The unit vector ˆr is along the radius vector from the centre to
the area element Now, since the normal to a sphere at every point is
along the radius vector at that point, the area element DS and ˆr  have
the same direction Therefore,
2
0
4
q
S
r
φ
ε
∆
=
∆
π
(1 29)
since the magnitude of a unit vector is 1