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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?
 
The current is in the east-west direction.
 
Applying the , we get that the magnetic field (at any point below or above the wire) turns  in a plane perpendicular to the wire, when viewed from the east end, and , when viewed from the west end.