Two long parallel wires separated by distance $d$ carrying currents $I_1$ and $I_2$: force per unit length $F/l = \mu_0 I_1 I_2/(2\pi d)$. Parallel currents (same direction) attract; antiparallel (opposite direction) repel. This is the basis for the definition of the ampere: 1 A is the current that produces a force of $2 \times 10^{-7}$ N/m between two parallel wires 1 m apart. Physical explanation: each wire creates a field that exerts a force on the other wire via $F = BIL$.