(b) Two thin horizontal wires are placed in a north- south direction. One wire is placed on a bench and the other wire is held $3.0cm$ directly above the first wire.

(i) When equal currents flow in the two wires, the force exerted on the bench by the lower wire decreases. Explain why this is so. What can you say about the directions of the currents in the two wires?

Two thin horizontal wires are placed in a north- south direction. One wire is placed on a bench and the other wire is held $3.0cm$ Directly above the first wire.

The magnetic flux density Bat a distance x from a long straight wire carrying a current i is given by the expression $B=2.0\times {10}^{-7}\frac{1}{x},$ Where X is in metres and i is in ampere. When the current in each wire is $4.0A$ Calculate the force per unit length on one wire due to the current in the other.