Catalogue overview
electromagnetic field
geomagnetic field
Magnetic field type
magnetic field
abstract
magnetic field
(Simple definition: there is a magnetic field in the space where magnetic force can be generated. Magnetic field is a special substance. There is a magnetic field around the magnets, and the interaction between magnets is mediated by the magnetic field. )
A special substance existing in the space around current, moving charge, magnet or changing electric field. Because the magnetism of a magnet comes from current, which is the movement of charge. Simply put, it is to generate a magnetic field by moving the charge or changing the electric field. The basic characteristic of magnetic field is that it can exert force on the moving charge, and the force or torque of magnetic field on current and magnet comes from this.
Similar to the electric field, the magnetic field is a vector field distributed continuously in a certain spatial area. The basic physical quantity describing the magnetic field is the magnetic induction intensity vector b, which can also be expressed by the magnetic induction line graph. However, as a vector field, the nature of magnetic field is very different from that of electric field. The total magnetic field generated by moving charge or changing electric field, or the sum of the two, is a passive and rotating vector field, and the magnetic lines of force are a closed family of curves, which are uninterrupted and do not cross. In other words, there is no source of magnetic field lines, and there is no tail that converges magnetic field lines. The closure of the magnetic field lines shows that the loop integral along the magnetic field lines is not zero, that is, the magnetic field has a spin field instead of a potential field (conservative field), and there is no scalar function similar to the potential.
Electromagnetic field is the medium of electromagnetic action and a unified whole. Electric field and magnetic field are closely related and interdependent. Changing electric field produces magnetic field, changing magnetic field produces electric field, and changing electromagnetic field propagates in space in the form of fluctuation. Electromagnetic wave propagation speed is limited, and energy and momentum are interchangeable. Interaction between electromagnetic waves and objects, mutual transformation between electromagnetic waves and particles, etc. It is proved that electromagnetic field is an objective substance, and its "special" is that it has no static mass.
Magnetic phenomenon is one of the earliest physical phenomena recognized by human beings, and compass is a great invention in ancient China. Magnetic fields exist widely in the earth, stars (such as the sun), galaxies (such as the Milky Way), planets, satellites and interstellar and intergalactic spaces. In order to understand and explain many physical phenomena and processes, we must consider the important factor of magnetic field. In modern science and technology and human life, magnetic fields can be seen everywhere. Generators, motors, transformers, telegrams, telephones, radios, accelerators, thermonuclear fusion devices and electromagnetic measuring instruments are all related to magnetic phenomena. Even in the human body, with the activities of life, some tissues and organs will produce weak magnetic fields. The magnetic level of the earth is opposite to the poles of geography.
Magnetic field direction: specifies the direction of magnetic field force at a certain point in the magnetic field of the north pole of the small magnetic needle as the direction of electromagnetic field.
Magnetic induction line: draw some curves in the magnetic field so that the tangent direction of any point on the curve is the same as the magnetic field direction of that point. These curves are called magnetic field lines. The magnetic field lines are closed curves. The direction in which the north pole of the small magnetic needle points is defined as the direction of the magnetic field lines. The magnetic field lines around the magnet all come out from the N pole and enter the S pole. Inside the magnet, the magnetic field lines go from the S pole to the N pole.
electromagnetic field
electromagnetic field
The unity and floorboard of electric field and magnetic field have internal relations and interdependence. Time-varying electric field produces magnetic field, and time-varying magnetic field produces electric field, which is mutually causal and forms electromagnetic field. Electromagnetic field can be caused by charged particles moving at variable speed, or by currents with different intensities. No matter what the reason, the electromagnetic field always travels around at the speed of light, forming electromagnetic waves. Electromagnetic field is the medium of electromagnetic action, with energy and momentum, and it is a form of matter. The properties, characteristics and motion changes of electromagnetic field are determined by Maxwell equations.
geomagnetic field
geomagnetic field
Space magnetic field from the center of the earth to the top of the magnetosphere. The main research object of geomagnetism. The early understanding of the existence of geomagnetic field comes from the finger polarity of natural magnets and magnetic needles. The polarity of the magnetic needle is because the north magnetic pole of the earth (magnetic S pole) attracts the magnetic needle N pole, and the south magnetic pole of the earth (magnetic N pole) attracts the magnetic needle S pole. This explanation was first put forward by W Gilbert of England in 1600. Gilbert's hypothesis that the geomagnetic field comes from the earth itself is correct. This point has been confirmed by the first application of spherical harmonic function analysis by German mathematician C F Gauss in 1839.
The geomagnetic field is a vector field. To describe the intensity and direction of geomagnetic field at a certain point in space, three independent geomagnetic elements are needed. There are seven commonly used geomagnetic elements, namely, the total geomagnetic intensity F, the horizontal intensity H and the vertical intensity Z. X and Y are the north and east components of H, and D and I are the declination and inclination respectively. Among them, the observation history of magnetic declination is the earliest. In modern geomagnetic field observation, geomagnetic stations generally only record H, D, Z or X, Y and Z..
The geomagnetic field in near-earth space, like the magnetic field of a uniformly magnetized sphere, is less than 1 gauss near the ground poles, so the geomagnetic field is very weak. The unit of geomagnetic field intensity used to be gamma (γ), that is, 10 gauss. 1960 decided to adopt Tesla as the international magnetic measurement unit, 1 gauss = 10 Tesla (t), 1 gamma = 10 (-9) Tesla = 1 nanoteslas (nt for short). Although the geomagnetic field is very weak, it extends into the distant space, protecting the life on earth and human beings from cosmic radiation.
The geomagnetic field consists of two parts: the basic magnetic field and the changing magnetic field, the origins of which are completely different. The basic magnetic field is the main part of the geomagnetic field, which originates from the interior of the earth, is relatively stable and changes very slowly. The changing magnetic field includes all kinds of short-term changes of geomagnetic field, which mainly comes from outside the earth and is very weak.
The basic magnetic field of the earth can be divided into 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 magnetic field, which is produced by the electro-magnetohydrodynamic process in the liquid outer core of the earth, that is, the self-excited motor effect. Non-dipole magnetic field is mainly distributed in East Asia, West Africa, South Atlantic and South Indian Ocean, and its average intensity is about 10% of the magnetic field. Geomagnetic anomalies can be divided into regional anomalies and local anomalies, which are related to the distribution of rocks and ore bodies.
The variation of geomagnetic field can be divided into two types: quiet variation and disturbance variation. The quiet change is mainly a solar quiet daily change with a solar day as the cycle, and its field source is distributed in the ionosphere. Disturbance changes include magnetic storm, geomagnetic substorm, solar disturbance and geomagnetic pulsation. Field sources are various short-term current systems generated by the interaction between solar particle radiation and geomagnetic field in the magnetosphere and ionosphere. Magnetic storm is a strong magnetic disturbance that occurs simultaneously all over the world. The duration is about 1 ~ 3 days, and the amplitude can reach 10 NAT. Other interference changes are mainly distributed in the aurora region of the earth. In addition to the external source field, the changing magnetic field also has the internal source field. The endogenous field is generated by the current induced by the external field on the earth. Gaussian spherical harmonic analysis can be applied to the changing magnetic field and can distinguish the internal and external fields. According to the relationship between the internal and external fields of the changing magnetic field, the distribution of electrical conductivity in the earth can be obtained. This has become an important field of geomagnetism, called electromagnetic induction of the earth.
The variation of geomagnetic field is not only related to the electromagnetic process of magnetosphere and ionosphere, but also related to the electrical structure of crust and upper mantle, so it is of great significance in the study of space physics and solid geophysics.
Magnetic field type
1. A magnetic field whose strength and direction remain constant is called a constant magnetic field or a constant magnetic field, such as the magnetic field generated by ferromagnetic sheets and electrified electromagnets.
2. Magnetic fields with regular changes in the intensity and direction of alternating magnetic fields, such as those generated by power frequency magnetic therapy machines and heteropolar rotating magnetic therapy machines.
3. Pulsating magnetic field A magnetic field with a regular change in magnetic field intensity and a constant magnetic field direction, just like the magnetic field generated by a polar rotating magnetotherapy instrument and a pulsating DC electromagnet.
4. Pulse magnetic field Intermittent pulse current is generated by intermittent oscillator. When this current flows into the coil of electromagnet, pulse magnetic fields of various shapes can be generated. Pulsed magnetic field is characterized by intermittent magnetic field, and its frequency, waveform and peak value can be adjusted according to needs.
Constant magnetic field is also called static magnetic field, while alternating magnetic field, pulsating magnetic field and pulse magnetic field all belong to dynamic magnetic field. When the intensity of magnetic field in magnetic field space is equal or approximately equal, it is called uniform magnetic field, otherwise it is called non-uniform magnetic field. The farther away from the magnetic pole surface, the weaker the magnetic field and the greater the magnetic field intensity gradient.