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Magnetic Influence
Magnetic influence describes a phenomenon in which an external magnetic field acts on a body and magnetizes it for a certain period. The said body is not only attracted to the applied magnet due to its magnetic properties, but it also becomes a magnet itself.
The prerequisite for this is that it is a body made of a material that is ferromagnetic. Ferromagnetic substances are all those that have a high permeability (greater than 1). They therefore offer an externally applied magnetic field - thanks to the particularly high magnetic flux density inside them - a simpler path than the air surrounding the body (air or oxygen has a magnetic permeability of approximately 1). Correspondingly, the magnetic fields of the magnet and the ferromagnetic object are drawn into one another.
How do you recognize magnetic influence?
The effect of the prevailing magnetism becomes clear through a strong attraction. In addition, the object itself is also magnetized. This can be recognized, among other things, by the fact that a nail made of a ferromagnetic material that has been touched with a magnet attracts other nails of this type for a short time - even if the magnet has already been removed. Classic ferromagnetic materials are:
- Iron
- Nickel
- Cobalt
- Terbium
- Erbium
- Gadolinium
- Holmium
- Dysprosium
However, when the temperature rises to room temperature and beyond, they become increasingly less magnetizable. Furthermore, certain alloys made of iron, nickel, zinc, and other elements are also ferromagnetic. These include mu-metal, neodymium, and ferrite. Some of these materials even have a particularly high permeability of more than 1,000. Bodies made of diamagnetic materials with a permeability of less than 1 are insensitive to the magnetizing effect of applied magnetic fields. Typical diamagnetic materials are, for example, copper, glass, and zinc.
How exactly does magnetic influence arise?
Ferromagnetic bodies consist of the smallest elementary or molecular magnets, which, like small bar magnets, have a north and a south pole. Without the influence of an external magnetic field or an electrical charge, these molecular magnets are freely arranged in individual domains - the Weiss areas. However, they do not point in the same direction, so that the individual smallest magnetic fields compensate each other. So no inherent magnetism can be seen.
If the north pole of an external magnet is brought to the body, the south magnetic poles of the elementary magnets turn towards it. They arrange themselves in parallel, the boundaries of the Weiss districts shrink, fold over and the entire ferromagnetic object is attracted by the magnet. In addition, the polarization through the magnetic influence - i.e., the formation of a north and a south pole at the ends of the body - ensures that the body itself becomes a magnet. Magnetic field lines run towards each other from the poles, from which the direction and force of the resulting magnetic field can be read.
Once magnetized, substances remain magnetic?
The magnetism created by the influence remains even after the magnet has been removed for a certain time - depending on the material of the affected body. If you want to manufacture a permanent magnet, sintering or annealing is often necessary in addition to a particularly strong magnetic field. This special form of heating ensures the alignment of the elementary magnets, which is achieved with the help of the magnetic field, over a long period of time. According to the laws of physics, however, the magnetic influence can also be reversed (the magnetization can be canceled again). For this you either need:
- strong increases or decreases in temperature
- Vibrations in the form of shocks
- a coercive magnetic field positioned against the direction of magnetization
- the application of an external electromagnet or an opposing external voltage