It is useful to know the electric field for some simple continuous distributions of charge. The field has cylindrical symmetry. The field on the axis of a planar uniform ring of charge Q and radius r may be found by integration. This expression is approximately valid near the center of a finite disk for perpendicular distances small compared to the radius. Electric flux Electric field lines represent the flow of the vector force field.
Positive charges are sources.
Negative charges are sinks. An area element vector dA at some space point x representing a small planar patch of area has magnitude equal to the area of the patch and is directed normal to the patch. The differential flux is the inner product of the electric field vector at x with the differential area element. The area elements of a closed surface are all outward directed, by convention. Consider a simple closed surface near a charge Q. Each element of surface on the near side projects onto an element of surface on the far side.