, qn By the superposition principle, the electric field E at r
due to the system of charges is (as shown in Fig 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