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Magnetic field

If magnetic forces are detected, the cause is a so-called magnetic field. Since it is invisible to humans, it is modeled or described by field lines. Like a language, field lines say something about the magnetic field: if the lines are closer together, for example, the magnetic field is particularly strong. The field lines also have a direction, symbolized by arrows. This way you know where the North Pole and the South Pole are: The field lines always go outside the magnet from the South to the North Pole and are always perpendicular to the material.

How does a magnetic field work?

The forces of a magnet are transmitted through the magnetic field. If you sprinkle iron powder on a paper around a magnet, they arrange themselves along the magnetic field - structures are formed that are directly comparable with the field lines. In addition to the modeling language of the field lines, there are various physical quantities that describe the properties of magnets:

  • One is electromagnetic energy. The strength of the field is described by the magnetic field strength, analogous to the electrical field strength.
  • The magnetic energy on the other hand can be described by the energy product. The magnetic field strength flows squarely into the formula of this energy product. As a result, a magnet with twice the field strength exerts four times as much force.
Magnetic fields are a particularly important topic in electrical engineering because they are basically caused by moving charges. A simple wire through which a current flows is already surrounded by a magnetic field. As a pure dipole field, there are no individual magnetic poles in magnetic fields. This becomes clear from the fact that the field lines always have a direction: starting from a magnet, they run in a certain path and then go back into the magnet. But they don't stop there. The field lines continue in the magnet. Outside of this, the field lines always point from the south to the north pole, but inside from the north to the south pole.

How is a magnetic field generated?

There are two ways to do this:

  • Homogeneous magnetic field: This is, for example, inside a horseshoe magnet. It is generated by electrical currents. They are both equally strong and equally directed.
  • Inhomogeneous magnetic field: If the field lines are not parallel, one speaks of an irregular magnetic field. These can be found, for example, in bar/rod magnets.
  • Describing magnetic fields

In electrodynamics, the so-called Maxwell equations are used to mathematically describe the magnetic field. They indicate the direction of the magnetic field lines and say something about the density of the field lines. The Maxwell equations are used to calculate which direction a magnetic force has and how strong it is. Magnetic fields have no sources and sinks - on the contrary, electrical charges have sources, for example. This description is basically another description for a dipole field. A magnetic field is still a vortex field, because certain materials, electric fields and electric currents generate magnetic vortices.

True to the superposition principle, the field strengths add up when many small magnets are superimposed. It follows that a certain orientation of the many elementary magnets leads to a measurable magnetization. However, an arbitrary arrangement of the elementary magnets results in a magnetic field that cannot be measured from the outside.

Calculating magnetic fields

In physics, the magnetic field is specified in amperes per meter and is designated with the letter H - not with the letter B, which in turn describes the magnetic flux density and is measured in units of Gauss or Tesla. The following relationship applies to the magnetic field:

magnetfeld µ denotes the magnetic permeability of a material that is filled by the magnetic field. If, for example, a current-carrying coil has a certain magnetic field, this is amplified by inserting a certain material with the magnetic permeability µ by this factor:
  • For air µ is about one.
  • The value of iron can reach thousands.
  • μ0 on the other hand denotes the magnetic permeability of the vacuum.
The amplification of a magnetic field by a ferromagnetic material can be explained in that the individual elementary magnets align themselves with the magnetic field and also generate a rectified magnetic field after the alignment. The quadratic dependency between the magnetic field strength and the magnetic force explained at the beginning comes about through the magnetization and the attraction. For example, if a piece of iron has been magnetized by a magnet that is twice as strong as another magnet, the iron is magnetized on the one hand twice, but on the other hand it is also attracted twice by the magnet. This means that the magnetic energy as a whole is four times greater when the magnet is twice as strong.

How is a magnetic field shielded?

Shielding is only possible to a limited extent. However, redirection is possible. Materials that are easy to magnetize are suitable for this, for example:

  • Iron
  • Copper
However, substances that have no magnetic effect do not work.