What is magnetic field strength?

- Oct 31, 2017 -

Magnetic field strength in the history of the first by the magnetic charge point of view. In contrast to the Coulomb's law of charge, it is believed that there are positive and negative magnetic loads, and the Coulomb's law of magnetic charge is proposed. The force on the magnetic field in the magnetic field is called the magnetic field strength. Later, Amp proposed a molecular current hypothesis that there is no magnetic charge, the nature of magnetic phenomena is the molecular current. Since the intensity of the magnetic field is expressed by the magnetic induction intensity B. However, in the magnetization problem of the magnetic medium, the magnetic field strength H still plays an important role as an derived auxiliary quantity.


Basic interpretation

Magnetic field strength describes the physical quantities of the magnetic field properties. Expressed in H. The definition is H = B / μ0-M, where B is the magnetic induction intensity, M is the magnetization, μ0 is the permeability in vacuum, μ0 = 4π × 10-7 Weber / (rice). H units are Ann / m. The unit of H in the Gaussian unit is Oster. 1 ampere / m = 4π × 10-3 oersted.

The history of the magnetic field strength H is defined from the magnetic load point of view. The magnetic load is summed up from the study of the interaction of permanent magnets. At that time, it was not known about the relationship between magnetism and current. Since the strip magnets had N and S poles, and the isotropic magnetic poles were repelled, the opposite magnetic poles were attracted, and this was very similar to the interaction between positive and negative charges. Compared with the charged body, it is assumed that the magnetic pole is formed by the magnetic charge distribution. N pole of the magnetic charge is the magnetic charge, S pole on the magnetic charge is called negative magnetic charge. Homogeneous magnetic charge repulsion, the opposite sex magnetic charge. When the magnetic pole itself is much smaller than the distance between the positive and negative poles, the magnetic load on the magnetic pole is called the point magnetic charge.

Coulomb is experimentally obtained by the law of the interaction between two points of magnetic charge, called magnetic coulomb law, expressed as Fm = κqm1qm2 / γ2r, where κ is the proportional coefficient, and the amount of units in the selection, qm1, qm2 denotes the value of the magnetic charge at each point, γ is the distance between the two points of magnetic charge, and γ is the unit vector on the two lines. According to the magnetic load, the definition of the strength of the electric field is defined as the magnetic field strength H is such a vector: its size is equal to the single point of the magnetic load in the magnetic field of a force, the direction of the magnetic load at that point by the magnetic field force direction. The table is H = Fm / qm0, where qm0 is the magnetic charge of the magnetic pole of the test point, and Fm is the magnetic force of qm0 in the magnetic field. Obviously, the magnetic field strength formula of the point magnetic charge is H = κqm / γ2r corresponding to the electric field intensity formula E = 1 / 4πεθq / γ2r of the point charge. It is reasonable to call H a magnetic field from the magnetic load point of view, which corresponds to E. From the molecular current point of view, the magnetic field is the current (motion charge) generated, and to the current (motor charge) to force. From the current element, kinematic charge, etc. in the magnetic field at the angle of the magnetic field reflects the nature of the definition of B (B = F maximum / I2dl2, B = F maximum / qv⊥). Obviously, at this time B corresponds to the electric field intensity E. B is called the magnetic field strength, because the magnetic field strength has been occupied by the history of H, so B called magnetic induction intensity. Magnetic charge point of view in the history is completely in the analogy with the charge proposed, the experiment did not find a separate magnetic charge. In 1931 Dirac from the perspective of quantum mechanics proposed the existence of magnetic unipolar, has not yet found it, but did not deny its existence, is still a research topic. The molecular current view and the magnetic load point are different from the microscopic model, but the macroscopic results are exactly the same. Regardless of the existence of magnetic charge, in the discussion of permanent magnet problem using magnetic load point of view is often relatively simple, still has application value.

In the cis-magnetic and anti-magnetic medium B = μH holds. From the formula, B and H are proportional and consistent. In the case where H has a certain symmetry, H can be obtained by using the Ampere loop theorem when there is a medium. This method can also be used to approximate the calculation of H, B in soft ferromagnetic materials. In the hard magnetic material in general H, B, M direction are different, the relationship between them can only be used H = B / μ0-M said.

*The above information from Chinese Encyclopedia of Secondary Education.

Calculation formula


Magnetic field strength, the magnetic field strength is defined as:


Similar to the intensity of the electric field.

In the medium, the magnetic field strength is usually defined as:


The formula for the magnetization.

In the International System of Units (SI), the unit of the magnetic field strength is in [A] / m (), the dimension is; in the Gaussian unit system (CGS), the magnetic field strength unit is [ 1 ampere / meters equivalent to Austria. [Information from Electromagnetics and electrodynamics] 

Simple definition: the magnetic field at a point magnetic induction B and the dielectric permeability μ ratio is called the point of the magnetic field strength.

The magnetic field strength is defined by the magnetic induction intensity and the permeability, and it is important to understand both.

The magnetic field strength in the medium

The closed loop integral of the magnetic field strength in the constant magnetic field is only related to the conduction current of the loop link without the binding of the molecular current, ie [I你formation from Electromagnetics (third edition)]


In vacuum, the magnetic field strength


When there is a magnetic medium,


In its interior


And blob.png So the formulablob.png is the magnetic susceptibility; M is magnetization.

Maxwell equations

In the time-varying electromagnetic field, the closed loop integral of the magnetic field strength is related to the total current of the loop link, but still does not include the bound molecular current


The full current consists of conduction current and displacement current. The differential form of this type is [Information from Electronics and electrodynamics]


blob.png is the conduction current density in formula 


Is the time rate of change of the electric displacement vector, that is, the displacement current density, the area integral is.

Calculation Formula of Magnetic Field Strength in Magnetic Circuit

Magnetic field strength formula:


Where H is the magnetic field strength, in units of A / m; N is the number of turns of the excitation coil; I is the excitation current (measured value) in units of A; Le is the effective magnetic circuit length of the test sample in m.

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