Charging by rubbing

1. When two different materials are rubbed together, charge is moved from one surface to the other, so both surfaces become charged.
2. The two objects always end up with opposite charges, so one becomes positively charged and the other becomes negatively charged.
3. Charge that builds up on a surface and stays there is called static electricity, because it stays in one place instead of flowing.
4. Rubbing does not make new charge, it only separates charge that was already in the two materials.
5. Familiar examples include a balloon rubbed on dry hair, a plastic comb pulled through hair, a polythene rod rubbed with a duster and a jumper pulled off over your head.
6. Walking across a nylon carpet in rubber-soled trainers charges your body, and sliding across a car seat charges both you and the seat.
7. Static charge only builds up on insulators, such as plastic, rubber, glass, wool, nylon and dry hair.
8. In an insulator, charge cannot move through the material, so it stays on the patch of surface where the rubbing put it.
9. In a conductor, such as a metal, charge moves freely through the material.
10. If you rub a metal rod held in your hand, any charge on it flows through the metal and through your body to the ground, so no charge builds up.
11. A metal object can only hold static charge if it is insulated from the ground, for example by standing it on a plastic block.
12. Static effects are strongest on dry days, because in damp air a thin film of water forms on surfaces and lets the charge leak away.

Forces between charged objects

1. Charged objects push or pull on each other, and this push or pull is called the electrostatic force.
2. Objects with the same type of charge, called like charges, repel each other.
3. Two positively charged objects repel, and two negatively charged objects repel.
4. Objects with different types of charge, called unlike charges, attract each other.
5. A positively charged object and a negatively charged object attract.
6. The electrostatic force is a non-contact force, which means it acts across a gap without the objects touching.
7. The force is larger when the charged objects are closer together and becomes weaker as they move apart.
8. The force is also larger when the objects carry more charge.
9. The evidence that charged objects exert forces without contact comes from seeing them move before they touch.
10. A rubbed balloon held a few centimetres above small pieces of paper makes them jump up to it before the balloon reaches them.
11. Strands of hair lift towards a rubbed balloon held just above your head, even though the balloon does not touch the hair.
12. A thin, smooth stream of water from a tap bends sideways towards a charged comb held close to it.
13. A rubbed rod hanging from a thread swings round when another charged rod is brought near it, even though there is a clear gap between the rods.
14. In each case an object starts to move across a gap, and a change in motion needs a force, so a force must be acting at a distance.

Sparking

1. The longer and harder an insulator is rubbed, the more charge builds up on it.
2. When a large charge has built up, it can suddenly jump across an air gap to a nearby conductor, such as a metal tap, radiator or door handle.
3. This sudden flow of charge through the air is a spark, and it gives out light and a crackling sound.
4. Air is normally an insulator, but a large build-up of charge can make the air conduct for an instant so that the charge crosses the gap.
5. The bigger the charge, the wider the gap that the spark can jump.
6. Sparks usually jump just before contact, which is why you feel a sharp shock when your fingertip is a few millimetres from a metal door handle after walking across a nylon carpet.
7. Pulling off a synthetic jumper in a dark room often produces tiny sparks that you can see and hear crackling.
8. Lightning is the same effect on a huge scale, when charge built up in a thundercloud jumps through the air to the ground or to another cloud.