In the last topic we saw how static electricity is produced: friction moves electrons from one insulating object to another, leaving one with a surplus and the other with a shortage. Charge that is stuck in one place like this does two things, and only two. It makes objects stick to each other, and if enough of it collects, it eventually jumps across a gap as a spark. Everything in this topic, the trouble it causes and the work we set it to do, is one of those two things.
Effects of static electricity
The effects of static electricity that have a direct influence on our daily life can be presented as follows.
| Effect | Which of the two behaviours it is |
|---|---|
| Thunder falling, or lightning flashing | Charge jumping across a gap, on an enormous scale |
| Accidents such as fire in a fuel tanker | A small spark jumping, next to fuel vapour that will catch light |
| Chips in a computer being damaged quickly when static charge collects in them | Charge jumping through parts far too delicate to survive it |
| Dust or lint sticking to clothes and to electrical goods such as a television, a computer or a mobile | Charged surfaces attracting small neutral things towards them |
Look at the last row of that table and then look at a television in your own house. The top of a wooden almirah in the same room collects far less dust than the screen does, even though both are flat, both face the room and both are dusted about as often. The screen collects more because it is charged. While the television is running, charge builds up on the front of the glass, and once it is charged it attracts every neutral speck of dust in the air by pushing the charges inside the speck around, exactly the way a rubbed comb attracts a piece of paper. The almirah has no charge, so it only collects the dust that happens to settle on it. This is also why wiping a screen with a dry cloth makes it worse within minutes: the rubbing of a dry cloth on glass is a rubbing experiment, and you have just recharged it. A slightly damp cloth works far better, because water conducts and carries the charge away as it goes.
The chain hanging from a fuel tanker
When fuel is being filled into a tanker, the nozzle of the pipe that fills the fuel is joined to the fuel tanker on one side and its other part is joined to the ground. Why might that be? When fuel is being filled into a tanker, the static electric charge that can be produced flows away underground in the form of charge. Because of this, that static electric charge and the vapour of the fuel cannot come into contact, and a fire is avoided.
Notice what the earthing wire actually does, because the same idea turns up again and again for the rest of this unit. A spark only happens when charge has piled up with nowhere to go, and then finally jumps. So if you give the charge an easy road to the ground before it piles up, it will take that road quietly and there will never be a spark at all. That is the whole of earthing. Petrol rushing through a hose rubs against the walls of the hose, exactly as a comb rubs against hair, and the tanker would slowly build up charge with nothing to attract except the petrol vapour sitting above the fuel. One spark into that vapour and the tanker is on fire. The wire and the clamp remove the problem before it begins. Look for this same idea in the rest of the unit: the lightning rod on a building, the earth pin in a three pin plug, and the wrist strap a computer technician wears are all the same sentence written three more times. Give the charge a road, and it will never take a shortcut.
Uses of static electricity
Static electricity is used for various tasks in our daily life, which are as follows.
| Where it is used | What the charge is doing |
|---|---|
| Printers and photocopy machines | Making the ink or toner attract to the photocopy paper |
| Paint sprayers | Making the equipment that sprays paint when paint is being applied |
| Air filters | Used in the equipment that filters air |
| Dust removal equipment | Used in the equipment that removes dust |
Inside a photocopier
A photocopier is worth taking apart in your head, because it is nothing but the comb and paper experiment done very cleverly. Inside the machine there is a drum that can be given a charge all over its surface. Light from the page being copied is shone onto that drum. Wherever the original page was white, light lands on the drum and knocks the charge off that part. Wherever the original page had black ink, no light arrives, so the charge stays. The drum is now carrying an invisible picture of your page, drawn entirely in charge.
Then fine black powder called toner is brought near the drum. The toner carries the opposite charge, so it is attracted only to the charged parts and ignores the rest, and the invisible picture becomes a visible one. The paper is given a charge as well, stronger than the drum, so it pulls the toner off the drum and onto itself. Finally hot rollers melt the toner so that it sticks permanently, which is why a page fresh out of a photocopier is warm. Not one of those steps is new to you. Opposite charges attract, like charges repel, and a charged object can pick up something small. That is the entire machine.
Why a charged spray paints better
An ordinary spray produces droplets of many sizes that clump together and land unevenly, and a great deal of the paint drifts past the object and is wasted. An electrostatic sprayer gives every droplet the same charge as it leaves the nozzle, and that one change does two jobs at once. First, like charges repel, so the droplets push each other apart and arrive as an even fine mist instead of blobs. Second, the object being painted is earthed, so it takes on the opposite charge and pulls the droplets towards itself. Some droplets even curve round to the far side of the object, which is why an electrostatic sprayer can coat the back of a pipe or a chair leg without the sprayer being pointed at it.
Air filters and dust removal
An ordinary cloth or paper filter works by having holes too small for dust to pass through, and the trouble with that is obvious: the smaller you make the holes, the harder it becomes to push air through at all. A static air filter takes a completely different approach and has no holes to block. Dirty air is drawn past a wire that gives every speck of dust a negative charge, and further along the duct there are metal plates carrying a positive charge. Opposite charges attract, so the dust leaves the moving air and sticks firmly to the plates, and clean air passes out the other end with nothing in its way. From time to time the plates are washed and the dust is thrown away. The same design, made much larger, is used in factory chimneys, where it removes ash and smoke particles before they leave the chimney and reach the air we breathe.
Put the two lists side by side and something ought to look familiar. Dust sticking to a television screen is a nuisance, and toner sticking to paper is a photocopy, and they are the same event. Paint spreading out in a fine mist is useful, and hair spreading out on your head is annoying, and they are the same event too. A spark near petrol destroys a tanker, and a spark in the sky is lightning, and again they are the same event. Static electricity is not sometimes helpful and sometimes harmful, as though it were choosing. It always does exactly one thing, which is to attract, repel or jump, and whether we file that under useful or under dangerous depends entirely on whether we wanted it to happen there. You met this idea once before, in the unit on force, where friction was called both a friend and an enemy for exactly the same reason. It is worth noticing that science keeps returning to this point: nature has no opinion about what is convenient for us.
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