Electromagnetic field theory

Physical Theory

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Electromagnetic field theory is a research electromagnetic field Zhongge physical quantity The theory of the relationship between spatial distribution and temporal variation. Coulomb's law Reveals the static electricity between charges Force It is inversely proportional to the square of the distance between them. Ampere and others found that Current element The force between inverse ratio Relationship. Maxwell comprehensively summarized electromagnetics All the research results have established a complete electromagnetic field theory system. with Maxwell's equations The core of electromagnetic theory is Classical physics One of the most proud achievements.

Theoretical points: variable magnetic field can excite vortex Electric field, changing electric field can excite vortex magnetic field. Electric field and magnetic field are not isolated from each other. They are connected and excited to form a unified electromagnetic field.

The influence of electromagnetic field on matter is related to the nature of matter. Electromagnetic field theory is not only an important part of physics, but also the theoretical basis of electrical technology.

Chinese name: Electromagnetic field theory
Nature: theory

Properties: electromagnetic field
Characters: Faraday

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Research History

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it is noted that Electromagnetic phenomenon The first is from their mechanical effects. Coulomb's law It reveals that electrostatic interactions It is inversely proportional to the square of the distance between them. A. - M. Ampere and others found that Current element Between Force It also conforms to the inverse square relationship and proposes Ampere's loop law 。 Based on this and Newton Law of universal gravitation Very similar, S.D Poisson , C.F. Gauss, etc Gravitational theory , various field vectors are also introduced for electromagnetic phenomena, such as electric field intensity 、 flux Density（ Electric displacement vector ）、 magnetic field intensity 、 Magnetic flux density And express these quantities as Spatial coordinates Function of. But at that time, these quantities were just mathematical means proposed for the convenience of description. In fact, it was believed that the physical action between charges or currents was Action at a distance 。 Until M. Faraday, he thought that the field was a real physical existence, and electricity or magnetic force passed through the field Force line Gradually transmitted, and finally applied to the charge or current. He discovered the famous Law of electromagnetic induction , and use Magnetic line of force The law is successfully expounded by the model of. In 1846, M. Faraday also put forward the idea that light waves are force line vibrations. J. C. Maxwell inherited and developed these ideas of Faraday, imitated the method in fluid mechanics, adopted strict mathematical form electromagnetic field The basic laws of differential equation , people call it Maxwell's equations 。 Maxwell added Ampere's loop law to the equation Displacement current He believes that displacement current can also produce magnetic field. According to these equations, Maxwell also derived that the propagation of the field takes time propagation velocity Is a finite value and equal to the speed of light, so it is concluded that electromagnetic wave It has common properties with light waves and foresees its existence electromagnetic radiation Phenomenon. electrostatic field 、 Constant magnetic field And in conductors Constant current The electric field of is also included in Maxwell's equation as a special case that does not change with time.

electromagnetic induction

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Faraday's electromagnetic induction experiment Work And Electromagnetic energy The connection proves that the two can be transformed into each other. Maxwell further proposed that there are certain energy density That is, the energy is localized in the field. According to this theory, J.H Poynting In 1884, it was proposed that Poynting theorem , vector E × H represents Energy flow 。 These theories have opened the way for the wide application of electric energy and the manufacture of generators, transformers, motors, etc Electrotechnical equipment Laid a theoretical foundation.

The electromagnetic radiation predicted by Maxwell was generated by H. R. Hz It is confirmed by the experiment. Electromagnetic waves can transmit information and energy in space without relying on the connection of conductors. This is radio technology The extensive application of. The electromagnetic field theory gives the distribution and change law of the field. If the properties of the medium in the electric field are known, and appropriate mathematical means are used, the electrical equipment Structural design , material selection energy conversion 、 Operating characteristics So as to greatly promote the progress of electrical technology. The electromagnetic field theory involves a lot of charged particle Under joint action Statistical average As a result, it does not involve the inhomogeneity of material structure and energy change Discontinuity 。 It belongs to macro theory, or classical theory. Theories involving the properties and behaviors of individual particles belong to microscopic theories, which cannot only rely on electromagnetic field theory to analyze the electromagnetic phenomena of microscopic origin, such as electromagnetic properties of relevant media, laser, superconductivity, etc. This does not deny the correctness of the electromagnetic field theory in the macro sense. Electromagnetic field theory is not only an important part of physics, but also the theoretical basis of electrical technology.

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Maxwell is another bit set after Faraday electromagnetics Dacheng A great scientist in one. He comprehensively summarized all the achievements of electromagnetic research, and on this basis, he proposed“ Induced electric field ”And“ Displacement current ”Has established a complete electromagnetic field Theoretical system , not only scientifically predicted the existence of electromagnetic waves, but also revealed that light, electricity Magnetic phenomenon The internal connection and unity of the, completed another big synthesis of physics. His theoretical achievements are modern radio electronics industry Laid a theoretical foundation.

seeing the name of a thing one thinks of its function, Maxwell's equations Maxwell's four equations describing electric and magnetic fields.

Of equations Differential form , commonly referred to as Maxwell's equation. In Maxwell Equations The electric field and magnetic field have become an inseparable whole. The equations systematically and completely summarize the basic laws of electromagnetic field and predict the existence of electromagnetic waves.

Maxwell's Eddy electric field and Displacement current The core idea of the hypothesis is that the changing magnetic field can stimulate vortex Electric field, changing electric field can excite vortex magnetic field; Electric field and magnetic field are not isolated from each other. They are connected and excited to form a unified electromagnetic field. Maxwell further integrated all laws of electric field and magnetic field and established a complete theoretical system of electromagnetic field. The core of this electromagnetic field theory system is Maxwell's equations.

The position of Maxwell's equations in electromagnetics is similar to Newton's law of motion In mechanics. The electromagnetic theory with Maxwell's equations as its core is Classical physics One of the most proud achievements. What it reveals Electromagnetic interaction Maxwell's perfect unity has established such a belief for physicists that the interactions of matter should be unified at a higher level. In addition, this theory is widely applied to the technical field.

The differential form of Maxwell's equations is shown in Figure 1:

Maxwell's equations (1 sheet)

The above four equations can be explained one by one as follows: at any point in the electromagnetic field

（1） Electric displacement Of divergence Equal to this point Free charge Of Bulk density （ Gauss law ）；

(2) The divergence of magnetic induction intensity is equal to zero everywhere（ Gauss magnetic law ）；

(3) Electric field intensity curl It is equal to the negative value of the change rate of magnetic induction at this point（ Faraday's law ）；

(4) The curl of the magnetic field strength is equal to the conduction at this point current density And Displacement current density Vector sum of (Maxwell Ampere loop law).

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