Permeability (electromagnetism)

The magnetic permeability μ (also magnetic conductivity ) determines the permeability of matter for magnetic fields. It is a derived SI size with closely related to the magnetic susceptibility.

Basics

The permeability is the ratio of magnetic flux density B to the magnetic field strength H:

Μ0 is the magnetic constant field, a physical constant and is the magnetic permeability of the vacuum. Also, the vacuum is assigned a permeability, as also set there or magnetic fields can propagate electromagnetic fields. The permeability ĩr, formerly referred to as relative permeability, the ratio

For the vacuum, thus resulting in a relative permeability of 1 The dimensionless quantity ĩr is related to the magnetic susceptibility χ together on the relationship

Complex permeability, relative permeability

Time-dependent effects are used phasors for the fields and in accordance with a complex permeability, especially in electrical engineering for detection.

The real part of the complex permeability is the normal permeability. The imaginary part, describes the size of the hysteresis losses.

With the exception of the ferromagnetic material with a distinctly higher relative permeability than one, and the imaginary part of the complex permeability is negligible, as the frequency dependency of the permeability. The result is a scalar, frequency-independent permeability:

In ferromagnetic materials, the frequency dependence for many technical applications can not be neglected, it follows that:

Wherein the frequency of the alternating magnetic field is. The imaginary part is directly associated with the movement of domain walls in the material and at a resonance results in a maximum, usually in the range 10-1000 kHz.

How many physical properties of materials is the complex permeability in the generalized linear form actually a three-dimensional second order tensor. For most materials, the anisotropy of the magnetic properties but it is so small that a description as scalar, complex permeability is sufficient.

Classification

Magnetic materials can be classified based on their permeability.

Peculiarity of the permeability for materials with a magnetic order

The magnetization depends nonlinearly in ferromagnetic materials in general the external magnetic field from. It is possible to magnetize ferromagnetic materials to saturation. In addition, the magnetization of the previous magnetization depends, they say they have a memory. The behavior is described by a hysteresis loop. The simple description of the permeability increase as a linear magnetization curve is therefore not always applicable.

Depending on the application, different definitions of permeability are used. For technical applications it is 1324 part 2 eleven times defined in DIN with different calculations. In addition to the permeability μ as the ratio of magnetic flux density B in tesla (T) and magnetic field strength H in amperes per meter (A / m), especially the differential permeability, ie the slope of the hysteresis curve at a place of importance.

The problem assumed constant permeability can be seen from the hysteresis curve. The permeability μ corresponds to the slope

Due to the magnetic saturation and the magnetic remanence of the permeability is not constant. For small areas, the permeability is substantially greater than the near saturation.

The notation as tensor second stage is only limited in order to detect the magnetic anisotropy of the ferromagnetic materials. In particular, the anisotropy is not linear. Only in the special ( simplified ) case when linearity and isotropy is given, the permeability is a scalar constant of the material.