Does magnetism work in a vacuum
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Magnetic field in matter
So far we have looked at the magnetic field in air. Strictly speaking, the regularities that we have got to know here are only valid in a vacuum, i.e. in a space in which there is no matter. The magnetic fields behave almost exactly in air as they do in a vacuum. It is therefore permissible that we have not carried out our previous experiments in a vacuum, which would have been much more time-consuming. For example, we were able to describe the magnetic field of a coil without a core. On the other hand, if you slide an iron core into the coil, the conditions change. Let us now take a closer look at the magnetic properties of some materials.
Let us look again at the experiment in which a nail itself becomes a magnet through contact with a bar magnet. The property of the nail - like the bar magnet - to have magnetic poles is called magnetization. They say: the nail has a magnetization.
There are different mechanisms for the creation of magnetization in matter. The microscopic treatment of these processes (spins or magnetic moments of electrons and nuclei) is a topic of quantum physics and solid state physics. Nevertheless, we want to classify the magnetic properties of solids on a macroscopic level.
One size is no longer sufficient to describe the conditions in a magnetic field in which there is also magnetizable matter. The following quantities are therefore used for the description:
- The field:
- is a vector quantity and describes the magnetic field of free electrical currents and magnetic poles. With free electrical currents, for example, the currents in wires and coils are meant. Magnetic poles mean, for example, the poles on the end faces of a bar magnet where the field lines begin and end. In our previous considerations on magnetic field strength, we could have used instead. We will soon see that in a vacuum (approximately in air) both spellings are equivalent.
- The magnetization:
- is a vector quantity that describes the magnetic field due to bound circular currents inside magnetic materials. Bound currents mean that these currents cannot flow freely in any space, but are bound to the atoms of a crystal, for example. These bound currents can come about, for example, through electron spins.
- The field:
- is a vector quantity that we used earlier to describe the magnetic field strength. The field describes the totality of the field and magnetization. However, it can also be used to describe the field in material-free space, since it is then equivalent to the field. The following definition makes this clear:
with the field inside a solid.
The magnetization is outside (i.e. in a vacuum or air) and is therefore automatic
The magnetization of a body in turn depends on the external field at the time of magnetization. For the example of the magnetized nail, the following applies: The stronger the bar magnet with which the nail is magnetized, the stronger the magnet the nail itself becomes. For many materials, an approximate proportionality applies here:
In the following we want to consider a constant proportionality factor, which is approximately the case for many materials. However, more precise measurements show that it usually depends on the magnetic field itself. This is comparable to Ohm's law in electricity. Here, too, a closer look reveals that the electrical resistance is often not constant, but depends on the current strength, i.e.. Nevertheless, Ohm's law is an adequate approximation for most applications.
By inserting it you now get the relationship between -field and -field:
being the definition
They are called magnetic susceptibility and permeability.
Both quantities are dimensionless and describe the magnetizability of a magnetic substance. Susceptibility and permeability are redundant, i.e. it is sufficient to specify one of the two quantities.
Susceptibility or permeability can now be used to categorize the magnetic properties of various substances.
Diamagnetic substances have a susceptibility. Strictly speaking, all substances are diamagnetic. However, diamagnetism only comes to light if it is not superimposed by other magnetic effects that are orders of magnitude stronger (paramagnetism, ferromagnetism, ...). Diamagnetism is caused by induced atomic circular currents that are produced when a magnetic field is switched on or off. According to Lenz's rule, a diamagnetic substance experiences a force in the direction of decreasing field strength.
Paramagnetic substances have a susceptibility. Paramagnetism arises from permanent atomic circular currents. The decision as to whether a substance is paramagnetic is closely related to its spin configuration. The permanent circular currents are aligned in an external field in such a way that their magnetic field direction corresponds to that of the external field. If the paramagnetic substance is in a coil, the circulating currents have the same direction as the coil current. The heat movement of the atoms counteracts the alignment. is therefore clearly dependent on the temperature. Paramagnetic substances experience forces in inhomogeneous fields in the direction of increasing field strength.
Ferromagnetic substances have a susceptibility. There is an almost complete alignment of the circular currents, which is also partially irreversible. The irreversibility means that when the external, magnetizing field is switched off, the magnetization in the matter is partially retained (so-called remanence). A permanent magnet is created. Demagnetization is only achieved by applying an external opposing field (coercive field strength). The cycle of magnetization, remanence, coercive field strength and counter magnetization is described by the hysteresis curve.
To the right is the external field that can be generated and regulated, for example, by a coil. The field in the ferromagnetic material is applied to the top. The curve begins at the zero point when the material is initially unmagnetized. An increase in the coil current leads to saturation of the field (all circular currents aligned). The retentive field remains when it is switched off. Only after reversing the polarity of the coil is the material demagnetized again in a coil field with the coercive field strength (). A further increase in the coil current leads to saturation in the opposite direction than before. Switching off the coil accordingly generates a remanence in the opposite direction than before.
- Tab. 1
- Tab. 2
- Tab. 3
NB = normal conditions fl = liquid
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