The flux of a vector field through a surface area is the amount of
whatever the field represents passing through the area. Let **E** be
a constant vector field. The area
vector is **A** = A**n**. The length of this vector is the size of the
area, while its orientation **n** is perpendicular to the area. The
normal **n** to the surface can point into two different directions.
If the area is not enclosing a volume, we can choose either one of those
perpendicular directions. The flux of the field through the area is
Flux = **E**∙**A** = E A cos(angle), where angle is the
angle between the directions of **E** and **A**. The flux can be
positive of negative.

If **E** represents an electric field whose field strength is measured in
N/C and the sides of the area are measured in m, then the units if the flux
are Nm^{2}/C. If the field were represented by field lines,
then the number of field in the colored box would be proportional to the
flux. A red box indicates that the flux is positive and a blue
box indicates that the flux is negative.