Why do magnets attract iron but not wood and aluminum?


When you were young, you must have played magnets. You […]

When you were young, you must have played magnets. You must have discovered such a phenomenon: magnets can attract iron products such as nails and iron sheets, but they cannot attract wooden tables or aluminum pans. Why?
To understand the magnetic properties of solids, we must first understand the quantum properties of electrons - spin. Like charge and mass, spin is also the intrinsic property of microscopic particles. It is not caused by the rotation of microscopic particles, but is an inevitable characteristic of quantization. The electron spin produces a spin magnetic moment. The movement of electrons around the nucleus also produces an orbital magnetic moment. The direction of the magnetic moment is perpendicular to the plane of the loop, and the magnetic moment can be regarded as a small magnetic needle. If the ordered magnetic moments of the same magnitude are the same and the directions are the same, the material as a whole will exhibit magnetism.
Solenoid magnetic field
Experiments have shown that any substance is more or less magnetized in an external magnetic field, but with a different degree of magnetization. Iron is a ferromagnetic material. There is a certain interaction (restriction) between the magnetic moments of adjacent atoms or ions in a ferromagnetic material, which causes the magnetic moments in some regions to be aligned in the same direction, thus forming a magnetic domain and magnetic properties in the magnetic domains. Very strong. There are many such magnetic domains inside the material, but the magnetic moment orientations of the different magnetic domains are random, and the material as a whole does not exhibit ferromagnetism. The main reason for the magnet attracting iron is that the iron itself spontaneously magnetizes and forms a spontaneous magnetization region, that is, a magnetic domain. After the ferromagnetic material is magnetized, the internal magnetic domains are aligned neatly, so that the whole body exhibits magnetism and can be attracted by the magnet. In addition to iron, transition metals such as cobalt, nickel, ruthenium, osmium, and rare earth metals all have spontaneous magnetization properties. After the magnet is removed, some ferromagnetic materials can keep the magnetic domains in order to retain the magnetic properties, and some will quickly disappear. The magnetic properties of pure iron are not easy to maintain, and the magnetic properties of steel cast by impregnating a certain amount of carbon and other impurities are good. At present, the strongest permanent magnet is found to be a NdFeB material, which is called “Magnetic King” because of its excellent magnetic properties. It has important applications in modern industrial and electronic technologies.
There are no magnetic domains inside the wood, and the electron magnetic moments in the atoms cancel each other out. When subjected to an external magnetic field, a small magnetic moment against the external magnetic field is generated, so that it cannot be attracted. Aluminum is a paramagnetic substance. When it is not subjected to a magnetic field, the magnetic moments are disorderly arranged. After the external magnetic field is applied, the magnetic moment tends to be consistent with the external magnetic field, but the consistency is not strong with ferromagnetic materials, so ordinary magnets cannot attract. Aluminum block.
The magnetic properties of solid materials are also related to the temperature of the material. For ferromagnetic materials, above a certain temperature, the thermal vibration of the atom is too intense and disturbs the magnetic moments that are consistently arranged. The material will transition from ferromagnet to paramagnet, and the corresponding temperature point is called the Curie point. The household rice cooker uses the Curie point of the ferromagnetic body. When the rice in the pot is heated to around 103 °C, the Curie point of the ferromagnetic material under the rice cooker is reached, and the magnetism of the demagnetized magnet is greatly reduced, thereby automatically Disconnect the heating switch and let the rice cooker enter the heat preservation state so that the rice can be cooked and not smeared.