One cause behind every static effect

1. Every effect in this topic comes from the same cause: electrons have moved, either from one object to another or from one side of an object to the other.
2. When electrons move between objects, one object is left negative and the other positive, so the two objects can then attract or repel.
3. When electrons only shift within an object, the object stays neutral overall but one side becomes negative and the other becomes positive.
4. Charge that has built up on an insulator has no easy path away, so it stays there until something provides one.
5. When a path does appear, the charge moves all at once rather than gradually, and this electrostatic discharge is felt as a shock or seen as a flash.
6. The size of the effect depends on how much charge has built up, which is why static effects are strongest in dry air and on good insulators.

Shocks from everyday objects

1. Walking across a nylon carpet rubs your shoe soles against the carpet fibres, so electrons are transferred between them.
2. Your body ends up with an overall charge, and because your shoes and clothing are insulators that charge cannot flow away to the ground.
3. The charge keeps building as you walk, so the potential difference between you and any earthed metal object grows steadily.
4. Reaching for an earthed metal door handle gives the excess electrons a conducting path, so they move very quickly indeed.
5. That rapid movement of electrons is a brief current through your fingertip, and the nerve endings in your skin register it as a sharp shock.
6. If the charge is large enough, the electrons cross the last few millimetres of air before you touch the metal, which gives a small spark and an audible click.
7. The direction the electrons move depends on the sign of your charge: they flow from you to the handle if you are negative, and from the handle to you if you are positive.
8. The same sequence explains the crackle heard when a jumper is pulled off over the head, and the shock felt when sliding off a car seat and touching the door frame.
9. Shocks are worse in winter, because cold indoor air holds little moisture and a dry surface lets less charge leak away as it builds up.

Lightning

1. Inside a thundercloud, powerful updraughts carry water droplets, hail and ice crystals rapidly past one another.
2. Collisions between these particles transfer electrons from one particle to another, so charge becomes separated within the cloud.
3. The heavier particles that fall tend to carry negative charge downwards, so the base of the cloud becomes negatively charged while the top is left positively charged.
4. The negatively charged cloud base repels electrons in the ground below it, so the ground surface is left positively charged.
5. The charge continues to build up because both the cloud and the air beneath it are very poor conductors.
6. The potential difference between the cloud base and the ground therefore rises to many millions of volts.
7. Once the build-up is large enough, electrons are driven suddenly through the air along a narrow channel between the cloud and the ground.
8. This very large electrostatic discharge is the lightning stroke, and all of that charge moves in a small fraction of a second.
9. The moving charge heats the air in the channel until it glows brightly, which is the flash that is seen.
10. The same heating makes the air expand explosively, and the shock wave that spreads out from the channel is heard as thunder.
11. Discharges also occur between the negative base and the positive top of the same cloud, which is why some lightning is seen inside clouds and never reaches the ground.

Attraction by induction

1. A neutral object holds equal amounts of positive and negative charge, so it has no overall charge.
2. Bringing a charged object close does not add any charge to the neutral object, but it does make the electrons inside it shift position slightly.
3. If the charged object is negative, it repels electrons in the neutral object, and those electrons move to the far side.
4. The near side of the neutral object is then short of electrons, so it becomes slightly positive.
5. The near positive side is closer to the negative object than the far negative side is, and the electrostatic force is larger over a shorter distance.
6. The attraction from the near side therefore beats the repulsion from the far side, and the overall result is attraction.
7. If the charged object is positive, electrons in the neutral object are pulled towards it instead, so the near side becomes slightly negative and the object is attracted again.
8. This is why a charged object attracts a neutral object whichever sign of charge the charged object carries.
9. The shift of charge inside the neutral object is induced charge separation, and it disappears as soon as the charged object is taken away.
10. A balloon rubbed on a jumper and held against a wall causes this separation in the wall surface, and the resulting attraction is strong enough to hold the balloon up against its weight.
11. A comb rubbed through dry hair picks up small pieces of paper for the same reason: the side of each piece nearest the comb gains the opposite charge and is pulled towards it.
12. The pieces of paper have to be small, because the electrostatic force here is only a fraction of a newton and can lift only a very small weight.
13. Paper sometimes jumps off the comb again shortly after touching it, because electrons transfer on contact and the paper is then left with the same sign of charge as the comb, so it is repelled.