Have you ever wondered why an ordinary iron nail can attract other iron nails like it does after touching a magnet? What kind of magical power is this? In fact, this force is what we often call magnetism, and the substance that can be attracted is a ferromagnetic material. Why can ferromagnetic materials be attracted?
To understand why ferromagnetic materials can be attracted, we must first know what magnetism is. Simply put, magnetism is the ability of matter to generate or change its own magnetic field under the action of an external magnetic field. According to the different reactions of substances to external magnetic fields, we can divide substances into several different types:
Paramagnetic substance: This substance is slightly attracted by an external magnetic field and produces a weak magnetic field in the same direction as the external magnetic field. When the external magnetic field disappears, they also lose their own magnetic field. For example, oxygen, titanium, vanadium, etc.
Ferromagnetic substance: This substance is strongly attracted by an external magnetic field and produces a strong magnetic field in the same direction as the external magnetic field. When the external magnetic field disappears, they can still maintain their magnetized state and become magnetic. For example, iron, cobalt, nickel, etc.
Ferromagnetic substances: This substance is similar to ferromagnetic substances, but they are less magnetized and lose their magnetism at a certain temperature. For example, ferrite, nickel-zinc ferrite, etc.
We can see that only ferromagnetic and ferromagnetic substances can maintain their own magnetic properties, while other types of substances cannot. Why is there such a difference? This starts with the microstructure of matter.
Matter is made up of atoms, which in turn are made up of nucleons and electrons. Electrons are tiny particles with a negative charge that not only rotate around the nucleus, but also have an inner rotation, just as the Earth not only revolves around the sun, but also rotates. This intrinsic rotation is called electron spin. Since electric charges in motion create magnetic fields, both electron spin and ring nucleus motion produce a tiny magnetic dipole, like a miniature compass.
In most matter, the magnetic dipole directions created by the electron spin and ring nucleus motion are disorganized, so they cancel each other out, resulting in no macroscopic magnetic field. But in ferromagnetic substances, due to a special quantum mechanical effect, exchange, the spins of neighboring electrons tend to point in the same direction. In this way, many small regions are formed, in which all the electron spins are aligned in the same line, and these small regions are called magnetic domains. Each magnetic domain is equivalent to a small magnet with a certain saturation magnetization strength.
However, in the absence of an external magnetic field, the direction between different magnetic domains is inconsistent, so there is still no net magnetic field macroscopically. Only when an external magnetic field acts on a ferromagnetic substance can two phenomena occur:
Domain wall movement: The domain wall is the boundary between two adjacent magnetic domains in different directions, on which the direction of electron spin gradually changes. When an external magnetic field is applied, those domains that are consistent or close to the direction of the external magnetic field expand their range, while those domains that are opposite or differ significantly from the direction of the external magnetic field will reduce their range. This is equivalent to the domain wall moving under the impetus of an external magnetic field, which increases the magnetization of the entire substance.
Domain rotation: In addition to the movement of the domain wall, there is a more efficient way to increase the magnetization of a substance by turning the domain that would otherwise be inconsistent with the direction of the external magnetic field to the direction of the external magnetic field. This is equivalent to a small magnet rotating under the attraction of a large magnet, so that the magnetization of the entire substance reaches the maximum.
When ferromagnetic substances undergo domain wall movement and domain rotation under the action of an external magnetic field, they are magnetized, that is, they have their own persistent magnetic field. At this point, even if the external magnetic field disappears, they do not return to their original state, but maintain a certain degree of residual magnetization. This is why iron nails, after touching a magnet, can attract other iron nails just like it.
What if we want to make the nails stronger and be able to suck more nails? At this time, we need to use a special device - electromagnets.
An electromagnet is a device that uses an electric current to generate a powerful and controllable magnetic field. It consists of a soft and easily magnetized core and a coil wound around the core. When current is introduced into the coil, a strong magnetic field is generated in the core, which causes the core to be magnetized. When the current stops, the core loses its magnetism. In this way, we can control the magnetism of the electromagnet by controlling the switching and direction of the current.
If we place an iron nail near the electromagnet, it will be attracted by the electromagnet and magnetized. If we place another iron nail near the first iron nail, it will also be attracted by the first iron nail and magnetized. In this way, we can use electromagnets to make strings of iron nails, which are very magnetic.
The reason why ferromagnetic materials can be attracted is that I have deep physics behind them, not some magical power. I hope you have a deeper understanding and interest in magnetism.