Mathematically,
F21 = force on q2 due to  
1
2
1
21
2
21
ˆ
k
(q q )
q
r
=
r
where 
ˆr21
 is a unit vector in the direction from q1 to q2 and k =  
0
1
4 ε
π
is the constant of proportionality In SI units, the unit of charge is coulomb The experimental value of
the constant e0 is
e0 = 8 854 × 10–12 C2 N–1 m–2
The approximate value of k is
k = 9 × 109 N m2 C–2
6