CBSE Class 12 · 2005 · All India · Magnetic Effects of CurrentLong Answer

Parallel currents in the same direction have field lines that crowd between the wires and cancel there, drawing the wires together. Wire 1's field at wire 2 is $B_1=\frac{{\mu }_0{I}_1}{2\pi d}$, so the force per unit length on wire 2 is attractive for currents in the same direction. For the rectangular loop, only the two sides parallel to the wire give a net force; the nearer side is attracted more strongly (field $\propto 1/r$), so the loop experiences a net force directed towards the straight conductor, of magnitude $\frac{{\mu }_0{I}_1{I}_2}{2\pi }\ell \left(\frac{1}{d_1}-\frac{1}{d_2}\right)$.