# Lines of Force

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## Lines of force

Imaginary lines in fields of force whose tangents at any point give the direction of the field at that point and whose number through unit area perpendicular to the field represents the intensity of the field. The concept of lines of force is perhaps most common when dealing with electric or magnetic fields.

Electric lines of force are drawn to represent, or map, an electric field graphically in the space around a charged body. They are of great help in visualizing an electric field and in quantitative thinking about such a field. A magnetic field may also be represented by lines of force. Magnetic lines of force due to magnets originate on north poles and terminate on south poles, both inside and outside the magnet. *See* Electric field

## Lines of Force

imaginary curves in a field of force whose tangents at every point in space coincide in direction with the vector characterizing the given field—with the field strength vector in the case of electrical and gravitational fields and with the magnetic induction vector in the case of magnetic fields. The representation of fields of force by lines of force is a special case of the depiction of any vector field by flow lines.

Since the field strength or magnetic induction is a single-valued function of the point, only one line of force can pass through each point in space. The density of the lines of force is usually selected so that the number of lines passing through a unit area perpendicular to the lines is proportional to the field strength or magnetic induction in the area. Thus, lines of force give a graphic picture of the field distribution in space: the density and direction of the lines of force characterize the magnitude and direction of the field strength.

The lines of force of an electrostatic field are always open: they begin at positive charges and terminate at negative charges (or go out to infinity). The lines of force of the magnetic induction vector are always closed—that is, a magnetic field is a rotational field. Iron filings placed in a magnetic field align themselves along the lines of force; this phenomenon makes possible experimental determination of the shape of the lines of force of magnetic induction. The rotational electric field produced by a varying magnetic field also has closed lines of force.