There is interaction between magnetic poles, the same name magnetic poles repel and the different name magnetic poles attract. There is a substance around the magnet that can deflect the pointer. This substance is called a magnetic field in physics. The distribution of magnetic field is usually represented by magnetic induction lines.
Generation of magnetic force:
Magnetic domain theory is to explain the magnetization mechanism of ferromagnetic materials from a microscopic point of view with quantum theory. The so-called magnetic domain refers to the small areas inside the magnetic material, and each area contains a large number of atoms. The magnetic moments of these atoms are arranged neatly like small magnets, but the directions of atomic magnetic moments are different between adjacent regions.
The interface between domains is called domain wall. Generally speaking, macroscopic objects always have many magnetic domains. In this way, the directions of magnetic moments of magnetic domains are different, and the results cancel each other out. The vector sum is zero, and the magnetic moment of the whole object is zero, so it cannot attract other magnetic substances. In other words, magnetic materials will not show magnetism to the outside under normal circumstances. Only when magnetism
Magnetic materials can only show magnetism when they are magnetized. In the middle school physics textbooks, the PEP version of physics and the standard experimental textbooks for high schools are used in the experimental areas of curriculum reform (Shandong, Jiangsu, Hainan, Ningxia, Guangdong, etc.). ), using magnetic domain theory, and physics, a full-time high school textbook used in most areas, uses ampere's molecular current hypothesis to explain the magnetization principle.
There is a strong exchange coupling effect between adjacent electrons in ferromagnetic materials. In the absence of an external magnetic field, their spin magnetic moments can be "spontaneously" arranged in a small area, forming a small area of spontaneous magnetization, called magnetic domain. In unmagnetized ferromagnetic materials, although each magnetic domain has a definite spontaneous magnetization direction and is very magnetic, the magnetization directions of a large number of magnetic domains are different, so the whole ferromagnetic material is nonmagnetic. As shown in the figure.
When ferromagnetic materials are in an external magnetic field, the volume of magnetic domains whose spontaneous magnetization direction is at a small angle with the direction of the external magnetic field increases with the increase of the external magnetic field, and the magnetization direction of magnetic domains further turns to the direction of the external magnetic field. The volume of other magnetic domains whose spontaneous magnetization direction makes a large angle with the direction of external magnetic field gradually shrinks, and then ferromagnetic materials show macroscopic magnetism to the outside. When the external magnetic field is enhanced, the above effects are correspondingly enhanced until all magnetic domains along the external magnetic field are saturated.
Below Curie temperature, there are many small regions where spontaneous magnetic moment and magnetic moment are paired in ferromagnetic or ferrimagnetic materials. They are arranged in a disorderly direction. If they are magnetized without a magnetic field, the magnetic moment is zero as a whole. These small areas are called magnetic domains. The interface between magnetic domains is called domain wall. When there is an external magnetic field, some magnetic moments in the magnetic domains turn to the direction of the external magnetic field, so that the total magnetic moment near the direction of the external magnetic field increases, such magnetic domains become larger, while other magnetic domains become smaller, resulting in an increase in magnetization.
With the further increase of the external magnetic field intensity, the magnetization increases, but even if the magnetic moment orientation in the magnetic domain is consistent, it becomes a single magnetic domain region, and its magnetization direction is not completely consistent with the external magnetic field direction. Only when the intensity of the external magnetic field increases to a certain extent can the magnetization direction of the magnetic moment in all magnetic domains be completely consistent with the orientation of the external magnetic field. At this time, the ferromagnetic body reaches the magnetic saturation state, that is, saturated magnetization. Once saturation magnetization is achieved, even if the magnetic field is reduced to zero, the magnetic moment will not return to zero, leaving some magnetization effects. This residual magnetization value is called residual magnetic induction intensity (denoted by symbol Br). The saturation magnetization value is called saturation magnetic induction intensity (Bs). If a reverse magnetic field is applied to zero the residual magnetic induction intensity, the magnetic field intensity at this time is called coercive field intensity or coercive force (Hc).
Ampere molecular current hypothesis
Ampere thinks that there is a kind of annular current-molecular current inside the molecules that make up the magnet. Because of the molecular current, each magnetic molecule becomes a small magnet, and both sides are equivalent to two magnetic poles. Usually, the molecular current orientation of magnet molecules is disordered, and the magnetic fields generated by them cancel each other, so they are not magnetic to the outside world. When the external magnetic field acts, the orientations of molecular currents are almost the same, and the adjacent currents between molecules cancel each other, but the surface parts do not cancel each other, and their effects show macroscopic magnetism.
Ampere's molecular current hypothesis could not be confirmed when little was known about the material structure at that time, which contained quite a few speculative components; It has been learned today that matter is made up of molecules, and molecules are made up of atoms, in which electrons move around the nucleus. Ampere's molecular current hypothesis has real content and becomes an important basis for understanding the magnetism of matter.