Permeance, in general, is the degree to which a material admits a flow of matter or energy. Permeance is usually represented by a curly capital P: P.

## 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: ${\displaystyle {\mathcal {P))={\frac {\Phi _{\mathrm {B} )){NI))}$

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): ${\displaystyle {\mathcal {F))=\Phi _{\mathrm {B} }{\mathcal {R))=NI}$

where:

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.[a]

## Materials science

In materials science, permeance is the degree to which a material transmits another substance.