Two thin long parallel wires of same material are separated by a distance ' $r$ ' Both carry same current '…
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.)
Attract each other with a force $\mu_0 I^{2 /}2 r$ per unit length
Repel each other with a force $\mu_0 I^2 /(2 \pi r)$ per unit length
Attract each other with a force $\mu_0 I^2 /(2 \pi r)$ per unit length.
Repel each other with a force $\mu_0 I^2 /(2 r)$ per unit length
Solution
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 $\mathrm{i}_1=\mathrm{i}_2=\mathrm{i}$
$\therefore \mathrm{F}=\frac{\mu_0 \mathrm{i}^2}{2 \pi \mathrm{r}}$