Motor Effect — Current Balance

Aim: To investigate what determines the size and direction of the force on the wire.

Part A — Is there a force on the wire?

1. Set 1 magnet pair. With the current at 0.0 A, check that the balance reads 0.0 g.
2. Slide the current up to 2.0 A. Watch the wire and the balance display carefully — what happens to each?
3. Turn the current back down to 0.0 A. Does the balance return to zero?
4. The wire never touches the magnets or the balance. What must be acting between them for the balance reading to change?

Part B — What does the direction depend on?

1. Set 2.0 A again. Note the sign of the balance reading and which way the wire bows. Log the current direction and the field direction (the pole letters on the magnets) in your table.
2. Flip only the current direction switch. What happens to the sign of the reading and to the wire?
3. Put the current direction back, then flip only the field direction. What happens now?
4. Flip both at once and compare with your first reading. Record every combination in your table, with short notes in the direction columns.

Part C — How big is the force?

1. With 1 magnet pair, take balance readings at 1.0, 2.0, 3.0, 4.0 and 5.0 A and record each in your table.
2. Convert every balance reading into a force in newtons on paper and fill in the force column.
3. Now fix the current at 3.0 A and take readings with 1, 2 and 3 magnet pairs.
4. Use your table: what happens to the force when you double the current? What happens when you double the number of magnet pairs?
5. The direction pattern you found in Part B has a name — Fleming's left-hand rule. Look it up and check that it predicts every direction observation in your table.

Part D — Thinking further

1. The force you calculated acts on the wire, yet it is the balance under the magnets whose reading changes. What does that tell you about the forces between the wire and the magnets?
2. Before you set it up, predict the balance reading for 2 magnet pairs at 2.5 A with the current reversed. Then test your prediction and check it against your table.
3. Only about 5 cm of the wire sits between the poles. Suggest what would happen to the force if a longer length of the wire were inside the field, and how you could test that idea with this apparatus.