Two long parallel wires a distance d apart carry currents Iₐ and I_b. The magnitude of the force per unit length between them is:

1. Field of wire a at wire b: $B_a = \dfrac{\mu_0 I_a}{2\pi d}$. 2. Force on length $L$ of wire b: $F = I_b L B_a = \dfrac{\mu_0 I_a I_b}{2\pi d}L$. 3. Force per unit length: $f = \dfrac{F}{L} = \dfrac{\mu_0 I_a I_b}{2\pi d}$. 4. The $1/2d$ form omits the $\pi$ from the straight-wire field and is incorrect. _Source: syllabus Physics Ch 4 "Moving Charges and Magnetism", p.155_