Answer to Question #243972 in Electricity and Magnetism for kington

Question

A long, straight, current-carrying wire runs from north to south.

a. A compass needle placed above the wire points with its north pole toward the east. In

what direction is the current flowing?

b. If a compass is put underneath the wire, in which direction will the needle point?

Answer 1

Here below you have (a) a sketch of the magnetic-field vectors due to a current element and (b) the magnetic field lines in a plane containing the current element. Following the right-hand rule, we find out that the conductor has to have current flowing from south to north if a compass needle placed above the wire points has its needle toward the east. On the other hand, if the compass is placed below the conducting line, the compass will have its north pointing towards the west since that is the direction of the magnetic field lines.

In conclusion, (a) the current is flowing from south to north and (b) the needle of a compass will point to the west if it's placed below the conducting line.

Reference:

• Sears, F. W., & Zemansky, M. W. (1973). University physics.