Changing Voltage With Magnetism

Right back at the start, we saw how electricity is generated by subjecting coils of wire to a varying magnetic field. Well, you do not have to vary the magnetic field by physically moving the magnet. Instead, you could use a stationary electromagnet, and change the amount of magnetism that it produced by varying the current.

In other words, passing an alternating current into an electromagnet will give you a magnetic field that also alternates, flipping from north-south to south-north. If you wrap a coil of wire around this electromagnet, you will generate an alternating current in the coil.

If you take a piece of iron or steel and wrap a coil of wire around it, and pass a current through the coil, you have an electromagnet. Wrap another coil of wire around it, separate from the first, and a current will be generated in it by the magnetic field. But what's the point of turning electricity into magnetism, then turning it straight back into electricity again?

There's a nice little trick here. What if the two coils are not identical?

It turns out that if there are more turns of wire on the output coil than there are on the input coil, then you will get more voltage out than you put in! In fact, if there are ten times more turns of wire on the output coil than the input coil, you will get ten times the voltage out. This arrangement of coils and iron is called a step-up transformer, because it steps the voltage up. Of course, there's no such thing as a free lunch. The amount of power you get out is the same as you put in. (Well, actually between 98% and 99.5%, with the rest lost as heat.) So if  you get more voltage out than you put in, you must be getting less current out than you put in. If the voltage was multiplied by 10, the current must have been divided by 10.

Alternatively, you can make a step-down transformer to reduce the voltage. As you might guess, this has fewer turns of wire on the output coil than the input coil. It reduces the voltage, and therefore increases the current.

A transformer will only work on alternating current, as a varying magnetic field is needed. Applying a direct current to a transformer would not change the magnetic field, so no current would be generated in the output coil. This is why we use AC.

Now you can see the basic form of an electricity network. Electricity is generated in a power station, probably at around 25kV (25,000 Volts). It is passed through step-up transformers to increase the voltage to somewhere between 132kV and 400kV and transmitted over long distances with low power loss, then passed through step-down transformers and reduced to lower voltages for distribution around towns and cities. Electricity will be sent at 11kV to your local substation (a substation is just a step-down transformer and some circuit breakers), and be sent the final few tens of metres to your home at 230V ready for use. In a rural area, or in North America, there might be no substation. Instead there will be a smaller step-down transformer on a pole outside your house.

Next, we will sum up the basics of electricity.