Flux is the presence of a force field in a specified physical medium, or the flow of energy through a surface. In electronics, the term applies to any electrostatic field and any magnetic field . Flux is depicted as "lines" in a plane that contains or intersects electric charge poles or magnetic poles. Three examples of flux lines are shown in the illustration.
Drawing A shows the geometric orientation of the lines of flux in the vicinity of an electrically charged object. The intensity of the field is inversely proportional to the separation between the lines of flux. The flux density , and hence the electrostatic field strength, decreases as the distance from the charged object increases. Electrostatic flux density is inversely proportional to the distance from the charge center.
Drawing B illustrates flux lines surrounding a current-carrying conductor as they appear in a plane perpendicular to the conductor. As with the flux surrounding an electrically charged object, the separation between the flux lines increases as the distance from the conductor increases. Magnetic flux density is inversely proportional to the distance from a current-carrying conductor, as measured in a plane perpendicular to the conductor.
Drawing C shows the general orientation of the lines of flux of an electrostatic field between two oppositely charged poles in a plane containing the centers of both poles. In a magnetic field between opposite poles, the flux lines have the same general shape and orientation, so this drawing also applies to that situation. The flux density is greatest near the poles. The flux density is considerable along and near a line connecting the poles. As the distance from the line connecting the poles increases, the flux density decreases.
Flux lines are intangible; they cannot be seen. But they can be observed indirectly, and they produce demonstrable effects. If you place iron filings on a sheet of paper and place the paper on a magnet so both magnetic poles are near the paper, the filings line up in a pattern resembling illustration C. This demonstration is common in school science classes.