JEE MAIN - Physics (2002 - No. 50)
If a charge $$q$$ is placed at the center of the line joining two equal charges $$Q$$ such that the system is in equilibrium then the value of $$q$$ is
$$Q/2$$
$$ - Q/2$$
$$Q/4$$
$$ - Q/4$$
Paliwanag
For equilibrium of charge $$Q$$
$$K{{Q \times Q} \over {{{\left( {2x} \right)}^2}}} + K{{Qq} \over {{x^2}}} = 0 \Rightarrow q = - {Q \over 4}$$
$$K{{Q \times Q} \over {{{\left( {2x} \right)}^2}}} + K{{Qq} \over {{x^2}}} = 0 \Rightarrow q = - {Q \over 4}$$