What will happen to a compass needle when the compass is placed below a wire and a current is made to flow through the wire? Give a reason to justify your answer.

Hint: We know that a current carrying wire produces a magnetic field around it, from maxwell equations. Also, a compass needle is such when it is placed in an external magnetic field it has the tendency to move. So this problem needs to find out such relationships.

Complete step by step solution:
Imagine that we are fielding a current-carrying straight conductor in our right hand such that the thumb pointed towards the direction of the current. Then our finger will wrap around the conductor in the direction of the field lines of the magnetic field is known as the right-hand thumb rule.
Let us consider that a current through a horizontal power line flows in the east to west direction. Applying the right-hand thumb-rule, the direction of the magnetic field at a point above the wire is from north to south. The direction of the magnetic field at a point directly above the wire is from the south to the north.
Thus, when a magnetic compass is trough below in the current-carrying wire the compass needle shows deflection in the north-south direction.

Note: Compass shows the multiple deflections in the presence of a magnetic field. Presence of more than one field will tend the compass to move in a direction where the fields get added because the magnetic field is a vector quantity and it gets added up vectorially.