Experiments show that a steady current I in a long wire produces a magnetic field B that is tangent to any circle that lies in the plane perpendicular to the wire and whose center is the axis of the wire (as in the figure). Ampère’s Law relates the electric current to its magnetic effects and states that

where I is the net current that passes through any surface bounded by a closed curve C, and μ₀ is a constant called the permeability of free space. By taking C to be a circle with radius r, show that the magnitude B = |B| of the magnetic field at a distance r from the center of the wire is

. B dr dr ! B = 2mr