4. A horizontal overhead power line carries a current of 90 A in east to west direction. What is the magnitude and direction of the magnetic field due to the current 1.5 m below the line?
The magnitude of the magnetic field at a distance r from a long straight wire carrying current I is given by,
In this case (current=35A, distance= 0.2m, permeability of free space = 410-7 TmA-1)
The current in the overhead power line is going from the East to West direction and the point lies below the power line. Applying Maxwell's right-hand thumb rule we can see that the direction of the magnetic field will be towards the South.