Force between two parallel current carrying wires —
Two long wires 1 and 2 are kept paralicl to each other at distance $r$ and carrying current $I_1$ and $l_2$ respectively in the same direction.
The force per unit length of the wire 2 due to wire 1 is
$\frac{F}{l}=\frac{\mu_0 I_1 I_2}{2 \pi r}$
The same force acts on wire 1 due to wire 2 .
The wires attract each other if currents in the wires are flowing in the same direction and they repel each other if the currents are in opposite directions.