Electromagnets: forces

Class practical

An illustration of magnetic induction. Students could go on to investigate how an electromagnet's strength varies with the current.

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends
• C-cores, laminated iron, 2

• Power supply, low-voltage

Health & Safety and Technical Notes

Read our standard health & safety guidance

For this experiment to be effective it is imperative that there be no grit, such as iron powder, between the touching faces of the two C-cores. If necessary, slide a clean piece of paper between the faces (with the current off) and withdraw while gripping it gently with the faces, which are thus wiped clean. Alternatively, remove grit, dust, or iron filings by wiping the faces clean with the thumb.

Care should be taken to keep the C-cores in their original pairs and their faces undamaged, especially for accurate results in induction experiments.

Procedure

1. Take one iron C-core. Wind twenty turns of PVC-covered copper wire round one arm and connect to the d.c. terminals of the low-voltage power supply.
2. Switch on. Bring up the second C-core, and investigate the attraction between the two C-cores.
3. Switch off the current and investigate the attraction again.

Teaching Notes

• Students should observe that the iron of the C-cores retains no magnetism, unlike the permanent bar magnets. However, a current through a few turns of wire produces a magnetic field which magnetizes the iron strongly.
• The magnetic field of the first C-core induces opposite poles in the second C-core, so that they attract strongly. It will take a considerable force to separate the C-cores while there is a current through the coil. They will just fall apart when the current is switched off.
• Students could go on and investigate how an electromagnet's strength varies with the current. Attach small nails or paper clips, head to tail, from the electromagnet (the first C-core). Estimate the electromagnet's strength by counting the number of paper clips the C-core can support. Repeat this procedure for different values of current, then analyze the data.

This experiment was safety-tested in April 2006