The flux Df of electric field E through a small area element DS is
given by
 Df  = E DS
The vector area element DS is
DS = DS ˆn
where DS is the magnitude of the area element and ˆn  is normal to the
area element, which can be considered planar for sufficiently small DS For an area element of a closed surface, ˆn  is taken to be the direction
of outward normal, by convention 15