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

If there is to be no change in the deflection of the compass, the magnetic field due to current-carrying conductor should be perpendicular to the plane of the paper. For this, the conductor should lie in the plane of the paper. (In this case, the magnetic field due to current-carrying conductor is along the axis of the compass, i.e., in the vertical direction). The deflection is maximum when the conductor through A is perpendicular to the plane of the paper because in this case, its magentic field is maximum in the plane of the paper.