The bicycle dynamo

Figure 13.23 shows the parts inside a bicycle dynamo. When its knurled wheel is set to touch a bicycle tyre, the wheel spins quickly as the tyre turns, and so does the cylindrical magnet joined to it. As the magnet turns, the magnetic field linking the coil wound on a soft iron core changes, and so an emf is induced in the coil.

• Winding the coil on soft iron gathers the field lines and guides them through the coil, so more field lines link the coil and a larger emf is induced.
• As the magnet turns, the direction of the field reverses back and forth, so the induced current reverses too. So a bicycle dynamo gives an alternating current.
• When the bicycle is pedalled faster, the wheel turns faster, the dynamo wheel touching the tyre turns faster, and the magnet spins faster. The field linking the coil changes faster, the induced emf rises, and a larger current is given. That is why the bicycle lamp gets brighter.

Why pedalling feels harder with the lamp on

Riders notice that a bicycle is a little harder to pedal with the dynamo pressed on the tyre. The energy that lights the lamp is not free: it comes from the rider's legs. The current induced in the coil sets up its own magnetic field, which pulls against the turning magnet, so the rider has to push harder to keep it spinning. The faster you ride, the brighter the lamp and the more work your legs do. When you stop at a junction, the magnet stops, no emf is induced, and the lamp goes out — which is why many bicycles now also carry battery lights.