Static charge — AQA GCSE Physics

Only electrons move. They sit on the outside of the atom and are the only charged particles free enough to be scraped from one surface onto another, while the positive nuclei are locked in place in the solid. Friction supplies enough energy to strip some electrons from one material and deposit them on the other, so the two objects end up with equal and opposite charges: whatever one gains, the other has lost. Charge is conserved, so the pair together is still neutral. Mark schemes are strict here: any answer mentioning positive charge being transferred, or 'positive electrons', scores zero even when the rest is correct. Say that electrons move, name the direction, then say what that leaves behind.

A neutral object holds as many electrons as protons, so the charges cancel. Add electrons and that balance breaks: there are now more negative charges than positive ones and the object is negatively charged overall. The size of the charge depends on how many extra electrons it holds, and it stays charged until those electrons can flow away. This is what happens to a student touching the dome of a static generator: electrons are transferred onto her, every hair on her head gains the same negative charge, and because like charges repel, the hairs push each other apart and stand on end. A polythene rod rubbed with a duster behaves the same way, gaining electrons from the duster and being left negative.

Nothing positive ever moves. An object becomes positively charged by losing electrons, which leaves its protons outnumbering its remaining electrons, so the overall charge is positive. The size of that positive charge is exactly equal to the negative charge picked up by the other material, because every electron that left one surface arrived on the other; charge is conserved, and the pair together is still neutral. This is the answer examiners want for 'explain how an uncharged object may become positively charged': negatively charged electrons are transferred away from the object. Given a cloth and a rod, say which lost electrons, say electrons are negative, then conclude that the cloth now has more positive charges than negative ones.

Two charged objects push or pull on each other without touching. The force acts along the line joining them, it acts on both objects equally and in opposite directions, and it grows as they are brought closer together. You can see it and even measure it: a charged rod held above another charged rod resting on a balance changes the balance reading, because the rod on the balance is being pushed down or pulled up by the rod above it. Since nothing is touching, this is evidence that the force acts through the space between them, and that space is described as an electric field. Only the change in the balance reading matters in that experiment, so a zero error on the balance does not affect the result.

Like charges repel: two negative objects push apart, and so do two positive ones. In the standard demonstration a charged polythene rod is brought near an identical charged rod hanging from a thread, and the hanging rod swings away. The reason is that each rod sits in the electric field of the other, and a negative charge in the field of another negative charge is pushed outwards. Repulsion is the more useful test of the two, because attraction also happens between a charged object and an uncharged one, whereas only two like-charged objects repel. That is why the hair on a charged student's head stands up: every hair carries the same sign of charge, so the hairs repel and spread as far apart as they can.

Unlike charges attract: a positive object and a negative object pull towards each other along the line joining them. The pull is a genuine non-contact force and grows as the separation shrinks. Attraction is a weaker test for charge than repulsion, because a charged object also attracts an uncharged one. Bring a charged rod near a thin stream of water or a scrap of paper and the charges inside the neutral object shift slightly, with the opposite charge drawn to the near side and the like charge pushed to the far side. The near side is closer, so attraction beats repulsion and the object moves towards the rod. To decide the sign of an unknown charge, test it against a rod of known charge and look for repulsion.

A non-contact force acts between two objects that are not touching, and electrostatic attraction and repulsion are the clearest classroom examples. The other two you need are the gravitational force and the magnetic force; everything else at this level, including friction, air resistance, tension and the normal contact force, is a contact force. The mechanism is the same for all three non-contact forces: each object creates a field in the space around it, and the second object experiences a force because it sits in that field. An electric field is the region around a charged object where another charged object experiences a force. Non-contact forces still obey the usual rules, so the pair is equal in size and opposite in direction, and each is a vector.

Static electricity is produced when two different insulating materials are rubbed together. Friction causes negatively charged electrons to be transferred from one material to the other. The material gaining electrons becomes negatively charged, while the one losing electrons becomes positively charged. Evidence for non-contact forces can be seen when a charged rod is brought near another suspended charged rod: like charges repel and opposite charges attract without touching. As charge builds on a surface, the potential difference between it and an earthed conductor grows. When large enough, the air breaks down and becomes conducting. Electrons move through the air between the objects, creating a spark. The direction of electron flow depends on the sign of the charged object. The spark transfers energy, seen as light and heard as a crack.