Magnetic field strength H 

The magnetic field strength H is actually a physical quantity with no practical meaning. When people defined it earlier, they assumed that there was a magnetic charge, but later found that this thing does not exist, it is just the other side of the current.


Scientists made a series of revolutionary discoveries in the 1820s that started the modern theory of magnetism.


Danish physicist Hans Oersted discovered in July 1820 that the current in a current-carrying wire would exert a force on a magnetic needle, causing the magnetic needle to deflect. (Oersted experiment - magnetic effect of electric current).


In September, just a week after the news reached the French Academy of Sciences, Ampere successfully performed experiments showing that two parallel current-carrying wires attract each other if the currents they carry are flowing in the same direction; will be mutually exclusive.


In 1825 Ampere published Ampere's law, which is the law about the relationship between the current and the direction of the magnetic field lines of the magnetic field excited by the current.


Through the measurement of mechanics, it can be concluded that the distance from the outside of the long straight wire to the wire is equal, the strength of the "magnetic field" felt by the magnetic needle is the same, and the strength of the "magnetic field" at 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 measurement and current intensity. Its unit is A/meter m. In the Gauss unit system, the unit of H is Oe Oersted, 1A/m=4π×10-3Oe.


There are many explanations about the magnetic field strength H, and we can simply understand H as an external magnetic field (analogous to the electric field strength, for example, using a current I to apply a magnetic field H to an object).


Magnetic induction B

The magnetic field strength is only a magnetic field given by an external current, and for ferromagnetic substances in the magnetic field, in addition to being affected by the external magnetic field H, the particles inside the material will also generate an induced magnetic field under the action of the external magnetic field. The 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 vacuum, the magnetic induction intensity is proportional to the external magnetic field, B=μ0H, where μ0 is the vacuum permeability. The magnetic induction intensity B=μ0(H+M) inside the ferromagnetic material, that is, the total magnetic field is equal to the sum of μ0 multiplied by the "magnetic field H generated by the current" plus the "magnetic field M generated after the medium is 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, magnetic induction is the real "magnetic field strength" of the magnet, but since H has been called the magnetic field strength in history, we can only give B another name called magnetic induction. B and H both refer to "magnetic field strength", but due to the different ways of definition and derivation, their units are different (the unit of B under the Gauss system is Gauss Gs, and the unit of H is Oersted Oe, 1Oe =1×10-4Wb·m-2=1×10-4T=1Gs). The magnetic field strength H is the magnetic field of the empty space, it does not consider the matter in the space, it pays attention to the relationship between the magnetic field and the source of the magnetic field - the current, and the magnetic induction B is considered on the basis of the magnetic field H of the empty space The strength of the final magnetic field after adding the actual matter, it focuses on the actual strength of the magnetic field of the matter.


Magnetization M

Just now we have mentioned the magnetization M, which is an induced magnetic field generated by the particles inside the material under the action of the external magnetic field. Modern physics proves that every electron in an atom is doing orbital motion and spin motion around the nucleus, both of which generate magnetic effects. If the molecule is regarded as a whole, the sum of the magnetic effects produced by the electrons in the molecule can be represented by an equivalent circular current. This equivalent circular current is called molecular current, and its corresponding magnetic moment is called The molecular magnetic moment, represented by pm, is the vector sum of the magnetic moments and spin magnetic moments of each electron orbit in the molecule.


When there is no external magnetic field, the vector sum of all molecular magnetic moments in any volume element inside the magnetic medium is zero, and the substance does not show magnetism to the outside; while when the magnetic medium is in the external magnetic field, each molecule is subjected to a torque, which is The molecular magnetic moment is forced to turn to the direction of the external magnetic field, so under the action of the external magnetic field, the vector sum of all molecular magnetic moments in any volume element is not zero. In this way, the magnetic medium shows a certain degree of 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 vector M, which represents the vector sum of all molecular magnetic moments in a unit volume, and the unit is A/m (the unit of M under the Gauss system is Gauss Gs ).


 In order to study the relationship between the induced magnetic field M and the applied 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; a small magnetic susceptibility means that even if the external magnetic field is very large, the material inside is "too lazy to care about it" and has only a weak response. Magnetic susceptibility can be positive or negative. Positive magnetic susceptibility χ>0 means that the generated internal magnetic field M is in the same direction as the external magnetic field H. Negative magnetic susceptibility χ<0 means that the additional magnetic field M generated by H inside the material is in the opposite direction to the external field H.


Magnetic polarization J

Above we introduced the magnetic induction intensity B=μ0(H+M) =μ0H+μ0M, we call μ0M the magnetic polarization of the substance, that is, J=μ0M, and its unit is also T (Tesla). The magnetic polarization J is interpreted as the magnetic dipole moment per unit volume of the magnetic medium in a physical sense, also known as the intrinsic magnetic induction. The symbol is Bi or J. It is not difficult to see from J=μ0M that the difference between the magnetic polarization J and the thinning intensity M is only M multiplied by a constant μ0.


In soft magnetic materials, the value of the magnetic field strength is usually not more than 1000A/m, μ0 is 4×10-7H/m, and J=B-μ0H, so the difference between the magnetic induction B and the magnetic polarization 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.



Leave a Reply