Breaking a magnet

Class practical

This experiment could be used either to show that magnetic poles always come in north-south pairs, or to introduce a simple theory of permanent magnetism.

Apparatus and Materials

For each student group

• Steel rods, hardened, 2

• Iron filings in a pepper pot

• Sheets of paper, 2
• Plotting compasses, 2

Health & Safety and Technical Notes

Eye protection should be worn.

Read our standard health & safety guidance

The steel rods should be approx 1.5 mm diameter and 10 cm long. They should first be hardened, by heating until they are cherry-red and then plunging them into cold water. When cooled, the rods should be glass hard and unable to be bent. To magnetise them, use a 300-turn coil from a demountable transformer kit supplied with 2 amps from a 4 volt d.c. source. Place the steel rods inside the coil and switch on for a minute.

Procedure

1. You are going to try breaking a long thin magnet in half. Before you break your magnetized steel rod, test it with iron filings to see where its poles are.
2. Also bring each end of the rod near a small compass needle so that you know which pole is north-seeking.
3. Snap the rod in half and again look for poles. Try further snapping and testing.

Teaching Notes

• You might start this experiment by asking, "Can you make a magnet with a north pole at one end, and no south pole?"
• Students should dip their thin bar magnet into iron filings to check where the poles are, and then break the magnet into two pieces. When they have broken their magnet into two pieces they will find that they now have two smaller magnets. They can continue breaking their magnet into smaller and smaller pieces. Each time there is never a single pole in the broken magnet. That is different from the way in which electric charges behave. You can separate negative and positive charges by rubbing insulating materials together.

This experiment was safety-tested in April 2006