3(a)] We denote force on q1 due to q2 by
F12 and force on q2 due to q1 by F21 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