For an area element of a closed surface, ˆn  is taken to be the direction
of outward normal, by convention 15 Gauss’s law: The flux of electric field through any closed surface S is
1/e0 times the total charge enclosed by S The law is especially useful
in determining electric field E, when the source distribution has simple
symmetry:
(i) Thin infinitely long straight wire of uniform linear charge density l
0
ˆ
2
r
ελ
=
π
E
n
where r is the perpendicular distance of the point from the wire and
ˆn is the radial unit vector in the plane normal to the wire passing
through the point