By the
superposition principle, the total electric field due to the charge
distribution is obtained by summing over electric fields due to different
volume elements:
2
0
1
ˆ
4
all
V
V
'
r'
ρ
ε
∆
∆
≅
Σ
π
E
r
(1 27)
Note that r, r¢,  ˆ′r  all can vary from point to point In a strict
mathematical method, we should let DV®0 and the sum then becomes
an integral; but we omit that discussion here, for simplicity In short,
using Coulomb’s law and the superposition principle, electric field can
be determined for any charge distribution, discrete or continuous or part
discrete and part continuous