5)
where ˆ′r is the unit vector along OP¢ Work done
against this force from  r¢ to r¢ + Dr¢ is
2
0
4
Q'
W
r
εr
∆
= −
∆ ′
π
(2 6)
The negative sign appears because for Dr¢ < 0, DW is positive Total
work done (W) by the external force is obtained by integrating Eq