What are the common magnetic fields

Electromagnetic Field

Electromagnetic field (electromagnetic field) is the unity and general term of the electric and magnetic fields that are intrinsically linked and interdependent. The time-varying electric field produces a magnetic field, and the time-varying magnetic field produces an electric field. Electromagnetic fields can be caused by charged particles moving at a variable speed, or by currents with varying strengths. Regardless of the cause, the electromagnetic field always propagates around at the speed of light, forming electromagnetic waves. Electromagnetic field is the medium of electromagnetic action, it has energy and momentum, and it is a form of matter existence. The properties, characteristics and the law of motion change of the electromagnetic field are determined by Maxwell's equations.

The electromagnetic field is the medium of electromagnetic action, and it is a unified whole. The electric field and the magnetic field are its two sides that are closely related and interdependent. The changing electric field produces a magnetic field, the changing magnetic field produces an electric field, and the changing electromagnetic field propagates in space in the form of waves. . Electromagnetic waves propagate at a limited speed, have exchangeable energy and momentum, the interaction between electromagnetic waves and objects, the mutual transformation between electromagnetic waves and particles, etc., all prove that the electromagnetic field is an objectively existing substance, and its "special" is only that it has no static mass. .

In electromagnetism, magnets, magnets, currents, and time-dependent electric fields all generate magnetic fields. The magnetic material or current in the magnetic field will feel the magnetic force due to the action of the magnetic field, thus showing the existence of the magnetic field. A magnetic field is a vector field; it has a direction and magnitude at any location in space.

Main Application Areas

Electromagnetic field (or wave) is a form of energy and is the most important energy source in the world today. The research field involves the generation, storage, transformation, transmission and application of electromagnetic energy.

As the carrier of information, electromagnetic waves have become the main means of information release and communication. The research contents include information release, exchange, transmission, storage, processing, reproduction and application.

Electromagnetic waves are an important means to detect the unknown world. The main research fields are the interaction characteristics of electromagnetic waves and targets, target detection and the acquisition of their characteristics.

As a means of measurement and control and positioning technology, electromagnetic waves constitute the application basis of modern industry, transportation, national defense and other fields

Electric and magnetic phenomena are the most important communication phenomena in nature, and they are also the first physical phenomena that scientists care about and study. Among them, scientists such as Layton, Franklin, and Volta contributed the most.

Before the 19th century, electrical and magnetic phenomena were widely concerned and studied as two independent physical phenomena. It is precisely because of these studies that the foundation for the establishment of electromagnetic theory is laid. At the end of the 18th century, the German philosopher Schelling believed that the universe was alive, not dead, and believed that electricity was the vitality and soul of the universe; the phenomena of electricity, magnetism, light and heat were interconnected. Oster, a follower of Schelling, has been studying the relationship between electricity and magnetism since 1807. In 1820, it was found that the current acts on the magnetic needle by force. Ampere found that the direction of the force and the current, and the direction of the vertical line from the magnetic needle to the wire passing the current were perpendicular to each other, and quantitatively established several mathematical formulas. This shows that there is a close connection between current and magnetism. Faraday believed that electricity, magnetism, light, and heat were interconnected. After Oster discovered in 1820 that electric current acts as a force on a magnetic needle, Faraday was acutely aware that magnetism must also have an effect on electricity. In 1821 he began to explore the magnetoelectric effect. In 1831 he discovered that when a magnetic holder is inserted into a conductor coil; an electric current is generated in the coil. It shows that there is a close connection between electricity and magnetism. Maxwell delved into and explored the interaction between electricity and magnetism, developing the concept of fields. On the basis of Faraday's experiment, he summarized the laws of macroscopic electromagnetic phenomena, introduced the concept of displacement current, and proposed a set of partial differential equations describing electromagnetic phenomena, namely Maxwell's equations, and established the macroscopic classical electromagnetic field theory German scientist Hertz, 1887 Exciting one loop antenna with a spark gap and receiving with another loop antenna with a band gap confirmed Maxwell's predictions about the existence of electromagnetic waves, an important experiment that led to the later invention of wireless telegraphy. From then on, the era of application and development of electromagnetic field and electromagnetic wave theory began.

Geomagnetic Field

The geomagnetic field is the magnetic field in the space from the center of the earth to the magnetopause. The main research object of geomagnetism. Human's early understanding of the existence of the geomagnetic field comes from the polarity of natural lodestones and magnetic needles. The geomagnetic north pole is near the geographic south pole; the geomagnetic south pole is near the geographic north pole. The polarity of the magnetic needle is because the north magnetic pole of the earth (the magnetic S pole) attracts the N pole of the magnetic needle, and the south magnetic pole of the earth (the magnetic N pole) attracts the S pole of the magnetic needle. This explanation was first proposed in 1600 by the English W. Gilber. Gibb's assumption that the geomagnetic field originates from the Earth's body is correct. This has been confirmed in 1839 by the German mathematician C.F. Gauss first* using the spherical harmonic function analysis method.

The magnetic field lines of the geomagnetism and the geographic meridians are not parallel, and the angle between them is called the magnetic declination angle. Shen Kuo, a famous scientist in ancient China, was the first to notice the phenomenon of magnetic declination.

The basic magnetic field of the earth can be divided into several components: dipole magnetic field, non-dipole magnetic field and geomagnetic anomaly. The dipole magnetic field is the basic component of the geomagnetic field, and its intensity accounts for about 90% of the total intensity of the geomagnetic field. The non-dipole magnetic field is mainly distributed in several regions such as eastern Asia, western Africa, the South Atlantic and the southern Indian Ocean, with an average strength of about 10% of the geomagnetic field. Geomagnetic anomalies are further divided into regional anomalies and local anomalies, which are related to the distribution of rocks and ore bodies.

The earth's changing magnetic field can be divided into two types: calm change and disturbance change. The quiet change is mainly a solar day change with a solar day as a cycle, and its field sources are distributed in the ionosphere. Interference changes include magnetic storms, geomagnetic substorms, solar disturbance diurnal changes and geomagnetic pulsations, etc. The field sources are various short-lived current systems generated in the magnetosphere and ionosphere by the interaction of solar particle radiation and the geomagnetic field. Magnetic storms are strong magnetic disturbances that occur simultaneously around the world, with a duration of about 1 to 3 days and an amplitude of up to 10 nT (Nat). Several other disturbance variations are mainly distributed in the Earth's auroral region. In addition to the external source field, the changing magnetic field also has an internal source field. Intrinsic fields are generated by electric currents induced in the interior of the earth by extrinsic fields. Using Gaussian spherical harmonic analysis for the changing magnetic field, this inner and outer field can be distinguished. According to the relationship between the internal and external fields of the changing magnetic field, the distribution of electrical conductivity in the interior of the earth can be obtained. This has become an important area of geomagnetism, called Earth's electromagnetic induction.

The earth's changing magnetic field is not only related to the electromagnetic processes of the magnetosphere and ionosphere, but also to the electrical structure of the crust and upper mantle, so it is of great significance in the research of space physics and solid geophysics.