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 = An. 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 Nm2/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.