= Magnetic circuit
{wiki}
A <magnetic circuit> approximates a narrow core and its gaps by sections carrying a common <magnetic flux> $\Phi$. With negligible leakage and approximately uniform <magnetic flux density> across each section, <Gauss's law for magnetism> gives $B=\Phi/A$. The <magnetostatic Ampère law> then gives $NI=\int H\,dl=\Phi\int dl/(\mu_0\mu_rA)$. This requires an appropriate material relation; constant <magnetic permeability> is an approximation for an actual magnetic core.
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