Permeance

Electromagnetism

In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns. A magnetic circuit almost acts as though the flux is conducted, therefore permeance is larger for large cross-sections of a material and smaller for smaller cross section lengths. This concept is analogous to electrical conductance in the electric circuit.

Magnetic permeance P is defined as the reciprocal of magnetic reluctance R (in analogy with the reciprocity between electric conductance and resistance): $${\displaystyle \mathcal{P}=\frac{1}{\mathcal{R}}}$$

which can also be re-written: $${\displaystyle \mathcal{P}=\frac{{\mathrm{\Phi }}_{\mathrm{B}}}{NI}}$$

using Hopkinson's law (magnetic circuit analogue of Ohm's law for electric circuits) and the definition of magnetomotive force (magnetic analogue of electromotive force): $${\displaystyle \mathcal{F}={\mathrm{\Phi }}_{\mathrm{B}}\mathcal{R}=NI}$$

where:

• Φ_B, magnetic flux,
• I, current, in amperes,
• N, winding number of, or count of turns in the electric coil.

Alternatively in terms of magnetic permeability (analogous to electric conductivity): $${\displaystyle \mathcal{P}=\frac{\mu A}{\ell }}$$

where:

• μ, permeability of material,
• A, cross-sectional area,
• ℓ, magnetic path length.

The SI unit of magnetic permeance is the henry (H), equivalently, webers per ampere.