Permeability (electromagnetism)
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In electromagnetism, permeability is the degree of magnetisation of a material in response to a magnetic field. Absolute permeability is represented by the symbol μ. In SI units, permeability is measured in henrys per metre.
- <math>\mu = \frac {B} {H} \,<math>
where
μ is the permeability, measured in henry per metre
B is the magnetic flux density (also called the magnetic induction) in the material, measured in tesla
H is the magnetic field strength, measured in ampere per metre
Absolute permeability
Absolute permeability is represented by the symbol μ0 and is the permeability of the vacuum, where μ0 = 4π × 10−7 N A−2 (exactly).
Together with permittivity, permeability defines the speed of light.
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:
- <math>\mu_{r} = \frac{\mu}{\mu_{0}} <math>
| Relative permeability for some materials | |
|---|---|
| Medium | <math>\chi_m = \mu_r - 1 \,<math> |
| Hydrogen | 0.008 × 10-6 |
| Copper | −6.4 × 10-6 |
| Water | −8.0 × 10-6 |
| Aluminium | 22.2 × 10-6 |
| Platinum | 265 × 10-6 |
SI electricity units
| SI Base unit | |||
|---|---|---|---|
| Name | Symbol | Quantity | Notes |
| ampere | A | Current | |
| SI Derived units | |||
| Name | Symbol | Quantity | Notes |
| volt | V | Potential difference | |
| ohm | Ω | Resistance, Impedance, Reactance | |
| farad | F | Capacitance | |
| henry | H | Inductance | |
| siemens | S | Conductance, Admittance, Susceptance | =Ω−1 |
| coulomb | C | Electric charge | |
| ohm · metre | Ω · m | Resistivity | |
| siemens per metre | S / m | Conductivity | |
| henry per metre | H /m | Permeability | μ |
| farad per metre | F / m | Permittivity | ε |
| reciprocal farad | F−1 | Elastance | =F−1 |
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