It may further be noted that since magnitude of depends on the direction of dl, it is called the directional derivative. If is called the scalar potential function of the vector function .

Divergence of a Vector Field:

In study of vector fields, directed line segments, also called flux lines or streamlines, represent field variations graphically. The intensity of the field is proportional to the density of lines. For example, the number of flux lines passing through a unit surface S normal to the vector measures the vector field strength.

Fig 1.18: Flux Lines