Static electricity
Physics · GCSE · The School
Charging is electrons moving, never protons
Rub a polythene rod with a duster and electrons transfer from the duster to the rod. The rod gains electrons and becomes negative; the duster loses them and becomes positive. Protons are locked in nuclei and never move in this process — so every explanation in this topic is a sentence about where the electrons went.
Like repels like
Two negatively charged rods push apart. A negative rod and a positive one pull together. This is the rule everyone can state, and the useful case is the third one: a charged rod also attracts an uncharged object, because it pulls that object’s electrons towards or away from the near surface. Attraction alone therefore does not prove the other object is charged.
Fields, and why sparks happen at points
A charged object has an electric field around it — a region where another charge would feel a force — drawn as lines from positive to negative, closer together where the field is stronger. When the field grows strong enough it tears electrons off air molecules, the air conducts, and you get a spark. Fields are strongest at sharp points, which is why lightning conductors are pointed.
Where it is dangerous and where it is useful
Refuelling an aircraft builds charge through friction, and a spark near fuel vapour is a catastrophe, so the aircraft is bonded to earth first. The same effect is put to work in spray painting: the paint droplets are charged, repel each other into an even mist, and are attracted to an oppositely charged panel, so less paint is wasted and edges get coated.
Charging by induction, without touching
A charged rod held near an isolated metal sphere pushes its electrons to the far side without ever touching it. Earth the far side with a finger and those repelled electrons drain away; remove the earth first, then take the rod away, and the sphere is left with a permanent charge opposite to the rod's. Get the order wrong — remove the rod before the earth — and the induced charge simply flows back and the sphere ends up neutral.
A second use: cleaning smoke and toner
An electrostatic precipitator charges the dust in a factory chimney as it rises, then attracts it onto oppositely charged plates so far less soot reaches the air. A photocopier uses the same idea in miniature: a charged drum attracts powdered toner only where the image is dark, then transfers it to the paper. Both rely on the same rule as the paint sprayer — charge something, and it is pulled towards the opposite charge.