The total electric field is opposite to ˆp We have
E = – (E +q + E –q ) cosq  ˆp
= −
2+
4
2
2 3
2
rq a
a
o
π ε (
)
�
/
p
(1 17)
At large distances (r >> a), this reduces to
E
p
= −
>>
42
q a3
r
r
a
π εo
ˆ
(
)
(1 18)
From Eqs