V=-N(dΦ/dt)
Electricity and Magnetism

Magnet and coil

Practical Activity for 14-16 PRACTICAL PHYISCS

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
  • 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

Read our standard health & safety guidance


Procedure

  1. 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.
  2. Connect the long leads to the galvanometer.
  3. Move the magnet in the space in and around the coil, keeping an eye on the galvanometer. Summarize your observations.

Teaching Notes

  • 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

V=-N(dΦ/dt)

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