A compass with its needle pointing north sits on a table. We place a straight wire over the compass perpendicular to its needle. We then send a conventional current running west to east through the wire. How does the compass needle respond to the current?
A) The compass needle deflects toward the west.
B) The compass needle deflects toward the east.
C) The compass needle does not do anything.
D) The compass needle turns around to point south.

Question

Quizplus · Accepted Answer

The magnetic field produced by a current-carrying wire is circular and perpendicular to the wire. Since the compass is placed directly under the wire, the magnetic field at the location of the compass needle is horizontal and does not exert a force that would cause the needle to deflect. Therefore, the compass needle does not move.

A Compass with Its Needle Pointing North Sits on a Table