11.1.1 Charging insulators by friction

Charge, insulators and static electricity

1. Every atom contains protons, which carry positive charge, electrons, which carry negative charge, and neutrons, which carry no charge at all.
2. Charge is measured in coulombs, symbol $C$, and one proton and one electron carry equal amounts of charge with opposite signs.
3. An atom with as many electrons as protons is neutral, so an object built from neutral atoms has no overall charge.
4. Electrons sit in the outer parts of an atom and some of them can be pulled away, while protons are locked inside the nucleus and never leave during charging.
5. Only electrons move, so every explanation of static electricity is written in terms of electron transfer and never in terms of moving protons.
6. In an insulator such as polythene, perspex, rubber, nylon or dry hair there are almost no free charges, so charge that arrives at one part of the surface stays at that part.
7. Charge trapped in this way is static electricity, and it is why an insulator can stay charged for many seconds after it has been rubbed.
8. A metal held in your bare hand never appears charged, because a metal is a conductor and any charge given to it flows away through your body.

Charging by friction

1. Two insulating surfaces touch only at a small number of high points, and rubbing them together repeats that close contact thousands of times.
2. At each contact, electrons on the surface of one material are held less tightly than the atoms of the other material would hold them, so those electrons are transferred across.
3. Rubbing supplies the energy needed to pull the electrons away from one surface, but it does not make any new charge.
4. The transfer always runs the same way for a given pair of materials, so the same pair always ends up charged the same way round.
5. A polythene rod rubbed with a woollen duster gains electrons, so the rod becomes negatively charged and the duster is left positively charged.
6. A perspex or acetate rod rubbed with the same duster loses electrons, so the rod becomes positively charged and the duster is left negatively charged.
7. Which material becomes negative depends only on the pair chosen, which is why a question always tells you the materials or tells you the final charge on one of them.
8. Rubbing harder or for longer transfers more electrons, so the charge on the rod is larger, although the sign of that charge does not change.

Equal and opposite charges

1. Before rubbing, both materials are neutral, so each holds as much negative charge as positive charge.
2. Rubbing transfers a number of electrons off one material, and exactly that number of electrons arrives on the other material.
3. The material that gains electrons now has more negative charge than positive charge, so its overall charge is negative.
4. The material that loses electrons is left with more positive charge than negative charge, so its overall charge is positive.
5. Because the electrons lost by one object are the same electrons gained by the other, the two charges are equal in size and opposite in sign.
6. Calling the charge on one object $-Q$ and the charge on the other $+Q$ makes this clear, since $(-Q)+(+Q)=0$ and the pair together is still neutral.
7. No charge has been made or destroyed anywhere in the process; charge has only been separated between the two objects.
8. This is why a question that gives the charge on the rod as $-6nC$ expects the charge on the cloth to be $+6nC$.

11.1.2 Forces between charges

Forces between charged objects

1. Any two charged objects exert an electrostatic force on each other, even when there is a gap between them.
2. The force is a non-contact force, so the objects do not have to be touching for it to act.
3. The force acts on both objects at once, equal in size and opposite in direction, so a charged rod that pushes a charged balloon away is pushed away by the balloon just as hard.
4. Whether that force is a push or a pull is decided only by the signs of the two charges.
5. Like charges repel, so two positive charges are pushed apart and two negative charges are pushed apart.
6. Unlike charges attract, so a positive charge and a negative charge are pulled together.
7. The rule works either way round: a negative charge next to a positive charge attracts in exactly the same way as a positive charge next to a negative one.
8. Repel means the force pushes the objects apart, and attract means the force pulls them towards each other, so an answer must name one of these rather than just saying that a force acts.

What changes the size of the force

1. The sign of each charge decides the direction of the force, while the size of each charge and the separation between the objects decide how large that force is.
2. Increasing the charge on either object increases the force, which is why a rod that has been rubbed for longer makes a suspended rod swing further.
3. Increasing the separation decreases the force, which is why a charged rod has no measurable effect from the far side of the bench.
4. The decrease with separation is steep, so halving the gap between two charged objects makes a much larger difference than doubling one of the charges.
5. Both objects always feel the same size of force, even when one of them carries far more charge than the other.
6. At this level you are asked to state the direction of the force and whether it becomes larger or smaller, rather than to calculate a value in newtons.

Using the rule to work out charges

1. Repulsion is the reliable test for charge, because only two charged objects can push each other apart.
2. If two suspended objects move apart when brought close, both must be charged and both charges must have the same sign.
3. If two charged objects move together, their charges must have opposite signs.
4. Attraction on its own does not prove that both objects are charged, because a charged object can also attract an object that carries no overall charge.
5. Two rods of the same material, rubbed with the same cloth in the same way, always repel each other, because identical treatment gives them the same sign of charge.
6. A rod that repels a known negative rod must itself be negative, and a rod that attracts it must be positive, so a rod of known charge can be used to identify an unknown one.

11.1.3 Electrostatic phenomena

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.

11.1.4 Earthing and removing charge

Earthing needs a conducting path

1. A charged object has an imbalance of electrons: either more electrons than it needs to be neutral, or fewer.
2. The positive charge in a solid sits in the protons inside atomic nuclei, and those nuclei are fixed in the structure of the material.
3. The only charge that can travel through a solid is therefore the electrons, so every explanation of earthing is written in terms of electron flow.
4. The earth acts as an enormous reservoir of charge: it can accept or supply huge numbers of electrons without its own charge changing noticeably.
5. Earthing removes excess charge only if a conductor joins the object to the ground, because electrons need free charges in order to move.
6. A copper wire, a metal chain, a metal water pipe or a person's body all provide such a path, while dry plastic, rubber or glass does not.
7. Once the path exists, electrons flow until the object has no excess charge left, and the flow then stops on its own.
8. The object is now neutral, which means the number of electrons again balances the number of protons.

Earthing a negatively charged object

1. A negatively charged object has gained electrons, so it holds more electrons than it needs to balance its protons.
2. Connecting it to earth through a conductor gives those extra electrons a route away from the object.
3. The extra electrons repel one another, so they flow from the object to the earth.
4. As they leave, the amount of negative charge on the object falls.
5. The flow stops when the electrons again balance the protons, so the object ends up neutral.
6. The electrons have not been destroyed; they are now spread through the earth, where the effect of adding them is far too small to measure.

Earthing a positively charged object

1. A positively charged object has lost electrons, so it holds fewer electrons than it needs to balance its protons.
2. It does not have extra protons, and no protons move at any stage.
3. Connecting it to earth lets electrons flow from the earth to the object.
4. Each electron that arrives cancels part of the positive charge, because it replaces a missing electron.
5. The flow stops once enough electrons have arrived for the object to be neutral.
6. Electrons therefore flow towards a positive object and away from a negative one, so you must state the sign of the charge before you state the direction of flow.

Why insulators are harder to discharge

1. Charge on an insulator cannot travel through the material, so touching an earthed wire to one point removes charge from that point only.
2. The rest of the charge stays where it was placed, which is why a charged polythene rod is still charged after being held at one end.
3. A conductor discharges completely and almost instantly when it is earthed, because its free electrons can move throughout the whole object.
4. This difference is why a metal object held in the hand never appears charged, while a plastic one does.
5. Charge does leak away slowly from an insulator in damp air, because a thin film of moisture on the surface behaves as a weak conductor.
6. Warming and drying an insulator before charging it therefore makes static effects last longer and appear stronger.