Field lines are a basic way of representing a force visually, for diagramming and for the purpose of explanation. (We draw field lines and we talk of field lines as if they were really what a force is composed of.)

     Here are two basic diagrams involving field lines, which created as examples for this website:

Charge inside an irregularly shaped shell with field lines drawn.

     The figure on the left, it should be noted, has two lines, which are not, strictly speaking, field lines, added to the object being squeezed for the purpose of highlighting how it is the the object is squeezed by the gravitational field.

     It is important to be aware that the force which the field lines represent actually exists between the field lines and that the field lines are drawn to show the direction and, to some degree, the strength of the force. The arrows, of course, indicate direction, while the distance between lines is indicative of the field strength. Obviously, in the above diagrams, the field is strongest close to the source of the force and farther out as the field lines diverge, the force weakens. Within any given diagram, the number of field lines coming out of a charge or mass should be (to the extent possible) proportional to the magnitude of the force being represented. Thus, a charge of 2q should have twice the number of field lines as a charge q.

     However, field lines of this sort only give a 2-dimensional view of forces that really operate in three dimensions. The diagram to the right shows field lines which point away from the viewer (into the screen/page) and towards the viewer (out from the screen/page) respectively.

     What remains difficult is showing any angle to the field lines in the dimension of depth. These field lines can only be used to show the magnitude of the field in this third dimension. However, this convention is immensely useful for diagrams that need to show the flux through a surface.