Storm in a Teacup: The Physics of Everyday Life(78)

So the basic setup used both to discover the electron itself and to run the Large Hadron Collider at CERN, a controlled stream of charged particles in a vacuum, also sat in a corner of many homes until very recently. These days, the bulky CRT televisions have been replaced almost entirely by flat screens. In 2008, sales of flat panel displays overtook CRT screens worldwide, and the world has never looked back. The switch made laptops and smartphones possible because it made them portable. The new displays are also controlled by electrons, but in a much more sophisticated way. The screen is split into many tiny boxes called pixels, and electronic control of each pixel determines whether or not it gives out light. If you have a screen resolution that is 1280 × 800 pixels, that means that you’re looking at a grid made up of just over a million individual dots of color, each separately switched on and off with tiny voltages, and each updated at least sixty times every second. It’s an astonishing feat of coordination, but it’s still trivial compared to what your laptop gets up to.

Let’s return to the magnets. A magnetic field can push electrons around, and so it can control electric currents. But that’s not the limit of the interlinking of electricity and magnetism. Electric currents also make their own magnetic fields.


AS WE SAW in chapter 5, toasters heat toast very efficiently using infrared light. But the real brilliance of a toaster isn’t that it provides lots of heat—your grill can do that; it’s that it knows when to stop. The universal rule of toasters is that the bread only disappears down into the innards of the toaster when you press on a lever at one side. If you don’t press it all the way down, it pops right back up. But if you push the lever all the way to the bottom, there’s a click and it stays put until it’s time for the hot toast to pop out of its mini-furnace. I don’t need to stand over it, checking how well browned the bread is. When the bread has turned into toast, there will be another mechanical click and the toast will pop up by itself. So as I prowl around the kitchen, looking for butter and jam, something is holding the bread in place.

There’s a beautiful simplicity about what the toaster is up to. When you put the bread in place, it rests on a spring-loaded tray. The springs underneath are pushing the bread up to its “popped” position, high above the heating elements. But you’re strong enough to push the bread down in spite of those springs. And once the tray reaches the bottom of the toaster, a protruding bit of metal fills the gap in not one, but two circuits. One of those circuits deals with the heating, so electricity starts to flow around the toaster to heat the bread.

But the other circuit is a lot more interesting. The electrons in that circuit shuffle along and around a section of wire that is wrapped around a small lump of iron. It’s a bit like a helter-skelter ride for electrons—they spiral around and around the iron and then out and along the rest of the circuit back to the plug socket. That’s all. But because magnetism and electricity are so deeply intertwined, when an electric current runs through a wire, it creates a magnetic field around that wire. Sending electrons around a coil of wire means that each time the electrons loop around, they’re adding to the same magnetic field. The iron core in the middle of the coil reinforces the magnetic field and makes it even stronger. This is an electromagnet. When an electric current is running through the wire, it’s a magnet. When the current stops, the magnetic field goes away. So when you push down the lever on the toaster, you’re switching on a magnetic field at the base of the toaster that wasn’t there before. Since the bottom of the bread tray is made of iron, it sticks to the magnet. In other words, while I’m poking about in the fridge, a temporary magnetic field is holding the bread tray in place. The toaster has a timer on the side, and the clock starts when the circuits are connected. When the time is up, the timer cuts the power to the whole toaster. Since there’s no power to the electromagnet, it stops being a magnet. Nothing is holding the bread down anymore, so the springs pop it up.

I sometimes forget that I’ve unplugged the toaster, but I find out pretty quickly. If I try to push the lever down, it pops straight back up, even if I push it down all the way. That’s because there’s no power to the electromagnet, so it can’t hold the bread tray down. It’s such a simple system, and stunningly elegant. Every time you make toast, you’re taking advantage of this very fundamental connection between electricity and magnetism.

Electromagnets are very common because it’s really useful to be able to switch magnets on and off. They’re in loudspeakers and electronic door locks and computer disk drives. They must be continually powered, otherwise the magnetic field vanishes. The kind of magnets you stick on your fridge are called permanent magnets—you can’t turn them on or off, or change the magnetism, but they don’t need any power. Electromagnets do exactly the same job as a fridge magnet when they’re turned on, but they can conveniently be turned off just by stopping the current.

We’re surrounded by small, local magnetic fields, some permanent and some temporary. They are almost all made by humans, either to do a useful job, or as a by-product of something that’s doing a useful job. The magnetic fields don’t reach very far, and so they’re only detectable very close to the magnet. But these are just tiny local glitches in a much bigger magnetic field that stretches around our planet, and this one is entirely natural. We can’t feel it, but we use it all the time.