Permeance

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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.

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:

• 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.^[a]

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

• Dielectric complex reluctance
• Reluctance

• Properties of Magnetic Materials (units of magnetic permeance)

• Bombaru, D., Jutras, R., and Patenaude, A., "Air Permeance of Building Materials". Summary report prepared by, AIR-INS Inc. for Canada Mortgage and Housing Corporation, Ottawa, 1988.