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.
As the electrons shuffle through the metal of the heating element, they’re being pushed by the electric field. That makes them speed up slightly, but the conductor is made up of lots of atoms, and so these sped-up electrons inevitably bump into things. When they bump, they lose energy, heating up whatever they bumped into. And so forcing lots of charge to move means that there’s lots of bumping and lots of heating. That’s all the kettle is doing—speeding up electrons so that they bump into things and pass on their energy as heat. The electrons themselves don’t travel very far at all—they might drift at about 0.04 inch per second. But it’s enough.
Boiling water has loads of extra energy, and it’s amazing that it gets there just from minuscule electrons shuffling about and bumping into things. Amazing, yet undeniable; my tea is ready, heated by electric fields pushing on electrons in a conductor. This is the simplest possible use for electrical energy: converting it directly into heat. But once people had worked out how to build circuits and power supplies and batteries, things got much more sophisticated very quickly.
There is a fundamental difference between the electron dance generated by batteries (any batteries) and what happens when you plug a device into the wall outlet. In any device powered by a battery, the electrons are always flowing in one direction only. This is called direct current, or DC. A standard AA battery will supply about 1.5 volts DC. But the wall current is different—it’s alternating current, or AC. That means it switches direction about a hundred times a second.** It turns out to be more efficient if you run your electricity supply like this.
You can switch between DC and AC, but it’s a bit of a nuisance. Anyone who carries around a laptop power cable will be familiar with this kind of nuisance—it’s the small heavy block that sits in the middle of the cable. It’s called an AC/DC adapter, and its job is to convert the AC current from the outlet into the kind of DC current that your laptop wants (which is what the laptop battery provides directly). To do that, it needs coils of wire and a bit of circuitry, and it’s still tricky to make all the necessary bits any smaller.?? So for the time being, we’re stuck with carrying around the adapters.