Q.Charge Q is divided into two parts which are then kept some distance apart. The force between them will be maximum if the two parts are
(a) Q/2 and Q/2
(b) Q/2 and 3Q/4
(c) Q/2 and 2Q/3
(d) e and (Q-e)
Sol: (a)
Let charge be q and Q-q
$ \displaystyle F= k\frac {q(Q-q)}{r^2} $
For F to be maximum
$ \displaystyle \frac{dF}{dq}= 0 $
Q-2q = 0
q =Q/2
Charges are Q/2 , Q/2