Question

The diagram shows two parallel wires carrying current in the downward direction. The magnitude of the current in wire A is greater than in wire B. It is also observed that the wires exert force on each other.

1. State the direction of the force experienced by wire A.
2. Name the law/laws used to come to the conclusion in (a).
3. When a compass is placed between the two wires, it does not remain along the direction of the magnetic field of wire A or wire B at one of the points P, Q, or R. State, with a reason, at which point you will observe this.

Answer 1

1. Both wires carry current in the same direction, so they attract each other. Therefore, wire A experiences a force directed towards wire B.
2. The laws used are the right-hand thumb rule, to find the direction of the magnetic field produced by wire B at the position of wire A, and Fleming's left-hand rule, to find the direction of the force on wire A.
3. This happens at point R. Between the wires, the magnetic field due to wire A and the magnetic field due to wire B act in opposite directions. Since the current in A is greater than in B, the field of A is stronger. So the point where the two fields cancel must lie closer to the weaker-current wire B. In the diagram, this point is R. At R, the two fields are equal and opposite, so their resultant is zero, and the compass needle does not point along the field of either wire.