Physical Definition of Magnetic Field Lines

Magnetic field lines, also known as magnetic field lines, are some curves that depict the distribution of the magnetic field. The tangent direction of each point on the curve is consistent with the direction of the magnetic field at this point. The direction of the magnetic induction intensity is the same as the tangent direction of the magnetic field line at this point, and its magnitude is proportional to the density of the magnetic field line. Understanding the basic characteristics of magnetic lines of force is the basis for mastering and analyzing magnetic circuits.

Magnetic field lines are artificial, hypothetical curves. There are innumerable magnetic lines of force. The magnetic lines of force are three-dimensional. All the magnetic lines of force do not cross. The magnetic lines of force always start from the N pole and enter the nearest S pole and form. These hypothetical curves are based on an interesting little experiment that requires nothing more than a bar magnet and some iron filings to be displayed on a piece of flat glass.

The magnetic field lines are all closed, see the magnetic flux continuity theorem in electromagnetism.

Similar to current, magnetic field lines always take the path with the least reluctance (maximum magnetic permeability), so the magnetic field lines are usually straight or curved, and there are no magnetic field lines that turn at right angles. Any two magnetic field lines in the same direction repel each other, so there is no intersecting magnetic field lines.

When the ferromagnetic material is not saturated, the magnetic field lines are always perpendicular to the polar plane of the ferromagnetic material. When a ferromagnetic material is saturated, the magnetic field lines in the ferromagnetic material behave the same as in non-ferromagnetic media such as air, aluminum, copper, etc. Since magnetic field lines have such fundamental properties, the magnetization state of a medium depends on the magnetic properties and geometry of the medium. Obviously, under normal circumstances, the medium is in a non-uniform magnetization state, that is to say, the magnetic field lines inside the medium are usually curved and unevenly distributed; in addition, although there are electrical insulators in nature, there are no magnetic insulators ( Except for superconducting substances), so that there is magnetic leakage in the usual magnetic circuit. The medium is in a non-uniform magnetization state and the magnetic circuit has two characteristics of magnetic flux leakage, which determines that the accurate calculation of the magnetic circuit is very complicated.

Suppose that the small magnetic needle is placed in the magnetic field of the magnet, and the small magnetic needle is affected by the magnetic field, and its poles point to a certain direction when it is stationary. At different points in the magnetic field, the small magnetic needles are not necessarily pointing in the same direction when they are stationary. This fact shows that the magnetic field is directional. It is physically stipulated that at any point in the magnetic field, the force direction of the N pole of the small magnetic needle is the direction of the magnetic field at that point.

The concept of magnetic field lines was first invented and introduced by the famous physicist Faraday. In the electric field, electric field lines can be used to describe the direction of the electric field at each point, and in the magnetic field, the magnetic field direction of each point can also be described visually with the magnetic field lines. The magnetic field lines are drawn in the magnetic field and do not actually exist. Some directional curves (also straight), the tangent direction of each point on these curves is consistent with the direction of the magnetic field at that point.

Magnetic field lines are artificially imaginary curves to visually study the magnetic field, not the real curves that exist objectively in the magnetic field.