Thus, the potential due to the
dipole is the sum of potentials due to the
charges q and –q
V
rq
rq
=
−




41
0
1
2
πε
(2 9)
where r1 and r2 are the distances of the
point P from q and –q, respectively Now, by geometry,
2
2
2
1
2
r
r
a
ar
=
+
−
cosq
2
2
2
2
2
r
r
a
ar
=
+
+
 cosq
(2 10)
We take r much greater than a (
r a
) and retain terms only upto
the first order in a/r
  r
r
a
r
ra
1
2
2
2
2
1
2
=
−
+




cosθ
  ≅
−

r
a
r
2
1
2
cosθ
(2