11.2.1 Uses of electrostatic charge

Charging a spray to control where it lands

1. An uncharged spray relies on the nozzle alone, so each droplet travels in whatever direction it happens to leave the nozzle and much of the liquid misses the target.
2. An electrostatic sprayer gives every droplet an electric charge as it forms, so the forces between charges then control where the spray goes.
3. All of the droplets are given the same sign of charge, which makes them like charges, so they repel one another.
4. That repulsion pushes the droplets apart, so they spread out into a fine, even cloud instead of joining together into large drops.
5. Small, well separated droplets coat a surface more evenly and cover a larger area for the same volume of liquid.
6. The target is given the opposite charge, either directly or by earthing it, so the droplets and the target are unlike charges and attract.
7. Because that attraction is a non-contact force, the droplets curve towards the target instead of travelling in straight lines.
8. Droplets are therefore pulled onto surfaces the nozzle is not pointing at, including the sides and the far face of the object.
9. Once part of a surface is coated, the charge already sitting there repels further droplets towards the bare areas, so the coating tends to even itself out.
10. The two forces have separate jobs: repulsion spreads the spray out, and attraction delivers it to the target.

Insecticide sprayers

1. A crop sprayer charges the insecticide droplets as they leave the nozzle, so they repel one another and spread across the full width of the spray boom.
2. The crop is connected to the ground through its stems and roots, so it behaves as though it carries the opposite charge to the droplets and attracts them.
3. Droplets are pulled onto the undersides of the leaves as well as the top surfaces, which matters because many pests feed underneath.
4. Less insecticide drifts away on the wind or falls onto the bare soil between the rows.
5. A smaller volume of insecticide gives the same protection, so the cost to the farmer falls.
6. Less chemical reaches the soil, hedgerows and nearby watercourses, so contamination of the environment is reduced.
7. The spray still has to be used with care, because charge cannot hold onto droplets that have already drifted well away from the charged crop.

Paint sprayers

1. In a spray booth the paint is broken into a mist of tiny droplets and each droplet is charged as it is formed.
2. The droplets all carry the same sign of charge, so they repel one another and spread into an even mist rather than arriving as blobs.
3. The metal object being painted, such as a car body or a bicycle frame, is earthed or given the opposite charge.
4. The droplets are attracted to every part of that metal surface, including edges, curves and the back of each tube, which a directed spray would miss.
5. Paint that would otherwise be lost as overspray is drawn onto the object instead of settling on the floor and walls of the booth.
6. The layer of paint is more even in thickness, so fewer coats are needed and the finish is smoother.
7. Wasting less paint saves material and reduces the volume of solvent released into the air of the workshop.

Photocopiers and laser printers

1. A laser printer starts by giving a rotating drum an even coating of charge across its whole surface.
2. A laser beam is then scanned across the drum and removes the charge from the areas that will be left blank on the page.
3. The charged pattern left on the drum is an image of the print, held in place because the drum surface is a poor conductor.
4. Toner powder is given the opposite charge to the drum, so it is attracted to the charged areas and stays away from the discharged ones.
5. The paper is then charged more strongly than the drum, so the toner is attracted off the drum and onto the sheet as it passes.
6. Heated rollers melt the toner so that it fuses into the paper fibres, and the image no longer depends on charge once it has cooled.
7. The same rule does all of the work: unlike charges pull the powder to the places that should be printed, and like charges keep it away from the rest.

11.2.2 Dangers of sparking

How a spark forms

1. Static charge builds up whenever insulating materials rub against each other, and it stays where it is because there is no conducting path away.
2. Liquid fuel flowing quickly along a hose or pipe rubs against the inside wall, so electrons are transferred between the fuel and the pipe and both become charged.
3. Powders such as flour, sugar or grain dust charge up in the same way as they are poured, tipped or blown along ducting.
4. Clothing, plastic containers and vehicle bodywork all charge up through ordinary movement and contact.
5. As the charge collects, the potential difference between the charged object and nearby earthed metal grows larger.
6. When the potential difference across the air gap becomes large enough, the air stops behaving as an insulator and charge crosses it.
7. That brief flow of charge through the air is a spark, and it lasts only a tiny fraction of a second.
8. All of the stored energy is transferred in that very short time, so the air in the spark channel reaches a high temperature and glows.

Why sparks are dangerous

1. A spark is dangerous mainly because it is a source of ignition in places where flammable substances are present.
2. Petrol vapour, diesel vapour, solvents, aerosol propellants, methane and fine dusts all catch light readily once they are ignited.
3. A vapour or dust mixed with air burns far more easily than the liquid or solid it came from, because the fuel is already spread through the oxygen it needs.
4. Even a small spark can transfer enough energy to start that reaction.
5. If the mixture burns inside an enclosed space, the hot gases expand rapidly and the result is an explosion rather than a steady fire.
6. Filling a car with petrol is a familiar risk, because petrol evaporates easily and vapour collects around the open filler neck.
7. Charge can build up on the driver, on the car body, on the nozzle or on a plastic fuel container during filling.
8. Transferring fuel between a road tanker and an underground storage tank moves large volumes very quickly, so far more charge can be separated.
9. Aircraft refuelling, solvent and paint handling, and grain, flour and sugar silos all carry the same hazard.
10. Sparks are also a hazard around sensitive electronics, because a discharge through a microchip can destroy it permanently.

Preventing the build-up by earthing

1. Earthing deals with the hazard at its source by stopping charge from collecting in the first place.
2. A conducting path lets electrons flow to or from the earth as quickly as the charge is separated, so the charge never reaches a dangerous level.
3. With no large build-up, the potential difference across any air gap stays small and no spark can form.
4. A road tanker is connected to an earthed point with a bonding cable before any fuel is transferred, and the cable stays attached until the transfer is complete.
5. Aircraft are earthed and bonded to the fuel tanker for the same reason before refuelling begins.
6. Metal pipework, nozzles and filters in a fuel system are bonded together with conductors, so that no two parts of the system can sit at different potentials.
7. Fuel must be dispensed into a metal container standing on the ground rather than a plastic one resting on a car boot, because the metal container can be earthed and the plastic one cannot.
8. Drivers are advised to touch the metal bodywork before picking up the nozzle, so any charge on them is removed before they are close to the vapour.
9. Antistatic additives in the fuel, conducting hoses and conducting flooring all do the same job by making otherwise insulating parts of the system slightly conducting.
10. Limiting the flow rate of the fuel also helps, because slower flow rubs less and separates less charge.
11. Humidifying the air in a factory reduces static build-up as well, because a thin film of moisture on surfaces lets charge leak away steadily.

11.2.3 Electric fields

What an electric field is

1. A charged object changes the space around it, so that any other charge placed in that space experiences a force.
2. That region of space is the object's electric field, and the field is there whether or not a second charge is present to feel it.
3. The field is what explains how two charged objects can push or pull each other without touching.
4. The field is strongest close to the charged object and becomes weaker as the distance from it increases.
5. A field has a direction as well as a strength, so it is drawn on diagrams using arrows rather than shading.
6. By agreement, the arrow on an electric field line shows the direction of the force that would act on a positive charge placed at that point.
7. A negative charge placed at the same point feels a force in exactly the opposite direction to the arrow.
8. Field lines are never drawn crossing one another, because a single point cannot have two different field directions.
9. A charged object small enough for all of its charge to be treated as acting at one place is called a point charge.

Fields around point charges

1. The field around an isolated point charge is radial, which means the field lines are straight and spread out evenly in every direction.
2. Around a positive point charge the lines point away from the charge, because a positive charge placed nearby would be repelled.
3. Around a negative point charge the lines point towards the charge, because a positive charge placed nearby would be attracted.
4. Every field line meets the surface of a charged sphere at right angles to that surface.
5. Close to the charge the lines are packed tightly together, and further away they fan out so the gaps between them widen.
6. That widening is how the diagram shows the field getting weaker with distance, which is why the force on a charge falls as it is moved away.
7. The pattern is the same shape for both signs of charge, so only the arrows tell you whether the central charge is positive or negative.
8. Putting more charge on the sphere strengthens its field at every distance, which is drawn as a larger number of field lines rather than longer ones.

Fields between parallel plates

1. Two flat metal plates placed parallel to one another and given opposite charges set up a field in the gap between them.
2. Between the plates the field lines are straight, parallel to one another, equally spaced, and at right angles to the plate surfaces.
3. The lines run from the positive plate towards the negative plate, matching the direction of the force on a positive charge.
4. Equal spacing means the field has the same strength at every point in the gap, so it is a uniform electric field.
5. A positive charge released between the plates is pushed towards the negative plate, in the same direction as the field lines.
6. A negative charge released in the same gap is pushed the other way, towards the positive plate.
7. The field is only uniform well inside the gap; near the edges of the plates the lines curve outwards and the field is weaker there.
8. Increasing the potential difference between the plates, or moving the plates closer together, makes the uniform field stronger.

Reading field strength from a diagram

1. The spacing of the field lines is what shows the strength of the field, and nothing else on the diagram does.
2. Lines drawn close together show a strong field, so the force on a given charge placed there is large.
3. Lines drawn far apart show a weak field, so the force on that same charge is smaller.
4. Widely spaced lines still mean a field is present, only a weaker one; a region with no lines at all is the only region with no field.
5. In a radial field the lines are closest at the surface of the charge, so that is where the field is strongest.
6. In the uniform field between parallel plates the spacing never changes, so no point in the gap is stronger than any other.
7. The length of a field line carries no meaning, and neither does the thickness of the line.
8. When two diagrams are compared, the one with the more crowded lines at a given point represents the stronger field at that point.

11.2.4 Electric fields and static electricity

Why the field model is needed

1. Every static effect involves a force between objects that are not touching, so it has to be a non-contact force.
2. Saying only that charges attract or repel does not explain how one object can affect another across an empty gap.
3. The electric field fills that gap: the first charge sets up a field in the space around it, and the second charge feels a force because it is sitting in that field.
4. The size of the force on the second charge depends on its own charge and on the strength of the field where it sits, not on any contact between the objects.
5. Because the field grows weaker further from its source, the force falls as the objects are separated, which is exactly what is observed.
6. Both objects set up fields and both sit in the other's field, which is why the two forces always come in a pair of equal size.
7. The field is a model, so it cannot be seen directly, but every prediction it makes about direction and strength can be tested.

Attraction and repulsion as field effects

1. Near a positive charge the field points away from it, so a second positive charge placed there is pushed outwards and the two objects repel.
2. A negative charge placed in that same field feels a force opposite to the arrows, so it is pulled inwards and the two objects attract.
3. The like-and-unlike rule is therefore not a separate fact to memorise; it follows from the direction of the field and the sign of the charge placed in it.
4. Moving a charged rod closer to another charged object moves it into a stronger part of the field, so the force it feels grows.
5. Rubbing a rod for longer puts more charge on it, which makes its field stronger at every point and increases the force on anything nearby.
6. The field also explains why the effect fades with distance rather than stopping abruptly, because the field lines fan out gradually.

Explaining attraction of uncharged objects

1. A field acts on the charges inside a neutral object as well, even though that object carries no overall charge.
2. Electrons in the neutral object are pushed one way by the field while the fixed positive nuclei are pushed the other way, which produces induced charge separation.
3. The side of the object that ends up with the charge opposite to the nearby charged object is also the side closest to it.
4. That near side sits in the stronger part of the field, so the force there is larger than the force on the far side.
5. The attraction on the near side therefore beats the repulsion on the far side, and the neutral object is pulled towards the charged one.
6. The field model explains this attraction, which the like-and-unlike rule on its own cannot do because the neutral object has no overall charge.
7. The effect disappears as soon as the charged object is removed, because the field that separated the charges has gone and the electrons move back.

Explaining sparks and shocks

1. As charge collects on an object, the electric field in the air around it becomes stronger.
2. Air molecules contain charges of their own, so a strong enough field pulls electrons away from them, and this is ionisation of air.
3. The freed electrons and the charged molecules left behind are both free to move, so the ionised air has become a conductor.
4. Charge then flows through that narrow conducting channel all at once, which is the spark, and the object loses its charge.
5. This is why a spark needs either a large build-up of charge or a small gap, because both of those make the field in the air stronger.
6. Field lines crowd together at sharp points and thin edges, so the field is strongest there and sparks tend to start from corners and points rather than flat surfaces.
7. The same reasoning covers every scale, from the click felt when touching a door handle to a lightning stroke several kilometres long.