Force on a wire carrying a current in a magnetic field

Class practical

This effect is the basis of all electric motors.

Apparatus and Materials

For each student group

• Iron yoke
• Magnadur (ceramic) slab magnets, 2
• Copper wire, stiff, bare, SWG 32 and SWG 26
• Clamp, or wooden support blocks
• Crocodile clips, 2
• Leads, 4 mm, 2

• Power supply, low-voltage

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Make a long rectangular loop of thin copper wire.
2. Clamp it in a wooden support block with wing nut, or between two pieces of wood in the jaws of a clamp. The closed end of the loop will project out horizontally, sagging a little.
3. Connect the ends of the copper wire to the low-voltage DC supply, using cleaned crocodile clips and 4 mm leads.
4. Place the slab magnets on the yoke, ensuring that opposite poles are facing each other. Bring it near the free end of the loop when a current is flowing.
5. Find the position in which the magnets have the greatest effect on the current-carrying wire.

6. Now, using two 5-cm lengths of the thicker copper wire, make a pair of parallel horizontal rails. Clamp them as shown, and connect up to the power supply, or clamp them directly to the DC terminals of a Westminster pattern power supply.
7. Place a third piece of copper wire across the rails.
8. Bring up the magnets; how should they be held to produce a force on the third wire?
9. Investigate what happens if you reverse the current, or if you reverse the magnets.

Teaching Notes

• In this experiment, students may use the knowledge that a current-carrying wire has an associated magnetic field. When the wire is placed in a magnetic field it is likely that these two fields will interact.
• In practice, students will see the motion and know that there must be forces at play, but the three-dimensional geometry will remain obscure.
• Students will find that there is a force on the wire at right angles to both the current and the magnetic field. (If the current-carrying wire is not at right angles to the field, then only a component of the current will create a force.) If the wire lies along the magnetic field, there will be no force. If the wire is perpendicular to the magnetic field then the force will be maximum. A reversal of the current or of the field will reverse the direction of the force.
• You could introduce the left hand rule here in order to summarize what students have discovered.

This experiment was safety-tested in July 2007