Explanation

(b) When two infinitely long parallel conductors carrying current $I_{1}\ and\ I_{2}$ are placed a distance $r$ apart, then force on the unit length of a conductor due to the other conductor is given by
$F = \frac{\mu_{0}}{4\pi}\ .\frac{2I_{1}I_{2}}{r}$
The force is attractive, if current in two conductors is in same direction; and repulsive, if currents are in opposite directions.