b
${{\text{F}}_3}{\text{& }}{{\text{F}}_4}\,{\text{cancel}}\,{\text{each}}\,{\text{other}}{\text{. }}$
${\text{Force on PQ will be }}{{\text{F}}_1} = {2_B}{\text{Ia}}$
${\text{ = }}{{\text{I}}_2}\frac{{{\mu _0}{I_1}}}{{2\pi {\text{a}}}}{\text{a}}$
$ = \frac{{{\mu _0}I}}{{2\pi {\text{a}}}}{\text{a}}$
$ = \frac{{{\mu _0}{I_1}{I_2}}}{{2\pi }}$
Force on $RS$ will be $F_{2}=I_{2} B_{2} a$
${=\mathrm{I}_{2} \frac{\mu_{0} \mathrm{I}_{1}}{2 \pi 2 \mathrm{a}} \mathrm{a}}$
${=\frac{\mu_{0} I_{1} I_{2}}{4 \pi}}$
Net force $=\mathrm{F}_{1}-\mathrm{F}_{2}=\frac{\mu_{0} I_1 \mathrm{I}_{2}}{4 \pi}$ repulsion
