The direction of a vector is specified by a unit
vector along the vector To denote the direction from 1 to 2 (or from 2 to
1), we define the unit vectors:
ɵ
21
21
21
= r
r
r
, ɵ
ɵ
ɵ
12
12
21
12
12
,
=
−
r
r
r
r
r
Coulomb’s force law between two point charges q1 and q2 located at
r1 and r2, respectively is then expressed as
ɵ
1
2
21
21
2
21
41
ε
=
π
F
r
o
q
rq
(1 3)
Some remarks on Eq (1