Magnetic field due to a long close-wound coil
Practical Activity for 14-16
Iron filings show that a long, closely wound current-carrying coil behaves just like a bar magnet.
Apparatus and Materials
- Copper wire, PVC-covered, 100 cm with bare ends
- Cardboard, rectangle
- Plotting compass (optional)
- Power supply, low-voltage ('Westminster pattern' very-low-voltage supplies are best)
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.
- Make a coil of twenty or thirty turns by winding the wire around a pencil. (Leave enough wire free at either end to make connections to the power supply.) The coils should be wound firmly and closely on the pencil.
- Lay the coil on the cardboard and sprinkle iron filings onto the board.
- Switch on the current, tap the board, and observe the pattern.
- Try using a plotting compass after you have tried the iron filings. Investigate what happens if the connections are reversed.
- The long, closely wound coil behaves just like a bar magnet.
- The direction of the magnetic field reverses when the current is reversed.
This experiment was safety-tested in April 2006