Question

A compass placed below a long straight wire carrying current in the direction South to North, while the compass needle pointing in the north-west direction. If the direction of current is reversed, then thcompass needle will point in which direction?

Answer 1

The arrow of a compass thatis placed below a current-carrying wire is orientating itself in the direction
of the magnetic field, which orbits the wire and is, thus, perpendicular.
Normally, under a wire carrying current South to North, the arrow would point
perpendicularly to the West. As the direction of the current changes, the arrow
would reverse to point perpendicularly to the East.
From the fact that it points in the North-West direction, we assume some
inertia of the compass arrow itself. The North component of it we preserve,
making the resulting orientation North-East.

Answer: North-East.