Static Electricity

Explained

Static electricity, and the particle that actually moves

Rub two insulating materials together and electrons are transferred from one to the other. The material that gains electrons becomes negatively charged. The material that loses them is left with more protons than electrons, so it becomes positively charged.

The two charges are equal in size and opposite in sign, because every electron that arrives on one object left the other. No charge is created; it is moved.

Only electrons move

This is the single most important point in the topic, and it is where most marks are lost. Protons sit in the nuclei of atoms and do not move between materials. An object never becomes positive by gaining positive charge. It becomes positive by losing electrons.

So when you describe charging, name electrons, say which way they went, and say what each material is left with.

Forces between charges

Like charges repel and opposite charges attract. Two negatively charged rods push apart; a negative rod and a positive rod pull together.

A charged object also attracts uncharged materials, which looks like an exception but is not. The charge on the rod pulls the electrons in the nearby object towards it or pushes them away, so the near surface ends up with the opposite charge. Opposite charges are closer than like charges, so the attraction wins. This is induction, and it explains why a charged comb picks up small pieces of paper.

Why insulators and not metals

Charge stays put on an insulator because the electrons cannot flow through it. On a metal held in your hand, any charge immediately flows away to earth through you, so nothing builds up. This is why the standard demonstration uses a plastic rod and a cloth.

Uses and dangers

Static is useful when the attraction is what you want. In electrostatic spray painting, the paint droplets are charged, so they repel each other and spread into an even cloud, and are attracted to the oppositely charged object so that little is wasted and even the far side gets coated. Photocopiers and inkjet printers work on the same principle.

Static is dangerous when the charge discharges suddenly. Refuelling an aircraft or a tanker can build up charge as fuel flows, and a spark near fuel vapour could ignite them, so the vehicle is earthed with a bonding wire that lets the charge flow safely away.

Lightning is the same process at a very large scale, and a lightning conductor works by giving the charge an easy path to the ground rather than through the building.