# Magnet and coil

##### Class practical

An introduction to the dynamo principle.

#### Apparatus and materials

For each student group

Permanent bar magnet

Copper wire, insulated with bare ends, 200 cm

#### Procedure

a Wind a coil of 10 to 20 turns with long leads (say 50 cm). The coils should be such that a permanent bar magnet can pass freely through.

b Connect the long leads to the galvanometer.

c Move the magnet in the space in and around the coil, keeping an eye on the galvanometer. Summarize your observations.

#### Teaching notes

1 You might introduce this experiment by saying:

A dynamo or generator is a carefully-designed piece of equipment. There is a coil of wire and a magnetic field. There is motion. Electricity (a voltage) is generated. You can understand the principle of the dynamo by starting with a simpler situation: you have a coil and a magnet, and you can move them. What will you discover?

2 The students should find out that:

• the current flows only when the magnet and the coil are moving relative to each other;
• the current changes direction when the magnet is inserted into the coil and then removed from the coil;
• more turns on the coil produce bigger currents provided the total length (i.e. the total resistance) of the wire remains the same;
• the faster the magnet is moved, the greater is the maximum deflection.

This experiment was saftey-checked in April 2006