For that we must
calculate the work done in bringing a unit positive
test charge from infinity to the point P For Q > 0,
the work done against the repulsive force on the
test charge is positive Since work done is
independent of the path, we choose a convenient
path – along the radial direction from infinity to
the point P At some intermediate point P¢ on the path, the
electrostatic force on a unit positive charge is
2
0
1 ˆ
4
'
Q
εr
×
′
π
r
(2