Question

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

Answer 1

In the plane of the paper itself. The axis of the compass is vertical and the field due to the conductor is also vertical. It could result in a dip of compass needle which is not possible in this case (dips result only if the axis of the compass is horizontal). The deflection is maximum when the conductor through A is perpendicular to the plane of the paper and the field due to it is maximum in the plane of the paper.