question_answer

Three charges +Q, q, +Q are placed respectively, at distance, 0, \[d/2\] and d from the origin, on the x-axis. If the net force experienced \[by+Q\], placed at \[x=0\], is zero then value of q is: [JEE Main 09-Jan-2019 Morning]

A)             +Q/2

B) -Q/4

C)             +Q/4

D) -Q/2

Correct Answer: B

Solution :

[b]

\[\frac{kQq}{{{\left( \frac{d}{2} \right)}^{2}}}\,\,=\,\,\frac{k{{Q}^{2}}}{{{d}^{2}}}\]

\[q\,\,=\,\,\frac{Q}{4}\,\,\frac{{{d}^{2}}}{{{d}^{2}}}\]

\[q\,\,=\,-\,\frac{Q}{4}\,\,\]

Option (2) is correct.