N35 Permanent Magnet
The basic concepts of magnetism H, B, M, and J are related but easy to confuse. H is the magnetic field excited by the current, and B is the total magnetic field felt by the material. M is the magnetization intensity of the material, and J is the magnetic polarization intensity. The four have different definitions and units, which are very important to practitioners in the magnetic material industry.
Magnetic field intensity H, magnetic induction intensity B, magnetization intensity M and magnetic polarization intensity J are four very important basic concepts in magnetism. They are related but sometimes easily confused. It is very important for practitioners in the magnetic material industry to distinguish these four concepts N35 Permanent Magnet. Today we will explain their concepts and relationships in detail.
Magnetic field strength H
Magnetic field strength H is actually a physical quantity without practical meaning. When people defined it before, they assumed that there was such a thing as magnetic charge, but later they found that this thing did not exist N35 Permanent Magnet. It was just the other side of the electric current.
In the distant 1820s, scientists made a series of revolutionary discoveries that opened up modern magnetic theory.
In July 1820, Danish physicist Hans Oersted discovered that the current in a current-carrying wire would exert a force on a magnetic needle, causing the magnetic needle to deflect in direction. (Oersted experiment – magnetic effect of electric current)
In September, just one week after the news reached the French Academy of Sciences, Ampere successfully conducted an experiment to show that if the currents carried flow in the same direction N35 Permanent Magnet, two parallel current-carrying wires will attract each other; otherwise, if the currents flow in opposite directions, they will repel each other.
In 1825, Ampere published Ampere’s law, which is a rule about the relationship between the direction of the current and the magnetic flux lines of the magnetic field excited by the current.
Through mechanical measurements, we can conclude that the “magnetic field” strength felt by the magnetic needle is the same for points with equal distances from the long straight wire N35 Permanent Magnet, and the “magnetic field” strength of points with different distances is inversely proportional to the distance. In this way, we define the physical quantity of magnetic field strength H through mechanical measurements and current intensity. Its unit is ampere/meter A/m. In the Gaussian unit system, the unit of H is Oe Oersted, 1A/m=4π×10-3Oe.
There are many explanations for the magnetic field strength H. We can simply understand H as an external magnetic field (analogous to the electric field strength, for example, using current I to apply a magnetic field H to an object).
Magnetic induction intensity B
Magnetic field intensity is only a magnetic field given by an external current. For ferromagnetic materials in a magnetic field, in addition to being affected by the external magnetic field H N35 Permanent Magnet, the particles inside the material will also generate an induced magnetic field under the action of the external magnetic field. Magnetic induction intensity B means that a particle “feels” the total magnetic field, which is the sum of the external magnetic field H and the induced magnetic field M at this time.
In a vacuum, the magnetic induction intensity is proportional to the external magnetic field, that is, B=μ0H, where μ0 is the magnetic permeability of the vacuum. The magnetic induction intensity inside the ferromagnetic material is B=μ0(H+M) N35 Permanent Magnet, that is, the total magnetic field is equal to μ0 multiplied by the sum of “the magnetic field H generated by the current” plus “the magnetic field M generated by the medium after being magnetized by H”. The unit of B is Tesla T, and the unit in the Gauss unit system is Gauss Gs, 1T=10KGs.
In fact, the magnetic induction intensity is the real “magnetic field intensity” of the magnet, but because H has been called the magnetic field intensity in history, B can only be given another name called magnetic induction intensity N35 Permanent Magnet. B and H both refer to “magnetic field intensity”, but due to different definitions and derivation methods, their units are different (under the Gaussian system, the unit of B is Gauss Gs, the unit of H is Oersted Oe, 1Oe＝1×10-4Wb·m-2＝1×10-4T＝1Gs).
The magnetic field intensity H is the magnetic field of the empty space. It does not consider the matter in the space. It focuses on the relationship between the magnetic field and the current that generates the magnetic field N35 Permanent Magnet. The magnetic induction intensity B considers the strength of the final magnetic field after adding actual matter to the magnetic field H of the empty space. It focuses on the actual magnetic field strength of the matter.
Magnetization intensity M
We have just mentioned the magnetization intensity M, which is the induced magnetic field generated by the particles inside the material under the action of the external magnetic field. Modern physics has proved that each electron in an atom orbits and spins around the nucleus, and both of these movements produce magnetic effects. If the molecule is regarded as a whole N35 Permanent Magnet, the sum of the magnetic effects generated by each electron in the molecule can be represented by an equivalent circular current.
This equivalent circular current is called the molecular current, and its corresponding magnetic moment is called the molecular magnetic moment, represented by pm, which is the vector sum of the orbital magnetic moment and the spin magnetic moment of each electron in the molecule.
When there is no external magnetic field, the vector sum of the magnetic moments of all molecules in any volume element inside the magnetic medium is zero. When the magnetic medium is in an external magnetic field, each molecule is subjected to a torque N35 Permanent Magnet, which forces the molecular magnetic moment to turn to the direction of the external magnetic field. Therefore, under the action of the external magnetic field, the vector sum of the magnetic moments of all molecules in any volume element is not zero.
In this way, the magnetic medium shows a certain magnetism to the outside, or the magnetic medium is magnetized. In order to describe the magnetization state (magnetization degree and magnetization direction) of the magnetic medium, we introduce the magnetization intensity vector M, which represents the vector sum of the magnetic moments of all molecules in a unit volume, and the unit is A/m.
In order to study the relationship between this induced magnetic field M and the external field H, we define the magnetic susceptibility χ=M/H. A large magnetic susceptibility means that the same external magnetic field can generate more internal induced magnetic fields N35 Permanent Magnet; a small magnetic susceptibility means that even if the external magnetic field is large, the material inside is “too lazy to care about it” and only responds weakly.
The magnetic susceptibility can be positive or negative. A positive magnetic susceptibility χ>0 means that the direction of the internal magnetic field M generated is the same as the external magnetic field H N35 Permanent Magnet. A negative magnetic susceptibility χ<0 means that the additional magnetic field M generated by H inside the material is opposite to the direction of the external field H.
Magnetic polarization intensity J
Above we introduced the magnetic induction intensity B=μ0(H+M)=μ0H+μ0M. We call μ0M the magnetic polarization intensity of the substance, that is, J=μ0M, and its unit is also T (Tesla). Magnetic polarization intensity J is interpreted in a physical sense as the magnetic dipole moment of unit volume of magnetic media, also known as intrinsic magnetic induction intensity. The symbol is Bi or J. In Gaussian system μ0=1, so J=M N35 Permanent Magnet.
In soft magnetic materials, the value of magnetic field intensity is usually no more than 1000A/m, μ0 is 4×10-7H/m, and J=B-μ0H, so the difference between magnetic induction intensity B and magnetic polarization intensity J is very small; But in hard magnetic materials, this difference is very significant, so two relationship curves, B=f(H) and J=f(H), are usually given.
In magnetism, the terms magnetic field intensity (H), magnetic induction intensity (B), magnetization intensity (M), and magnetic polarization intensity (J) describe different aspects of the magnetic behavior of materials. Here’s a breakdown of each term:
1. Magnetic Field Intensity (\(H\))
Definition: The magnetic field intensity \(H\) is a measure of the strength of the magnetic field generated by currents and magnetic materials. It represents the amount of magnetizing force applied N35 Permanent Magnet.
Units: The unit of \(H\) is ampere-turns per meter (A/m) in the International System of Units (SI) N35 Permanent Magnet.
Formula: It can be derived from Ampère’s Law for a long straight conductor as \( H = \frac{I}{2 \pi r} \), where \(I\) is the current and \(r\) is the distance from the wire.
2. Magnetic Induction Intensity (\(B\))
Definition: The magnetic induction intensity \(B\), also known as the magnetic flux density, describes the total magnetic field, including the effects of both the external field and the material’s response to it.
Units: The unit of \(B\) is the tesla (T) in the SI system. One tesla is equal to one newton per ampere-meter N35 Permanent Magnet.
Relationship with \(H\)**: \(B\) and \(H\) are related by the equation \( B = \mu_0 (H + M) \), where \( \mu_0 \) is the permeability of free space, and \( M \) is the magnetization of the material.
3. Magnetization Intensity (\(M\))
Definition: Magnetization \(M\) refers to the magnetic moment per unit volume within a material. It represents the degree to which a material is magnetized in response to an applied magnetic field.
Units: The unit of \(M\) is A/m (amperes per meter) in the SI system.
Formula: Magnetization can be expressed as \( M = \frac{\text{magnetic moment}}{\text{volume}} \).
4. Magnetic Polarization Intensity (\(J\))
Definition: Magnetic polarization intensity \(J\) is another term sometimes used interchangeably with \(M\), particularly in older literature. It represents the same concept as magnetization: the magnetic moment per unit volume within the material.
Units: Like \(M\), the unit of \(J\) is also A/m in the SI system.
Relationships Between the Quantities
1. \(B\) and \(H\):
\[ B = \mu_0 (H + M) \]
Where \( \mu_0 \) is the permeability of free space, approximately \(4 \pi \times 10^{-7}\ \text{T·m/A}\).
2. \(M\) and \(H\):
For linear materials, the relationship between \(M\) and \(H\) can be given by N35 Permanent Magnet:
\[ M = \chi_m H \]
Where \( \chi_m \) is the magnetic susceptibility of the material.
In summary:
– \( H \) is the applied magnetic field intensity.
– \( B \) is the resultant magnetic induction or flux density.
– \( M \) (or \( J \)) is the magnetization or polarization intensity within the material.
These concepts help in understanding how materials respond to magnetic fields and are fundamental in the study of magnetism and electromagnetic theory.
