The dc generator

A direct current (dc) generator is another device that produces a potential difference. A simple dc generator consists of a coil of wire rotating in a magnetic field. However, it uses a split ring commutator rather than the two slip rings found in alternating current (ac) generators. Some bike lights use a type of dc generator called a dynamo to run the lamps while the wheels are turning.

The dynamo

A bicycle dynamo. The wheel of the dynamo rubs against the bicycle tyre to turn a magnet sited within a coil of wire. This generates electricity to power the bicycle's lamps.In a bike dynamo, the magnet rotates inside a fixed coil of wire

In a dynamo, a split ring commutator changes the coil connections every half turn. As the induced potential difference is about to change direction, the connections are reversed. This means that the current to the external circuit always flows in the same direction.

Dynamo output on a graph

The output of a dynamo can be shown on a potential difference–time graph. The graph shows a sine curve that stays in the same direction all the time. The maximum potential difference or current can be increased by:

  • increasing the rate of rotation
  • increasing the strength of the magnetic field
  • increasing the number of turns on the coil

The diagram shows four different positions of the coil in a dynamo, and the corresponding voltage produced.

An alternator is rotating clockwise. Underneath there is a graph. At A, C and A, the curve should be at 0, and at B and D the curve is at its peak.The voltage–time graph for a dynamo

A – The coil is at 0°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.

B – The coil is at 90°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum.

C – The coil is at 180°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.

D – The coil is at 270°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the same direction as at B.

A – The coil is at 360°, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.