The transformer

How can you use the same power outlet to charge your phone and power your TV? The wall socket has 230 volts, but the phone doesn't need more than five. Why doesn't the phone burn up with that high voltage? No, it doesn't go up in flames so something must happen on the way to the phone. It's time to open the case and find out what's inside.

We want to reduce the voltage from 230 volts in the socket to five, to charge the phone. To help us reduce the voltage, we have a piece of iron. And around this iron core, there's some copper wire wound up on each side. If you look carefully, you'll see that there are more turns on the left than on the right. When a conductor is wound like this, it's called a coil.

Now, let's put the plug in to get some voltage over to the left coil. A current is now moving through the copper wire, and a current moving through a conductor creates a magnetic field around that conductor. If you have ever built an electromagnet, you have seen how this works. By winding the copper wire several turns in the coil, the magnetic field gets amplified and becomes really strong inside the coil. The more turns we have, the stronger the magnetic field is.

And since we feed an alternating current into the coil, it changes direction back and forth. So, the magnetic field does the same thing. The iron core inside the coil makes the magnetic field even stronger, and allows the magnetic field to spread to the other side where there is also a coil. Now the magnetic field changes inside the right coil as well. The coil gets so enthused by this, that it dances on along.

The electrons inside the copper conductor sensed the magnetic field, and it started to move. And moving electrons make up an electric current. We have induced a current in the right coil. On the left, a current creates a magnetic field, and on the right, the magnetic field creates a current. The iron core increases the magnetic field, and transfers it from one coil to the other.

Pretty neat, huh? If all we wanted was to transfer a current, we might as well have wired a conductor from left to right and be done. But, we want to reduce the voltage too. And this is why it matters how many times the conductor winds around the core in each coil. The side with the most turns will have the highest voltage.

And we want to reduce the voltage, so there should be fewer turns in the right coil than in the left one. The left coil, which the current hits first, is called the primary coil, and the right one is called the secondary coil. We have 230 volts in the primary coil. If we wanted to divide that in half, we would need half as many turns in the secondary coil as in the primary one. But we want to go from 230 volts to five, so we write that as a fraction.

Secondary voltage divided by primary voltage. If we know that the primary coil has 460 turns, how many turns should the secondary coil get? Call the number of turns in the secondary coil X and solve the equation. The secondary coil must have ten turns to reduce the voltage, so that we could charge the phone. Now, we have transformed the voltage from 230 to five volts.

If we wind the right coil using more turns - say, 920 turns- then we increase the voltage. But wait a moment; where does that increased voltage come from? We can't create extra energy from thin air, can we? Well, when we increase the voltage, the current is reduced. Doubling the voltage means halving the amperage.

Well, not exactly. Some of the energy is leaking out as heat. Feel for yourself. A transformer, a good thing to have if you want to increase or decrease the voltage of an alternating current. Or, to warm your feet on a cold morning.