= Magnetic field at the center of a hairpin wire
{title2=$B=\frac{\mu_0I}{4\pi a}(\pi+2)$}
A semicircular current arc of radius $a$ connected tangentially to two parallel semi-infinite wires has the displayed <magnetic field> at its centre. The arc contributes $\mu_0I/(4a)$ and each straight segment $\mu_0I/(4\pi a)$ by the <Biot-Savart law>. For the continuous hairpin current all three <magnetic fields> have the same normal direction by the <right-hand rule>; they add rather than cancel.
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