ELECTROMAGNETIC FIELD THEORY (EMT) MAXWELL EQUATION
The first case is to give the
distribution of charge and current. Solve the excited electromagnetic field;
the second case is to give the external electromagnetic field and solve the
motion of the internal charged particles and current. Sometimes these two
situations will be merged into one, but the processing method at this time can
only be carried out in order: first determine the movement of the charged
particles in the external field while ignoring the radiation, and then use the
trajectory of the moving particles as the radiation source Distribution
calculation of electromagnetic radiation. Obviously, this treatment can only be
approximately correct in electrodynamics. Furthermore, although Maxwell's
equations are linear in their own right, the interaction between charges and
currents and the electromagnetic fields excited by them in some
electro-mechanical systems cannot be ignored. For such systems, we cannot yet
use electrodynamics. Fully understand. Despite a century of effort, people have
not yet been able to obtain a set of widely accepted classical equations
describing the motion of charged particles, and they have not been supported by
any useful experimental data.
Electromagnetic Field Theory
Electromagnetic
field theory is a theory to study the relationship between various physical
quantities in the electromagnetic field and its spatial distribution and time
change. Coulomb's law reveals that the electrostatic force between charges is
inversely proportional to the square of the distance between them. Amper et al.
found that the force between current elements also conforms to the inverse
square relationship. Maxwell comprehensively summarized all the results of
electromagnetic research and established a complete theoretical system of
electromagnetic fields. Electromagnetic theory with Maxwell's equations at its
core is one of the most proud achievements of classical physics.
Theoretical point: The changing
magnetic field can excite the vortex electric field, and the changing electric
field can excite the vortex magnetic field. The electric field and the magnetic
field are not isolated from each other.
Research History
People
noticed that electromagnetic phenomena started with their mechanical effects.
Coulomb's law reveals that the electrostatic force between charges is inversely
proportional to the square of the distance between them. A.-M. Ampere and
others found that the force between current elements also conformed to the
inverse square relationship, and proposed the Ampere loop law. Based on this is
very similar to Newton’s law of universal gravitation, Poisson, Gauss, and
others modeled the theory of gravity and introduced various field vectors to
electromagnetic phenomena, such as electric field strength, electric flux
density (electric displacement vector), magnetic field strength, magnetic Flux,
etc., and express these quantities as a function of spatial coordinates.
However, these quantities were only mathematical methods proposed for the
convenience of description. In fact, the physical interaction between charges
or currents is considered to be an over-distance effect. It was not until M.
Faraday that he believed that the field was a real physical existence, and
electric power or magnetic force was gradually transferred through the lines of
force in the field, and finally acted on the electric charge or current. He
discovered the famous law of electromagnetic induction in 1831, and
successfully elaborated the law with a model of magnetic field lines. In 1846,
M. Faraday also proposed the idea that light waves are vibrations of force
lines. J.C. Maxwell inherited and developed these ideas of Faraday. Following
the methods in fluid mechanics, using strict mathematical form, the basic laws
of electromagnetic field were reduced to four differential equations, which
were called Maxwell's equations. In the equation, Maxwell added the effect of
displacement current to Ampere's loop law. He believes that displacement
current can also generate a magnetic field. According to this set of equations,
Maxwell also derived that the propagation of the field takes time, and its
propagation velocity is a finite value and is equal to the speed of light, so
as to conclude that electromagnetic waves and light waves have common
properties, and predict the existence of electromagnetic radiation.
Electrostatic fields, constant magnetic fields, and electric fields with
constant currents in conductors are also included in Maxwell's equations, just
as special cases that do not change with time.
Electromagnetic Induction
Faraday's electromagnetic
induction experiment connected mechanical work and electromagnetic energy,
proving that the two can be transformed into each other. Maxwell further
proposed that there is a certain energy density in the electromagnetic field,
that is, the energy is localized in the field. According to this theory, J.H.
Poynting proposed Poynting's theorem of energy propagation in a time-varying
field in 1884. The vector E×H represents the energy flow through the unit area
and unit time in the field. These theories opened the way for the widespread
application of electrical energy and laid the theoretical foundation for the
manufacture of electrical equipment such as generators, transformers, and
motors.
The electromagnetic radiation
predicted by Maxwell was confirmed by H.R. Hertz's experiment in 1887.
Electromagnetic waves can propagate information and energy in space without
relying on the connection of conductors. This creates conditions for the
widespread application of radio technology.
The theory of electromagnetic field gives the
distribution and change law of the field. If the nature of the medium in the
electric field is known, then appropriate mathematical methods can be used to
analyze and calculate the structural design, material selection, energy
conversion, operating characteristics, etc. of the electrical equipment. The
content of electromagnetic field theory belongs to the statistical average
result of a large number of charged particles. It does not involve the
non-uniformity of the material structure and the discontinuity of energy
changes.
The theories involving the
properties and behavior of individual particles belong to microscopic theories,
and we cannot rely solely on electromagnetic field theory to analyze the
electromagnetic phenomena of microscopic causes, such as the electromagnetic
properties of the medium, lasers, and superconductivity. This does not negate
the correctness of electromagnetic field theory in a 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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