Science Puzzle
How Does a Bike Dynamo Make Electricity?
An old-fashioned bicycle dynamo is a little bottle-shaped gadget clipped to the frame, with a small wheel pressing against the tyre. Ride along and the lamp lights up; the faster you go, the brighter it shines. Stop, and it goes out.
There's no battery. Where does the electricity come from?
The Answer
From a spinning magnet. Inside the dynamo, the tyre turns a magnet next to a coil of copper wire, and a moving magnet pushes electricity round a wire. The energy comes from your legs.
As the magnet spins, its field sweeps back and forth through the coil. Whenever the magnetic field passing through a coil of wire changes, it pushes the electrons in the wire along, making a current. A magnet sitting still next to the coil does nothing; it's the change that counts. That's why the lamp goes out when you stop and shines brighter as you speed up, because a faster spin means a faster change.
Nothing is free, though. The current flowing in the coil makes its own magnetic field, which pushes back against the spinning magnet. You feel that as a little extra effort on the pedals, and that effort is where the lamp's energy really comes from.
Michael Faraday discovered this in 1831. Almost all the electricity in the world is made the same way: power stations and wind turbines spin magnets and coils past each other, on a far bigger scale than your bike.
The principle: Electromagnetic induction. A changing magnetic field through a coil of wire pushes a current round it, so spinning a magnet next to a coil turns movement into electricity.