124. Magnetic Induction Lines

The magnetic induction vector \(~ \overrightarrow{B}\,\) at each point has a certain magnitude and direction. A visual picture of the magnetic field can be obtained by drawing so-called magnetic induction lines. Magnetic induction lines or magnetic lines are lines tangent to which are directed in the same way as magnetic induction vector\(~ \overrightarrow{B}\,\) at a given point of the field. In this sense, the induction lines are similar to the electrostatic field power lines. There is, however, a significant difference between them.

Let us build, for example, magnetic induction lines for the magnetic field of a straight-line conductor with an electric current. From the previous experiments and for reasons of symmetry it follows that magnetic lines in this case will be concentric circles lying in a plane perpendicular to the current. The center of circles is on an axis of the conductor (fig. 1). As in the case of power lines of the electric field, magnetic induction lines can be arranged so that their density characterized the quantity of vector\(~ \overrightarrow{B}\,\) in this place. For example, in figure 1 concentric circles are thickened to the center. This should mean that the magnetic induction near the wire is greater than far away from it.

Let's give another picture of the magnetic field of the coil with electric current (solenoid). A picture of magnetic induction lines built with the help of magnetic arrows is shown in figure 2 (solenoid given in cut).

The picture of magnetic lines is easy to make visible using small iron filings. In a magnetic field, each piece of iron poured onto a sheet of cardboard is magnetised and behaves like a small magnetic arrow. The presence of such a large number of arrows makes it possible to determine the direction of the magnetic field in a large number of points, and therefore to find out more precisely the location of the induction lines.

An important feature of magnetic induction lines is that they have neither beginning nor end. They are always closed or go from infinity to infinity. Remember that this is not the case with the electrostatic field. The power lines of the electrostatic field start at positive charges and end at negative charges.

The closed magnetic lines represents fundamental property of a magnetic field. It consists in that the magnetic field has no sources. There are no magnetic charges like electric charges in nature. A field of this type is called a vortex field. The magnetic field is a vortex field.