A magnetic compass needle is placed in the plane of paper near point A as shown in figure. In which plane should a straight current carrying conductor be placed so that it passes through A and there is no change in the direction of the compass? Under what condition is the deflection maximum and why?

Explanation

The Straght current conductor should be placed in the plane of paper so that it passes through A. This produces undeflected due to vertical magnetic field perpendicular to plane of paper. The compass needle remains undeflected due to vertical magnetic field produced by wire since compass neelde are free to rotate only in horizontal plane. The deflection in compass needle is maximum when the conductor through A is perpendicular to the plane of paper and the magnetic field due to straight current conductor lies in the plane of paper.