Question

Diagram alongside shows a freely pivoted magnetic needle. A copper conductor is held parallel to the magnetic axis of the needle. In which direction the north pole of needle moves in the following situations:

1. When the copper conductor is above the magnetic needle and current flows from A to B?
2. When the copper conductor is below the magnetic needle and current flows from A to B?
3. Why does the magnetic needle move in the situations (i) and (ii)?
4. Name and state the law, which helped you to find answer (i) and (ii).

Answer 1

1. The north pole of the needle moves towards the west.
2. The north pole of the needle moves towards the east.
3. A current-carrying conductor produces a magnetic field around itself. This magnetic field acts on the magnetic needle and exerts a force/torque on it, so the needle deflects from its north-south position. The deflection lasts only as long as the current flows through the conductor.
4. Ampere's Swimming Rule. It states: Imagine a swimmer swimming along the conductor in the direction of the current and always looking towards the magnetic needle, such that the current enters from his feet and leaves from his head. The direction in which the swimmer's left hand points gives the direction of motion of the north pole of the magnetic needle.