The formula is given by the Biot-Savart Law:
$dB = \frac{\mu_0}{4\pi} \frac{I(dl \times r)}{r^3}$
where $dB$ is the magnetic field, $\mu_0$ is the permeability of free space ($4\pi \times 10^{-7} Tm/A$), $I$ is the current, $dl$ is the length of the wire
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