
${\text{Force on PQ will be }}{{\text{F}}_1} = {2_B}{\text{Ia}}$
${\text{ = }}{{\text{I}}_2}\frac{{{\mu _0}{I_1}}}{{2\pi {\text{a}}}}{\text{a}}$
$ = \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
$(i)\,\,\left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{d\vec l\, \times \,\vec r}}{{{r^3}}}} \right)$
$(ii)\,\, - \left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{d\vec l\, \times \,\vec r}}{{{r^3}}}} \right)$
$(iii)\,\left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{\,\vec r \times d\vec l}}{{{r^3}}}} \right)$
$(iv)\, - \left( {\frac{{{\mu _0}i}}{{4\pi }}} \right)\left( {\frac{{\,\vec r \times d\vec l}}{{{r^3}}}} \right)$

