Solve the follwing example .
A current through a horizontal power line flows in east to west direction . What is the direction of magnetic field at a point directly below it and at a point directly above it ?

Explanation

The current is in the east - west direction . Applying the right - hand thumb rule , we get that the magnetic field ( at any point below or above the wire ) turns clockwise in a plane perpendicular to the wire , when viwed from the east end , and anticlcokwise, when view from the west end .
Hence , at a point directly above the horizontal power line , the magnetic field B points in the north direction and at point directly below it , magnetic field B point in the south direction.