Magnetic Circuits

Analyse cores like electric circuits: MMF drives flux through reluctance.

Intermediate

Electric circuit	Magnetic circuit
EMF (V)	MMF $\mathcal{F} = NI$ (ampere-turns)
Current $I$	Flux $\Phi$
Resistance $R$	Reluctance $\mathcal{R} = \dfrac{l}{\mu A}$

$\Phi = \frac{NI}{\mathcal{R}}$

An air gap has a reluctance thousands of times higher than the same length of steel, so even a small gap dominates — useful in inductors to prevent saturation, and the reason motors keep their air gaps as small as possible.

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