Bicycle dynamo

 

I found an old bicycle of a type I used in the 80s. It had a dynamo. I used to love these because your batteries never ran out. The little black wheel on it is turned because it sits on the tyre. The turning black wheel is attached to a permanent magnet that spins inside a coil of wire. By Faraday's Law, this induces an EMF and the EMF causes a current to flow. The faster you pedal, the higher the EMF and thus the higher the current and the brighter the light. This is also covered by Faraday's Law which says that induced EMF is proportional to the rate of change of flux linkage. The flux of the magnet links the coils and this changes as it spins round. The faster it spins, the bigger the EMF. Disadvantage is that when you stop at traffic lights, your lights go out, and they are dimmer going uphill!