How electricity makes things work
Without electricity our appliances are just lumps of plastic and metal. But what does electricity really do? How does it make things work?
We use it every day, but most of us haven't got a clue how electricity makes things work. What's going on in the wires that make motors move, and heaters heat?
Whether it's a toaster or an electric car, everything that electricity does comes down to one thing: what happens when you teach electrons to line dance.
When electrons are forced to move in synch, they can produce heat and — way more impressive — they turn the wire they're moving in into a magnet. Heat can boil water and make light bulbs glow, and magnets can make things move. And those two tricks are behind the 'magic' of every electrical appliance.
Getting electrons organised
The electrons that give our appliances their zing are in the wires that make up the circuits.
Wires are made of metal, and metals have always got loose electrons buzzing around throughout them. But if you can make those electrons move in an organised way, you've got an electric current flowing. That's all an electric current is — electrons moving in an organised way.
The energy to get the electrons moving in an organised way comes from either a battery or a generator.
When a battery organises electrons they all move in the same direction at the same time — the battery pumps electrons through the circuit wires from the negative terminal to the positive. Because they're all going in one direction, it's called a direct current (DC).
The electricity generators at power stations organise electrons in a slightly different way. They pump electrons, but they change the direction they're pumping them 100 times every second. So instead of moving along in one direction like in a DC circuit, the electrons stay pretty much where they are and constantly jiggle forwards and backwards. If you could see inside the power cord when an appliance is turned on, you'd think the electrons had just learned how to line dance — they're all constantly taking one step forward, one step backwards in synch. The constantly changing direction is what's behind its name, alternating current (AC).
So a current is just electrons moving in an organised way in a circuit. But how do electrons on the run make the heat that's behind toasting, drying and foot warming?
Hotting things up
All wires get a little bit hot when they've got a current running through them, because as the electrons move in the wire they bang into the metal atoms. And whenever they prang into an atom, energy from the moving electrons gets given off as heat.
We use copper for electrical wiring because it's easy peasy for electrons to move around in, so not too much energy gets wasted as heat. But if it's heat you want, say for your hairdryer/toaster/electric jug, it's dead easy to get. You just need to use a bit of metal that's really hard for electrons to move through, like nickel.
Heating elements like the ones in toasters or hairdryers are bits of wire made of a nickel/chromium alloy called nichrome. Run a current through nichrome and you'll get some serious heat. While the electrons in the copper wires can move around easily, the ones in the nichrome element are constantly banging into the nickel and chromium atoms and leaking heat all over the place. Which is just what you want on those wet-haired, stale bread days.
But heating is only one of the things electric appliances can do. Most of the other things involve making things move - and that involves a motor. So how do organised electrons make a motor spin?
Getting motors into a spin
Every appliance with moving parts more complex than a pop-up toaster has got an electric motor in it. And while they run thousands of different gadgets, electric motors really just do one thing — they spin whenever you turn on the power. And anything attached to them — like fan blades, wheels or washing tubs — spins too.
The spinning only happens when current is flowing — when electrons in the wire are organised into a current.
So how do moving electrons make a motor spin? They don't. They do something way more swanky — they turn wires into magnets. And as any five-year-old knows, magnets are great for making things move.
We've all mucked around with magnets, but what a lot of us don't realise is that magnets get their properties from the same thing that electricity does: organised electrons.
Electricity and magnetism … talk about co-dependent
Every electron is like a tiny, weak magnet. Most electrons hang around in pairs, and they cancel each other's magnetic property out. But some materials — like iron — have got some unpaired electrons around their atoms. And if you can get those unpaired electrons to line up so their magnetic fields are all pointing in the same direction, your piece of iron is suddenly a magnet. Which is exactly what happens when you stroke a needle or paperclip with a magnet — the magnetic field around your magnet pulls some of the unpaired electrons in the needle into lines, so all their mini-magnetism adds up to a full scale magnet.
But you can also make any metal into a temporary magnet — an electromagnet — just by running an electric current through it.
Electromagnets work because the charge on an electron can create a magnetic field too, but only when it's moving. So any time electrons in a wire are moving in synch (ie whenever a current is flowing), the wire becomes a magnet. It's too weak to be a useful magnet as it is. But if you coil the wire around a piece of iron, the weak magnetic field around the wire forces unpaired electrons in the iron to line up, and all their mini-magnetism adds up just like in a bar magnet.
But unlike a regular magnet, the wire is only magnetic while the current is flowing — once it stops, the electrons in the wire get back to acting like sub-atomic dodgems. And the piece of iron its wrapped around goes back to being a piece of iron.
And it's the ability of an electric current to turn wires into temporary magnets that makes it possible for us to have motors that can be switched on and off.
How motors work in 25 words or less …
If you've ever used one magnet to repel another, you already know the basics of how electric motors work. In fact, if you used the north end of one magnet to push the north end of another magnet around in a circle, you'd be doing pretty well the same thing an electric motor does. Except a motor doesn't have a giant hand pushing one magnet to repel another — it relies on a set of magnets in a ring surrounding a loop of wire.
When the current flows, the wire loop becomes an electromagnet. And the magnets around the electromagnet are set up so their attractive and repulsive forces cause the electromagnet to constantly spin until the power is cut.
When the off switch is hit, it's game over. Without the battery or generator to push them, the electrons are no longer organised, the wire is no longer magnetic, and the motor's spin comes to a halt. The pumps/fan blades/belts attached to the motor stop sucking, blowing and driving.
The electrical 'magic' stops, and the appliance is just a lump of plastic and metal until the next time we turn it on.
Thanks to Ian Sefton from the Physics Research Education Group at The University of Sydney.
Published 07 July 2010