Two long wires separate by a distance 2a = 4 cm, parallel to the x axis, are crossed by the same current i = 50A along the positive x-axis. Calculate the magnetic field B(z) on the axis of the two wires and at what distance from the center O a small magnetic needle oriented parallel to B is affected by a zero force.

Two long wires separate by a distance 2a = 4 cm, parallel to the x axis, are crossed by the same current i = 50A along the positive x-axis. Calculate the magnetic field B(z) on the axis of the two wires and at what distance from the center O a small magnetic needle oriented parallel to B is affected by a zero force.