question_answer

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. \[K\frac{4q.\,\,q}{{{l}^{2}}}+\frac{K\,\,Q.q}{{{(1/2)}^{2}}}=0\] \[Q=-q\]