Magnetic field inside an open coil

Class practical

Using a plotting compass and iron filings to explore the field pattern.

Apparatus and Materials

For each student group

• Cylinder, wooden
• Copper wire, PVC-covered, 150 cm with bare ends

• Iron filings in a pepper pot

• Plotting compass
• Cardboard, two slots cut into it

• Power supply, low voltage

Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Read our standard health & safety guidance

Procedure

1. You are going to explore the magnetic field inside a current-carrying coil of wire. Wind five spaced turns on the wooden cylinder.
2. Slide this coil off the cylinder and into slots on the board.
3. Connect the ends up to the DC terminals of the power supply.
4. Sprinkle iron filings onto the board, paying particular attention to the region inside the coil.
5. Switch on the current, tap the board, and observe the pattern.
6. Use a plotting compass to investigate the direction of the field inside and outside the coil. Investigate what happens if the current is reversed.

Teaching Notes

This is a chance to explore the field along the centre of the coil and around some of the coils as they enter the card. Students should see that the field inside the coil is in the opposite direction to the field outside the coil.

This experiment was safety-tested in July 2007