Surface Magnetism and Magnetic Flux
Remanence, coercive force, intrinsic coercive force and maximum magnetic energy product are the four most common parameters to measure the performance of magnetic materials. In addition, we will often see two other parameters in finished magnets – surface magnetism and magnetic flux (also called magnetic flux).
The simple understanding of surface magnetism is the magnetic induction intensity of the surface, which is defined as the magnetic flux passing through a unit area, so it is also called the surface magnetic flux density. The unit is Gauss Gs or Tesla T. The measuring instrument is a Gauss meter or a test with a Hall chip. instrument.
The surface magnetism refers to a measurement point, but it is actually the value of a small measurement area, and the magnetic field distribution of the magnet changes, so the surface magnetism at different points is generally different. For applications that require the use of a spatial magnetic field, surface magnetism or the magnetic induction value at a specified point is usually taken as an important technical requirement.
Magnetic field simulation diagram
The surface magnetism is related to the height-to-diameter ratio of the magnet (the ratio of the height to the diameter of the magnet, here the default height or thickness is the magnetization direction of the magnet). The larger the height-to-diameter ratio, the higher the surface magnetism, that is, the larger the surface area perpendicular to the magnetization direction , the lower the surface magnetism; the larger the size of the magnetization direction, the higher the surface magnetism. In addition, the Hall element on the Gauss meter of different manufacturers is different, and the surface magnetism measured by the same magnet is also slightly different.
Magnetic flux Φ
The definition of magnetic flux physics is the number of magnetic field lines passing through a certain area. Our simple understanding is the physical quantity that measures the overall magnetic size. The unit is Wb, expressed in Φ, Φ=B×S, and B represents the magnetic induction intensity , S represents the magnetic pole area, that is to say, the size of the magnetic flux depends on the magnetic pole area and the magnetic induction intensity.
The measuring instrument of the magnetic flux is a fluxmeter. With the Helmholtz coil, not only the magnetic flux can be measured, but also the magnetic moment can be calculated, because the measured value of the magnetic flux will vary with the parameters of the fluxmeter and the Helmholtz coil. Variety. Magnetic flux and magnetic moment are more like the difference between weight and mass. The weight is affected by the gravitational constant. The weight of the same object on the earth and Mars is different, but the mass is the same. Affected by the number of turns of the coil, the magnetic flux of the same magnet measured by different fluxmeters and coils may be different, but the magnetic moment must be the same. Surface Magnetism and Magnetic Flux
When the magnet is in an open circuit state, the actual remanence value Bdi (also called the intrinsic magnetic flux density) corresponding to the working point can be calculated by converting the magnetic flux into the magnetic moment, Bdi=Φ*coil constant/magnet volume.
Remanence, surface magnetism and magnetic flux are three very confusing concepts, here is a clearer one:
• Remanence is the essential property of materials. On the premise that self-demagnetization does not occur, the remanence of a magnet is constant. It is determined by the product raw material formula and preparation process. The test is carried out in a completely closed state
• Surface magnetism is the magnetic induction intensity value at the measurement position (a small area) when the magnet or magnetic component is in an open or semi-open state. The surface magnetism is a directional vector, and the surface magnetism data on different surfaces of the magnet are very different. We usually refer to the surface magnetic value perpendicular to the magnetic pole surface. The highest surface magnetism of a single magnet is one-half of the remanence. Note that it is a “single magnet”. In some magnetic components and magnet arrays, special magnetic circuit designs can be used to improve the surface magnetism of the magnet, and its value can even exceed the remanence. Surface Magnetism and Magnetic Flux
• The magnetic flux is the overall magnetic magnitude of the magnet measured by the coil test method, and usually the magnetic component is not suitable for testing the magnetic flux. The magnetic flux also has a direction requirement by default. Of course, the total magnetic flux value can also be measured by using a three-dimensional Helmholtz coil in actual measurement. Special attention should be paid to the test direction when measuring surface magnetism and magnetic flux.
Extended knowledge – magnetic circuit
A magnetic circuit refers to a combination of one or more permanent magnets and magnetically permeable materials in a certain shape and size to form a component with a specific working air gap magnetic field. Magnetically permeable materials play a role in controlling the flow of magnetic flux in the magnetic circuit, increasing the local magnetic induction intensity, preventing or reducing magnetic flux leakage, and improving the mechanical strength of the entire component.
Usually, the magnetic state of a completely independent single magnet (no magnetic conductive material and other permanent magnetic materials to cause the direction of the magnetic circuit to change) is called an open circuit state. The most typical is to place a magnet on a platform with no magnetic attraction at all. There is no influence of any magnetic material and other magnets around, and the magnet is in an open circuit state at this time. Surface Magnetism and Magnetic Flux
If the magnetic circuit direction of the magnet is completely locked in a built-in space and does not show magnetism to the outside, then the magnet is in a closed circuit state. The most typical one is a magnet that is in the process of measuring the demagnetization curve, and it hardly shows magnetism to the outside.
In addition to the open and closed states above, the rest can be called semi-open.