# The dc generator

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

## The dynamo

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 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.

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.