A long wire carrying 5 A is bent into a hairpin: two long straight parallel segments, 20 cm apart, are joined at their ends by a semicircular arc of…

A long wire carrying $5\ \mathrm{A}$ is bent into a hairpin: two long straight parallel segments, $20\ \mathrm{cm}$ apart, are joined at their ends by a semicircular arc of radius $10\ \mathrm{cm}$, so the current enters along one straight segment, goes round the arc and leaves along the other. Each straight segment extends to infinity and ends at the arc along a line through the centre of the arc. The magnetic field at the centre of the arc is nearly ($\mu_0=4\pi\times10^{-7}\ \mathrm{T\,m\,A^{-1}}$)