Storm in a Teacup: The Physics of Everyday Life(75)

This is why laptops, vacuum cleaners, and hair dryers heat up as you use them. The electrical energy must go somewhere, and if it’s not converted into other kinds of energy, heat is the inevitable end. Hair dryers use this to heat air; their circuits are arranged to dump energy as heat in a very concentrated way. But laptop manufacturers hate heat, because hotter circuits work less efficiently. There is no way of using electrical energy without paying a heat tax.#

So the electrons flow because an electric field is pushing on them. A battery doesn’t really provide electrons—there are plenty of those in the world. What it does is provide the electric field to move electrons. And if the circuit is complete, this electric field will push electrons around the loop. So far, so simple. But what are all those numbers on plugs and in tiny font on the safety warnings? Perhaps it’s best to take the typical British approach to all problems: Find the cookie tin and put the kettle on.

The most important thing about a tea break is that it involves both tea and a break. Some of my American coworkers never really understood this, and used to bring along work to continue discussing it over tea. But for the British, the act of “putting the kettle on” signifies a change of pace. I’m going to do it now, and in this case my kettle is an electric one that I simply fill with water and plug into the power outlet. My mind is allowed to stop working for a bit, while the kettle gets on with its job.

Pushing down on the switch does one very simple thing. It shifts a bit of metal and thereby slots the last segment of a circuit into place. Now there’s a route through the maze of the kettle, a path made entirely of electrical conductors that electrons can easily travel along. This path is now uninterrupted and it runs from one pin of the plug, through the kettle, and back to the second pin of the plug. In this case, the electric field comes not from a battery, but from a plug socket.

A standard three-pin plug has one long pin at the top. That’s called the ground pin. It’s completely separate from the rest of the circuit. Effectively, it’s doing the job that my car did on those cold snowy mornings—it’s there to provide an escape route if any electrons start to build up in the wrong place (say, on the outside of the kettle). So that’s not part of the path that’s going to power the kettle.

The other two pins, the smaller ones, are going to do the electron-pushing. One of them behaves like a fixed positive charge, and one like a fixed negative charge. As I press down on the switch, I connect up a path that now has an electric field running along it. Electrons along that path are feeling a push away from the negative side and a pull toward the positive side. So as I find the teapot and dig out teabags, the electrons start to shuffle. They’re jiggling around quite a bit anyway, but now they have a slight tendency to drift down the wire. And what that means is that overall, there’s a movement of electric charge from one pin of the plug, through the kettle, and out through the other pin of the plug.

On the bottom of my kettle, a label tells me that it’s designed to work at 230 volts (230V). The voltage is related to the strength of the electric field that’s pushing electrons along the circuit. The stronger the electric field, the more energy each electron has to get rid of along the way. That’s what a high voltage is telling you—it’s saying that this is the amount of energy available for use along the path of the circuit. In terms of the slide analogy from earlier on, the voltage is the height of the slide that the electrons have to shoot down before getting back to the other pin of the plug. The higher the voltage, the more energy each electron has to dump on the way.

I’ve swilled out the teapot and put the teabags in it, the milk and a mug are out and ready. Now I’m just waiting for the water to heat up. It only takes a couple of minutes, but when I’m thirsty, I’m very impatient. Hurry up! I know what the voltage of the electrical supply is, but that’s only part of the story. The higher the voltage, the more energy each electron can give up. But that doesn’t say anything about how many electrons are passing through. The fastest way to dump lots of energy in the water is to make sure that lots of electrons are flowing around the circuit. That’s what an electrical current is, and we measure it in amps. The higher the current, the more electrons are moving past one point in the wire in any one second. When you multiply the voltage of the supply by the current (in amps) flowing through the circuit, you get the total amount of energy deposited per second. My kettle runs from a 230V supply, and can draw a current of 13 amps, and 230 × 13 = 3,000 (approximately). The base of the kettle agrees—it says that the kettle power is 3,000 watts (3,000W), which equates to 3,000 joules of energy released per second. That’s enough to heat my water to boiling in just less than two minutes, but it will lose a bit of heat to the surroundings, so in practice it takes closer to two and a half minutes.

I’ve no intention of testing this out while I’m waiting for my tea, but they say “volts jolt, current kills.” The voltage difference between me and my car on that snowy day in Rhode Island was probably 20,000 volts. But only a tiny amount of electrical charge went anywhere, so it didn’t do me too much harm. The current was tiny and very little energy was transferred. If I connected up the path between the two plug terminals with my fingers, so that my body took the place of the kettle, it would be a different story. A high current means that there are lots of electrons, each carrying the same amount of energy. The total amount of energy is huge, because so many electrons are rushing through. It would be far more dangerous than the shock from the car, even though the voltage difference across the pins of the kettle is only about a hundredth of the voltage difference between me and my car. It’s the current that matters most when it comes to potential harm to you.