Static Electricity

Static electricity — electric charge that has built up on an insulator and stays there, rather than flowing as a current.

Charging by friction — rubbing two insulators together transfers electrons from one to the other. The material that gains electrons becomes negatively charged; the one that loses them is left positively charged.

Only electrons move. Protons are locked in the nuclei and go nowhere. An object is never "given positive charge" — it loses negatives. Saying protons moved is the single most common error in this topic.

The law of charges:

• Like charges repel — two negatives, or two positives, push apart.
• Unlike charges attract — a positive and a negative pull together.

Why a charged rod attracts uncharged paper. The rod's field pushes electrons within each scrap of paper slightly away (or draws them slightly closer), leaving the near face oppositely charged. Attraction to the near face beats repulsion from the far one because it is closer, so the paper lifts. The paper is still neutral overall — this is induced charge, not charging.

Dangers of static:

• Refuelling an aircraft or a tanker builds charge as fuel rubs through the pipe. A spark near fuel vapour is an explosion, so the tanker is earthed by a bonding cable before fuelling begins.
• Charge builds on a person walking across a nylon carpet; touching a metal door handle discharges it as a shock.

Earthing — providing a conducting path to the ground so charge flows away safely instead of building to the point where it jumps as a spark.

Uses of static:

• Electrostatic precipitator — smoke particles are charged, then attracted to oppositely charged plates in a chimney, removing them from the flue gases.
• Paint spraying — the paint droplets are given the same charge, so they repel each other into an even spread, and the object is given the opposite charge so the paint is attracted to it. Less waste, and it reaches round the back.
• Photocopier — a charged drum attracts toner powder only where the image is.

Example: a polythene rod rubbed with a duster gains electrons and becomes negative. Held near a thin stream of water from a tap, the stream bends towards it — the rod induces a slight positive charge on the near side of the polar water molecules.