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In [[electromagnetism]], '''permeability''' is the degree of magnetisation of a material that responds linearly to an applied [[magnetic field]]. Magnetic permeability is represented by the symbol [[Mu (letter)|μ]]. [[Oliver Heaviside]] coined this term in |
In [[electromagnetism]], '''permeability''' is the degree of magnetisation of a material that responds linearly to an applied [[magnetic field]]. Magnetic permeability is represented by the symbol [[Mu (letter)|μ]]. [[Oliver Heaviside]] coined this term in September, [[1885]]. |
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In [[SI]] units, permeability is measured in [[henry (inductance)|henry]]s per [[metre]]. The constant value <math>\mu_0 \,</math> is known as the '''magnetic constant''' or '''permeability of a free space'''. |
In [[SI]] units, permeability is measured in [[henry (inductance)|henry]]s per [[metre]]. The constant value <math>\mu_0 \,</math> is known as the '''magnetic constant''' or '''permeability of a free space'''. |
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Revision as of 01:47, 26 October 2005
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In electromagnetism, permeability is the degree of magnetisation of a material that responds linearly to an applied magnetic field. Magnetic permeability is represented by the symbol μ. Oliver Heaviside coined this term in September, 1885.
In SI units, permeability is measured in henrys per metre. The constant value is known as the magnetic constant or permeability of a free space.
Some substances, called ferromagnetic materials or ferromagnets, are naturally magnetic. They are not a large single natural magnet, but are composed of a large number of very small magnets 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 field from the ferromagnet plus the applied magnetic field are stronger than the applied magnetic field alone.
Permeability in linear materials owes its existence to the approximation:
where is a dimensionless scalar called the magnetic susceptibility.
According to the definition of the auxiliary field, H
where
- μ is the permeability of the material, 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 magnetic field strength, measured in amperes per metre
Permeability of free space
The permeability of free space, also known as the magnetic constant, is represented by the symbol μ0 and is the permeability of the vacuum, where μ0 = 4π×10−7 N A−2 (exactly).
The permittivity of free space (the electric constant) and the magnetic constant are related to the speed of light (c) by the formula
Relative permeability
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:
Magnetic susceptibility
Magnetic susceptibility is defined as:
(see magnetic susceptibility)
Table of values for magnetic susceptibility and magnetic permeability
| Magnetic permeability & susceptibility for selected materials | ||
|---|---|---|
| Medium | Susceptibility | Permeability |
| Hydrogen | 0.008 × 10-6 | 1.2566371 µN/A2 |
| Copper | −6.4 × 10-6 | 1.2566290 µN/A2 |
| Water | −8.0 × 10-6 | 1.2566270 µN/A2 |
| Aluminum | 22.2 × 10-6 | 1.2566650 µN/A2 |
| Platinum | 265 × 10-6 | 1.2569701 µN/A2 |
| Sapphire | −0.21 × 10-6 | 1.2566368 µN/A2 |
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