A wire with current $i=3.00 \mathrm{~A}$ is shown in Fig. $29-53 .$ Two semi-infinite straight sections, both tangent to the same circle, are connected by a circular arc that has a central angle $\theta$ and runs along the circumference of the circle. The arc and the two straight sections all lie in the same plane. If $B=0$ at the circle's center, what is $\theta ?$