The two point
charges q1 and q2 have been numbered 1 and 2 for
convenience and the vector leading from 1 to 2 is
denoted by r21:
r21 = r2 – r1
In the same way, the vector leading from 2 to 1 is denoted by r12:
r12 = r1 – r2 = – r21
The magnitude of the vectors r21 and r12 is denoted by r21 and r12,
respectively (r12 = r21) 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