Therefore, the total force on Q = 
43
0
2
πε
lQq
ˆ
ˆ
r
(−r
)
 = 0, where ˆr
is the unit vector along OA It is clear also by symmetry that the three forces will sum to zero Suppose that the resultant force was non-zero but in some direction Consider what would happen if the system was rotated through 60°
about O