Physics:Permeance

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): [math]\displaystyle{ \mathcal{P} = \frac{1}{\mathcal{R}} }[/math]

which can also be re-written: [math]\displaystyle{ \mathcal{P} = \frac{\Phi_\mathrm{B}}{NI} }[/math]

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): [math]\displaystyle{ \mathcal{F} = \Phi_\mathrm{B} \mathcal{R} = NI }[/math]

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): [math]\displaystyle{ \mathcal{P} = \frac{\mu A}{\ell} }[/math]

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.

Materials science

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

See also

• Dielectric complex reluctance
• Reluctance

References

Electromagnetism

• Properties of Magnetic Materials (units of magnetic permeance)

Material science

• 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.