Permeability (electromagnetism)
From Free net encyclopedia
In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. Magnetic permeability is represented by the symbol μ. This term was coined in September, 1885 by Oliver Heaviside.
In SI units, permeability is measured in henrys per metre, or newtons per ampere squared. The constant value μ0 is known as the magnetic constant or the permeability of vacuum, and has the exact value μ0 = 4π×10−7 N/A2.
Some materials, called ferromagnetic or ferromagnets, are highly magnetic by nature, relative to most materials. They are composed of a large number of very small magnetic units working together called domains. Domains are not always aligned, and they often act against each other to reduce the strength of the net magnetic field.
If one puts the ferromagnetic material into an externally applied magnetic field, the domains tend to line up, so that the sum of the fields from the ferromagnet and the applied magnetic field is higher in magnitude than the applied magnetic field alone.
Permeability in linear materials owes its existence to the approximation:
- <math> \mathbf{M}=\chi_m\mathbf{H}</math>
where <math>\chi_m \,</math> is a dimensionless scalar called the magnetic susceptibility.
According to the definition of the auxiliary field, H
- <math>\mathbf{B}=\mu_0 (\mathbf{H}+\mathbf{M})=\mu_0 (1 + \chi_m)\mathbf{H} = \mu \mathbf{H}</math>
where
- μ is the material's permeability, measured in henries per metre
- B is the magnetic flux density (also called the magnetic induction) in the material, measured in teslas
- H is the auxiliary field strength, measured in amperes per metre
- M is the material's magnetization, measured in amperes per metre
The permittivity of free space (the electric constant) and the magnetic constant are related to the speed of light (c) by the formula: <math>\varepsilon_0\mu_0 = \frac{1}{c^2}</math>
Relative permeability, sometimes denoted by the symbol μr, is the ratio of the permeability of a specific medium to the permeability of free space μ0:
- <math>\mu_{r} = \frac{\mu}{\mu_{0}} </math>
In terms of relative permeability, the magnetic susceptibility is:
- <math>\chi_m = \mu_r - 1 \,</math>
| Magnetic permeability & susceptibility for selected materials | ||
|---|---|---|
| Medium | Susceptibility | Permeability |
| Platinum | 2.65 × 10−4 | 1.2569701 µN/A2 |
| Aluminum | 2.22 × 10−5 | 1.2566650 µN/A2 |
| Hydrogen | 8 × 10−9 | 1.2566371 µN/A2 |
| Vacuum | 0 | 1.2566371 µN/A2 |
| Sapphire | −2.1 × 10−7 | 1.2566368 µN/A2 |
| Copper | −6.4 × 10−6 | 1.2566290 µN/A2 |
| Water | −8.0 × 10−6 | 1.2566270 µN/A2 |
See also
- SI electromagnetism units
- ferromagnetism
- antiferromagnetism
- diamagnetism
- paramagnetism
- electromagnetca:Permeabilitat
de:Permeabilität (Magnetismus) es:Permeabilidad magnética fr:Perméabilité magnétique nl:Magnetische permeabiliteit ja:透磁率 pl:Przenikalność magnetyczna pt:Permeabilidade (física) sl:Magnetna permeabilnost fi:Permeabiliteetti