Charging by friction

1. Static electricity is explained by the movement of electrons, the negatively charged particles in the outer part of every atom.
2. The protons carry the positive charge, but they are held inside the nucleus at the centre of each atom, so rubbing cannot move them.
3. When two different insulators are rubbed together, friction pulls some electrons off the surface atoms of one material and onto the surface of the other.
4. Different materials hold on to their electrons with different strengths, so the material that holds its electrons less tightly loses electrons and the other material gains them.
5. The material that gains electrons now has more electrons than protons, so it becomes negatively charged.
6. The material that loses electrons now has fewer electrons than protons, so it is left positively charged.
7. A positive charge is therefore caused by a shortage of electrons, never by extra protons.
8. Every electron that leaves one surface arrives on the other, so the two objects gain charges that are equal in size and opposite in type.
9. Charge is moved but not made or destroyed, so the total charge of the two objects is still zero after rubbing, just as it was before.
10. The direction of electron transfer depends on the pair of materials, so the same cloth can make a polythene rod negative but an acetate rod positive.

Why only insulators hold static charge

1. In an insulator, electrons cannot move freely through the material, so electrons gained by rubbing stay on the patch of surface where they arrived.
2. In the same way, a patch of an insulator that has lost electrons cannot quickly get them back from the rest of the material.
3. In a metal, some electrons are free to move through the whole object.
4. If a metal rod held in your hand gains electrons, they spread through the metal, into your hand and through your body to the ground, so the rod is left neutral.
5. If the metal rod loses electrons, electrons flow up from the ground through your body to replace them, so again the rod is left neutral.
6. A metal object standing on an insulating support keeps its charge, because the electrons have no path to or from the ground.

Discharging and sparks

1. A charged object loses its charge when electrons flow onto it or away from it until its numbers of protons and electrons are equal again.
2. A negatively charged object is discharged when its extra electrons flow away from it.
3. A positively charged object is discharged when electrons flow onto it to make up the shortage.
4. Connecting a charged conductor to the ground with a metal wire is called earthing.
5. The Earth is so large that it can give or take any number of electrons without its own charge changing noticeably.
6. When a negatively charged object is earthed, electrons flow from the object to the Earth.
7. When a positively charged object is earthed, electrons flow from the Earth to the object.
8. A spark is a burst of electrons crossing an air gap when a large charge has built up on an object.
9. If your body is negatively charged, the extra electrons jump from your fingertip to a metal door handle as you reach for it.
10. If your body is positively charged, electrons jump the other way, from the handle to your fingertip.
11. Either way, the flow of electrons stops as soon as your body is neutral again, which is why a static shock lasts only an instant.

Attracting neutral objects

1. A charged object can also attract a neutral object, such as a rubbed balloon sticking to a wall or a charged comb picking up scraps of paper.
2. When a negatively charged balloon is held near a wall, it repels electrons in the surface of the wall, pushing them a tiny distance away from the balloon.
3. The surface of the wall nearest the balloon is left with a small positive charge, while the displaced electrons sit slightly further away.
4. The balloon attracts the nearby positive charge more strongly than it repels the more distant negative charge, because the electrostatic force is stronger over shorter distances.
5. The overall force is therefore attraction, and the balloon sticks to the wall.
6. A positively charged object attracts a neutral object in the same way, except that it pulls electrons in the surface towards itself.
7. The wall as a whole stays neutral, because no electrons have left it and they have only shifted slightly within it.