Normally these dipoles, which in turn have north and south magnetic poles, are distributed with some degree of disorder within the volume of the material. The disturbance occurs less in materials with strong magnetic properties, such as iron, and greater in others with less obvious magnetism.
However, by placing the material in the middle of an external magnetic field, such as that produced inside a solenoid, the dipoles are oriented according to the field and the material is able to behave like a magnet.
Let M be the magnetization vector, defined as:
However, the intensity of magnetization in the material, product of immersion in the external field H , is proportional to this, therefore:
M ∝ H
The proportionality constant depends on the material, it is called magnetic susceptibility and is denoted as χ:
M = χ . H
The units of M in the International System are amperes / meter, just like those of H , so χ is dimensionless.
Orbital and rotational magnetic moment
Magnetism arises from electrical charges in motion; therefore, to determine the magnetism of the atom, the motions of the charged particles that make it up must be taken into account.
Starting with the electron, which is considered to be orbiting the atomic nucleus, it’s like a little spiral (closed circuit or closed current circuit). This movement contributes to the magnetism of the atom, thanks to the orbital vector of the magnetic moment m, whose magnitude is:
m = AI
Where I is the current intensity and A is the area enclosed by the loop. Therefore, the units of m in the International System (SI) are amplifiers per square meter .
Magnetic Rotation Moment
Apart from the orbital magnetic moment, the electron behaves as if it were rotating on itself. It doesn’t quite happen that way, but the resulting effect is the same; therefore, it is another contribution that must be taken into account for the net magnetic moment of an atom.
In fact, the magnetic rotational moment is more intense than the orbital moment and is primarily responsible for the net magnetism of a substance.
Rotation moments are aligned in the presence of an external magnetic field and create a cascading effect, successively aligning with neighboring moments.
Not all materials exhibit magnetic properties. This is because electrons with opposite rotational pairs form and cancel their respective magnetic rotational moments.
Only if it is missing is there a contribution to the total magnetic moment. Therefore, only atoms with an odd number of electrons are likely to be magnetic.
Protons in the atomic nucleus also make a small contribution to the total magnetic moment of the atom, because they also have rotation and therefore an associated magnetic moment.
But it inversely depends on the mass, and that of the proton is much larger than that of the electron.
Inside a coil, through which an electrical current passes, a uniform magnetic field is created.
And, as described in Figure 2, when you put a material there, its magnetic moments align with the coil’s field. The net effect is to produce a stronger magnetic field.
Transformers, devices that raise or lower alternative voltages, are good examples. They consist of two coils, the primary and the secondary, wound around a soft iron core.
A variable current is passed through the primary coil which alternately modifies the magnetic field lines within the core, which in turn induces a current in the secondary coil.
The frequency of the oscillation is the same, but the magnitude is different. In this way, higher or lower voltages can be obtained.
Rather than winding the coils onto a solid iron core, it is preferable to place a load of varnish-coated metal sheets.
The reason is due to the presence of eddy currents inside the core, which have the effect of overheating it a lot, but the currents induced in the plates are smaller and therefore the heating of the device is minimized.
A cell phone or an electric toothbrush can be charged by magnetic induction, known as wireless charging or inductive charging.
It works like this: a base or charging station is available, which has a solenoid or main coil, through which a variable current is passed. On the brush handle, another (secondary) coil is placed.
The current in the primary coil, in turn, induces a current in the cable coil when the brush is placed in the charging station and is charged by charging the battery that is also in the cable.
The magnitude of the induced current is increased when a core of ferromagnetic material, which may be iron, is placed in the main coil.
In order for the primary coil to detect the proximity of the secondary coil, the system emits an intermittent signal. Once a response is received, the described mechanism is activated and current begins to be induced wirelessly.
Another interesting application of the magnetic properties of matter is ferrofluids. They consist of small magnetic particles of a ferrite compound, suspended in a liquid medium, which can be organic or even water.
The particles are coated with a substance that prevents their agglomeration and thus remain distributed in the liquid.
The idea is that the flowability of the liquid is combined with the magnetism of the ferrite particles, which by themselves are not strongly magnetic, but acquire a magnetization in the presence of an external field, as described above.
The acquired magnetization disappears once the external field is removed.
Ferrofluids were originally developed by NASA to mobilize fuel inside a weightless ship, giving impulse with the help of a magnetic field.
Currently, ferrofluids have many applications, some still in the experimental phase, such as:
– Reduce friction on the speaker pad and headphones (avoid reverberation).
– Allow separation of materials with different density.
– Act as seals on hard drive shafts and repel dirt.
– As a treatment against cancer (in the experimental phase). Ferrofluid is injected into cancer cells and a magnetic field is applied that produces small electrical currents. The heat generated by these attacks on malignant cells and destroys them.