Magnetic field due to a coil carrying a current

Class practical

A plotting compass and iron filings show that the shape of this magnetic field resembles that of a permanent magnet.

Apparatus and Materials

• Copper wire, PVC-covered, 100 cm with bare ends
• Cardboard, two slots cut into it

• Iron filings in a pepper pot

• Plotting compass
• Cylinder, wooden

• Power supply, low-voltage ('Westminster pattern')

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

The power supplies used in this experiment must be able to allow a current of up to 10 amps to flow when a short piece of wire is connected across its 1 or 2 volt terminals. Many 0-12-volt power supplies will not pass such high currents and so the trip switch will cut out or, worse still, the power supply will be damaged. Students should be encouraged to switch off the power supply when they are not using it because the wires and coils will become hot.

Procedure

1. Wind five or six turns, closely spaced, onto the wooden cylinder. Twist the ends to prevent unwinding.
2. Slide it off the cylinder and into the slots cut in the board.
3. Connect the ends of the wire to the DC terminals of the power supply.
4. Investigate whether there is a field when there is no current.
5. Sprinkle the board lightly with iron filings, switch on the current, tap the board gently with a pencil, and note the pattern.
6. Remove the iron filings and repeat step 5 using the plotting compass inside and outside the coil.
7. Reverse the current and investigate the effect on the plotting compass.

Teaching Notes

• The best results with the compass needles will be found when the axis of the coil is aligned in a north-south direction.
• The pattern near to each of the vertical parts of the coil is the same as when a single vertical wire is connected to a power supply.
• The direction of the field can be found using a plotting compass. When the current is reversed, the direction of the magnetic field through the centre of the coil and around the wires is reversed.
• The magnetic field is strongest inside the coil. This is because the effects of both sides of the vertical part of the wires add up at the centre.
• The standard right hand grip rule can be modified to predict the direction of the magnetic field. If the right hand fingers curl round the coil in the direction of the current, then the thumb points in the direction of the magnetic field down the centre of the coil.

This experiment was safety-tested in July 2007