Two conducting strips, having infinite length in the z direction, lie in the xz plane. One occupies the region d/2 < x < b + d/2 and carries surface current density K = K₀a_z; the other is situated at −(b + d/2) < x < −d/2 and carries surface current density −K₀a_z.

(a) Find the force per unit length in z that tends to separate the two strips.

(b) Let b approach zero while maintaining constant current, I = K₀b, and show that the force per unit length approaches μ₀ I²/(2πd) N/m.