People have been exposed to the phenomena of electricity and magnetism for a long time, and they know that magnetic rods have North Pole and South Pole. /kloc-in the 0/8th century, it was found that there are two kinds of charges: positive charge and negative charge. Charge and magnetic pole repel each other and opposites attract. The direction of the force is on the connection line of charges or magnetic poles, and the magnitude of the force is inversely proportional to the square of the distance between them. Similar to gravity in these two points. At the end of 18, it was found that charge could flow, which is current. But the connection between electricity and magnetism has never been discovered.

In the early19th century, Oster discovered that current could deflect a small magnetic needle. Then Ampere found that the direction of force and the direction of current, as well as the vertical direction from the magnetic needle to the wire passing through the current, were perpendicular to each other. Soon after, Faraday found that when a magnetic rod is inserted into a coil, a current will be generated in the coil. These experiments show that there is a close relationship between electricity and magnetism. After the connection between electricity and magnetism was discovered, people realized that the essence of electromagnetic force was similar to gravity in some aspects, but different in others. For this reason, Faraday introduced the concept of electric field lines, thinking that current produces magnetic field lines around wires, and charge produces electric field lines in all directions, and on this basis, the concept of electromagnetic field was produced.

Now people realize that electromagnetic field is a special form of matter. The electric charge generates an electric field around it, which exerts a force on other charges. Magnets and current generate a magnetic field around them, which acts on other magnets and objects with current inside. Electromagnetic field also has energy and momentum, and it is the medium to transmit electromagnetic force, which permeates the whole space.

/kloc-In the second half of the 9th century, Maxwell summarized the law of macroscopic electromagnetic phenomena and introduced the concept of displacement current. The core idea of this concept is: changing electric field can produce magnetic field; Changing the magnetic field can also produce an electric field. On this basis, he put forward a set of partial differential equations to express the basic laws of electromagnetic phenomena. This set of equations, called Maxwell equations, is the basic equation of classical electromagnetism. Maxwell's electromagnetic theory predicted the existence of electromagnetic waves, and its propagation speed was equal to the speed of light, which was later confirmed by Hertz's experiments. So people realized that Maxwell's electromagnetic theory correctly reflected the law of macroscopic electromagnetic phenomena, and affirmed that light is also an electromagnetic wave. Because electromagnetic field can act on charged particles with a force, a moving charged particle is acted by both electric field and magnetic field. Lorentz attributed the force of electromagnetic field on moving charge to a formula, which is called Lorentz force. Maxwell equations and Lorentz force, which describe the basic laws of electromagnetic field, form the basis of classical electrodynamics.

Inspired by a series of experiments such as Oster's current magnetic effect experiment, Ampere realized that the essence of magnetic phenomenon is current, and attributed all kinds of interactions involving current and magnet to the interaction between currents, and put forward the basic problem of finding the interaction law of current elements. In order to overcome the difficulty that isolated current elements can not be measured directly, Ampere carefully designed four zero-indicating experiments after careful theoretical analysis, and obtained the results. However, due to the concept of over-distance action of ampere on electromagnetic action, the assumption that the force between two current elements follows the connecting line is imposed in theoretical analysis, and Newton's third law is expected to be obeyed, which makes the conclusion wrong. The above formula is the modified result, which abandons the assumption that the wrong force is along the line. From the point of view of short-range action, it should be understood that one current element generates a magnetic field and the magnetic field exerts a force on another current element.