Magnet and coil
Practical Activity for 14-16
An introduction to the dynamo principle.
Apparatus and Materials
For each student group
- Permanent bar magnet
- Copper wire, insulated with bare ends, 200 cm
- Galvanometer, sensitive to e.g. 3.5–0–3.5 mA., 10 ohm resistance (see note below)
Please note: Strictly speaking, we generate e.m.f. but frequently measure the current through the load resistor (i.e. the wire) using a galvanometer (not an ammeter).
Health & Safety and Technical Notes
- 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.
- Connect the long leads to the galvanometer.
- Move the magnet in the space in and around the coil, keeping an eye on the galvanometer. Summarize your observations.
- 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?"
- 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