Knowing the size of the magnet, can the surface magnetism be calculated? What is the relationship between surface magnetism and magnet performance
Concept and measurement of surface magnetism
Surface magnetism, also known as surface field and surface magnetic field, refers to the magnetic induction intensity at a certain point on the surface of the magnet, in Gauss Gs or Tesla T (1T=10000Gs).
Surface magnetism is the most easily measured parameter in daily life. When the size of the magnet is fixed, people often judge and compare the performance of the magnet by comparing the surface magnetism. For some magnets with very large or small size and special shape, which are not suitable for routine measurement, it is very important to measure the magnetism of the meter. There are two things you need to know about surface magnetism:
The surface magnetism is the value measured when the Gauss meter is in contact with a certain point on the surface of the magnet. It is a data reflection of the magnet itself on the measuring tool, and does not represent the overall performance of the magnet.
The surface magnetism of different positions on the surface of the magnet is different. For non-multi-pole magnetized and regular-shaped magnets, the center surface magnetism is generally measured. The surface magnetism is easily affected by the external environment. For the same magnet, using Gaussmeters from different manufacturers, the measured central surface magnetism may be different; for the same magnet, measured in different environments, the measured surface magnetism may also be different. . (The central surface magnetism of the N pole and S pole of the same magnet is also different~)
From the above two points, it can be seen that the surface magnetic measurement is not objective, and it is not a parameter that can fully reflect the performance of the magnet. It is not recommended as an evaluation index when trading products.
The relationship between surface magnetism and magnetic properties
What is the relationship between surface magnetism and magnet performance parameters (such as remanence Br, coercive force Hc and maximum magnetic energy product (BH)max, etc.)? Can you find the mathematical formula? These two questions are frequently asked by readers.
The answer to the first question is yes, but some people have only made empirical statistics in the past. For example, for a cylinder with an aspect ratio of 1, the remanence Br is 2~3 times that of its surface field, but this statement cannot be strictly established. Quantitative Relationship.
The relationship between surface magnetism and remanence
Remanence refers to the magnetic induction intensity retained in the ferromagnet when the external magnetic field is used to magnetize the ferromagnet to a saturation state and then gradually reduce the external magnetic field to zero, which is called the residual magnetic induction intensity (Br). The remanence is determined by the characteristics of the magnet itself. The remanence of the same magnet is constant under certain conditions and has a single value.
The remanence determines the surface magnetism of the magnet to a certain extent, but the surface magnetism of the magnets with the same remanence is not the same. The surface magnetism is also affected by the shape, size and magnetization of the magnet.
For two magnets with the same shape, performance and size, the one with higher remanence has stronger surface magnetism. For two magnets with different shapes, or different performances and sizes, the size of the residual magnetism cannot be determined simply by the level of the surface magnetism.
The surface magnetism of the magnet is lower than the remanence
The remanence is tested in a closed-circuit state, while the surface magnetism is tested with a Gauss meter in an open-circuit condition, and the magnet itself has a demagnetization field, so the maximum surface magnetism of a single magnet is much smaller than its remanence. At present, the maximum remanence of sintered NdFeB magnets is around 14000Gs, so we can say with certainty that the maximum surface magnetism of a single NdFeB magnet cannot exceed 14000Gs. (Note that it is "single magnet", in some magnetic assemblies and magnet arrays, the surface magnetism of the magnet can be improved by special magnetic circuit design)
