Two charges\[4q\]and\[q\] are placed at a distance\[I\] another charge Q is placed at the midpoint of line joining the charges\[4q\]and\[q\] If net force on charge\[q\] is zero, then value of \[Q\] is:
A)\[+q\]
B) \[-q\]
C)\[+2q\]
D) \[3q\]
Correct Answer:
B
Solution :
On charge Q, both 4q and q charge apply force. Net force of Q should be zero. \[K\frac{4q.\,\,q}{{{l}^{2}}}+\frac{K\,\,Q.q}{{{(1/2)}^{2}}}=0\] \[Q=-q\]