Relative permeability

In physics, in particular in magnetostatics, the relative permeability is an intrinsic property of a magnetic material. It is usually denoted by μ_r. For simple magnetic materials, using SI units, μ_r is related to the proportionality constant between the magnetic flux density B and the magnetic field H, namely B = μ_r μ₀ H, where μ₀ is the magnetic constant. The relative permeability describes the ease by which a magnetic medium may be magnetized.

A related quantity is the magnetic susceptibility, denoted by χ_m, related to the magnetic permeability in SI units by:

$\mu _{r}=1+\chi _{m}\ .\ \mathrm {SI\ units} \ .$

The relative permittivity μ_r may be greater than or less than 1. For simple media, if μ_r < 1 (χ_m negative), the medium is termed diamagnetic; if μ_r > 1 (χ_m positive) paramagnetic. Only classical vacuum has μ_r = 1 (exact). A typical diamagnetic susceptibility is about −10⁻⁵, while a typical paramagnetic susceptibility is about 10⁻⁴.

Relation to forces

(PD) Image: John R. Brews
Infinitesimal current elements in two closed current-carrying loops

See also Ampère's equation.

Similar to Coulomb's law describing the electric force between electric charges, the magnetic force between two magnetic poles of pole strength given by p₁ and p₂ and separated by distance r in a medium of relative permeability μ_r has magnitude F:

$F={\frac {\mu _{r}\mu _{0}}{4\pi }}\ {\frac {p_{1}p_{2}}{r^{2}}}\ ,$

and is directed along the line joining the poles. It is attractive if the poles are opposite in nature (one north pole and one south pole) and repulsive if they are the same.

It happens, however, that magnetic poles always occur as opposite pairs, so the force law between single poles cannot be observed directly. However, magnetic dipoles do occur in nature and can be directly observed. A planar wire loop conducting a steady electric current resembles a magnetic dipole at distances large compared to the dimensions of the loop.

The magnetic force between two charges moving in a medium of relative permeability μ_r is provided by the Lorentz force acting upon one charge via the magnetic flux density generated by the other as predicted by the Biot-Savart law. Let the charges be q₁, q₂, their separation be r₁₂, and their constant velocities be v₁, v₂. The magnetic field generated by charge 2 at a position P is:

$\mathbf {B} ={\frac {\mu _{r}\mu _{0}}{4\pi }}\ {\frac {q_{2}\mathbf {v_{2}\ \times \ {\hat {u}}} }{r^{2}}}\ ,$

where û is a unit vector pointing from the charge to the field evaluation point P, and r is the distance between the charge and the field point.

Then the force upon charge 1 located at P₁ due to charge 2 according to the Lorentz law is:

$\mathbf {F_{12}} =q_{1}\mathbf {v_{1}\ \times \ B} (P_{1})$

$={\frac {\mu _{r}\mu _{0}}{4\pi }}\ {\frac {q_{1}q_{2}}{r_{12}^{2}}}\left(\mathbf {v_{1}\ \times \ } (\mathbf {v_{2}\ \times \ {\hat {u}}_{12}} )\right)\ ,$

where û₁₂ is a unit vector pointing from charge 2 to charge 1 and r₁₂ is their separation. This relation is closely connected to the force between two current elements, as described by Grassmann's adaptation of Ampère's equation, described next.

The force exerted between infinitesimal current elements from two loops carrying currents I_i and I_j and immersed in a magnetic medium with relative permeability μ_r is changed by a factor μ_r. The infinitesimal current elements have vector components i_idℓ_i and i_jdℓ_j where the incremental lengths are directed along the wire at the location of the element, and pointing in the direction of the current. Grassmann's law for the force upon current element i exerted by current element j is:

$d^{2}\mathbf {F_{ji}} =-i_{i}i_{j}{\frac {\mu _{r}\mu _{0}}{4\pi }}{\frac {1}{r_{ij}^{2}}}\left(\ (d{\boldsymbol {\ell _{i}\cdot }}d{\boldsymbol {\ell _{j}}})\mathbf {{\hat {u}}_{ij}} -(\mathbf {{\hat {u}}_{ij}\cdot } d{\boldsymbol {\ell _{i}}})d{\boldsymbol {\ell _{j}}}\right)\ ,$

as is readily derived from the Biot-Savart law, and is consistent with space-time symmetry. Here û_ij is a unit vector pointing from element j to element i along the line joining them, and r_ij the length of this line. The force element is second order because it is a product of two infinitesimals.

Background

Maxwell reduced electrodynamics to the knowledge of charge and current distributions, and introduced the notion of material properties (constitutive relations) relating magnetic flux density B to magnetic field H and electric displacement D to electric field E. In implementing these relations, it should be noted that the use of a constant as the relative permeability of a substance is an approximation, even for quantum vacuum. A more complete representation recognizes that all media exhibit departures from this approximation, in particular, a dependence on field strength, a dependence upon the rate of variation of the field in both time and space, and a dependence upon the direction of the field. In many materials these dependencies are slight; in others, like ferromagnets, they are pronounced.