84.109.51.121 (talk) No edit summary |
looks like "applied" was meant here, not "resulting" |
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Some materials, called [[ferromagnetic]] or [[ferromagnet]]s, 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. |
Some materials, called [[ferromagnetic]] or [[ferromagnet]]s, 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. |
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If |
If the ferromagnetic material is put 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. |
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Permeability in linear materials owes its existence to the approximation: |
Permeability in linear materials owes its existence to the approximation: |
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Revision as of 10:27, 28 May 2007
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 Greek letter μ. This term was coined in September, 1885 by Oliver Heaviside.
In SI units, permeability is measured in henries per metre, or newtons per ampere squared. The constant value is known as the magnetic constant or the permeability of vacuum, and has the exact or defined value = 4π×10−7 N·A−2.
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 the ferromagnetic material is put 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:
where is a dimensionless scalar called the magnetic susceptibility.
According to the definition of the auxiliary field, H
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 magnetic field intensity, 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:
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:
In terms of relative permeability, the magnetic susceptibility is:
χm, a dimensionless quantity, is sometimes called volumetric or bulk susceptibility, to distinguish it from χp (magnetic mass or specific susceptibility) and χM (molar or molar mass susceptibility).
| Magnetic permeability & susceptibility for selected materials | |||
|---|---|---|---|
| Medium | Susceptibility | Permeability | |
| Mu-metal | 20,000 [1] | 25,000 µN/A2 | at 0.002 T |
| Permalloy | 8000 [1] | 10,000 µN/A2 | at 0.002 T |
| Transformer iron with ρ=0.01 µΩ·m | 4000 [1] | 5000 µN/A2 | at 0.002 T |
| Steel | 700 [1] | 875 µN/A2 | at 0.002 T |
| Nickel | 100 [1] | 125 µN/A2 | at 0.002 T |
| soft ferrite with ρ=0.1 Ωm | source, ferroxcube | 5000 µN/A2 | < 0.1 mT |
| soft ferrite with ρ=10 Ωm | source, ferroxcube | 2500 µN/A2 | < 0.1 mT |
| Platinum | 2.65 × 10−4 | 1.2569701 µN/A2 | |
| Aluminum | 2.22 × 10−5 [2] | 1.2566650 µN/A2 | |
| Hydrogen | 8 × 10−9 or 2.2 × 10−9 [2] |
1.2566371 µN/A2 | |
| Vacuum | 0 | 1.2566371 µN/A2 | |
| Sapphire | −2.1 × 10−7 | 1.2566368 µN/A2 | |
| Copper | −6.4 × 10−6 or −9.2 × 10−6 [2] |
1.2566290 µN/A2 | |
| Water | −8.0 × 10−6 | 1.2566270 µN/A2 | |
Permeability varies with flux density. Values shown are approximate and valid only at the flux densities shown. Moreover, they are given for a zero frequency; in practice, the permeability is generally a function of the frequency.
Note that the magnetic permeability has an exact value in SI units (i.e. there is no error bar or uncertainty in its value), a fact that is intimately related to the above formula , and the definition that the speed of light is exactly 299 792 458 meters/second. The agreed upon international definitions and best determinations of the values of the fundamental physical constants in SI are given by the CODATA database supported on the web by NIST
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