1 9)
E(r)  =  E1 (r) +  E2 (r)  + …  +  En(r)
        =
41
41
41
0
1
1
2
1
0
2
2
2
2
0
2
π
π
π
ε
ε
ε
rq
rq
q
r
n
n
n
P
P
P
P
P
P
ˆ
ˆ ˆ
r
r
r
+
+
+
E(r) =
=∑
 
41
0
P
i P
πε
q
r
i
i
i
n
2
1
ˆr
(1 10)
E is a vector quantity that varies from one point to another point in space
and is determined from the positions of the source charges