is the charging of an object without direct contact.
Charge induction by grounding can be illustrated using a metal sphere hanging from a nonconducting string.
A charge redistribution is induced by the presence of the charged rod. The net charge on the sphere is still zero.
Touching the sphere removes electrons by contact and the sphere is left positively charged.
The positively charged sphere is attracted to a negative rod.
When electrons move onto the sphere from the rod, it becomes negatively charged by contact.
When we touch the metal surface with a finger, charges that repel each other have a conducting path to a practically infinite reservoir for electric charge—the ground.
When we allow charges to move off (or onto) a conductor by touching it, we are grounding it.
Charging by induction occurs during thunderstorms.
The negatively charged bottoms of clouds induce a positive charge on the surface of Earth below.
Most lightning is an electrical discharge between oppositely charged parts of clouds.
The kind of lightning we are most familiar with is the electrical discharge between clouds and oppositely charged ground below.
If a rod is placed above a building and connected to the ground, the point of the rod collects electrons from the air.
This prevents a buildup of positive charge by induction.
The primary purpose of the lightning rod is to prevent a lightning discharge from occurring.
If lightning does strike, it may be attracted to the rod and short-circuited to the ground, sparing the building.
