The so-called magnetic field refers to the space where magnetic force exists. The magnetic field is one of the basic forms of the existence of matter, which exists in the space around the magnet, the space around the moving charge, and the space around the current. The most common phenomenon in a magnetic field is that two magnetic objects attract or repel each other, so the interaction between the magnets is through a magnetic field. The magnetic field also exerts a force on the energized conductor, indicating that the magnetic field has a force property. When the energized conductor moves within the magnetic field, the magnetic field force will work on the energized conductor, indicating that the magnetic field has energy. These manifestations illustrate the physical properties of the magnetic field.
There are several ways to express the strength of a magnetic field.
(1) Magnetic force lines Magnetic lines are curves that visually represent the strength, direction, and distribution of a magnetic field, as shown in Figure 1.
FIG 1 to obtain permanent magnet and a solenoid magnetic field lines distribution
The degree of density of the magnetic lines of force indicates the strength of the magnetic field. The direction of the magnetic lines of force is defined as follows: starting from the N pole outside the magnet, entering the S pole through the space, and forming a closed curve from the S pole to the N pole inside the magnet. The direction of the magnetic field line is the direction of the magnetic field, and the tangential direction of a point on the magnetic line is the direction of the magnetic field strength of the magnetic field at that point.
(2) Magnetic induction The magnetic field has a powerful effect on the moving charge or current-carrying conductor. The physical quantity that describes the nature of a magnetic field by the force of a magnetic field on a current-carrying wire is called magnetic induction.
The magnitude of the magnetic induction is defined as the maximum magnetic moment experienced by the test coil of the unit magnetic moment at a point in the magnetic field, ie
Â
B = | Mo |
Pm |
Where M o — the maximum magnetic moment experienced by the test coil in the magnetic field;
P m — The magnetic moment of the coil itself, equal to the product of its current intensity and the area it encloses, ie P m =IS. [next]
Magnetic induction In the "utility unit system", when the coil with a magnetic moment of 1 ampere-meter 2 is located at a certain point in the magnetic field, if it receives a maximum magnetic moment of 1 Newton-meter, the magnetic induction at that point is 1 Weber / m 2 , called Tesla (T), referred to as "special". In numerical value
1 Weber / m 2 = | 1 Newton·m | = 10 4 Gauss ( G s ) |
1 ampere meter 2 |
(3) Magnetic field strength Magnetic field strength is a physical quantity used to measure the strength of a magnetic field. The definition of the magnetic field strength is: in any magnetic medium (a substance that can strengthen or weaken a magnetic field due to magnetization in an external magnetic field is called a magnetic medium), the ratio of the magnetic induction B at a point in the magnetic field to the magnetic permeability at the same point. , the magnetic field strength at this point, denoted by the symbol H, ie
H = | B |
μ |
The unit of the magnetic field strength is determined as follows: In the above formula, the unit of the magnetic induction B is Weber/m 2 (Wb/m 2 ), and the unit of the magnetic permeability μ is Weber/(A·m) [Wb/(A·m) )], the unit of magnetic field strength is ampere/meter 2 (A/m). The unit of A/m is very small. In practical applications, the common units are ampere/cm (A/cm) and Oe (Oe). The conversion relationship between them is:
1A | = | 10 -2 A | =4 π× 10 -3 Oe |
m | Cm |
Both the magnetic field strength and the magnetic induction are physical quantities indicating the direction and strength of the magnetic field. There is a connection between them (B=μH) and a difference. Since the magnetization of the magnetic medium in the magnetic field has an influence on the magnetic field, in the case of a uniform magnetic medium, the magnetic field generated by the magnetization of the medium is represented by the magnetic induction B. The magnetic field caused by pure current or moving charge is expressed by the magnetic field strength H.
In the case of the same magnetic field, if different magnetic media are placed, there is a different magnetic induction B, but the magnetic field strength does not change. For example, in a magnetic field, a piece of iron with a magnetic permeability μ that is much larger than the magnetic permeability μ 0 of air is placed. Most of the magnetic lines of force pass through the iron block intensively, and this change can be detected with iron filings. This shows that the distribution of the magnetic induction intensity has changed, but the magnetic field strength has not changed.
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