Two thin long parallel wires of same material are separated by a distance ' \(r\) ' Both carry same current ' \(I\) ' in the direction. Hence they will ( \(\mu_0=\) permeability of free space.)

The force per unit length between two long current carrying carrying conductors is \(\mathrm{F}=\frac{\mu_0 \mathrm{i}_1 \mathrm{i}_2}{2 \pi \mathrm{r}}\)
where \(\mathrm{d}\) is the distance between the conductors and \(\mathrm{i}_1\) and \(\mathrm{i}_2\) are the currents carried by the conductors.
The conductors attract each other if \(i_1\) and \(i_2\) are in the same direction. Magnetic field created by conductor 1 at the location of the conductor 2 and conductor 2 gets attracted towards conductor 1.
Since \(\mathrm{i}_1=\mathrm{i}_2=\mathrm{i}\)
\(\therefore \mathrm{F}=\frac{\mu_0 \mathrm{i}^2}{2 \pi \mathrm{r}}\)