Two spheres carrying charges $+6 \mu C$ and $+9 \mu C$ are separated by a distance $d$ and experience a force of repulsion $F$. When a charge of $-3 \mu C$ is added to both spheres and they are kept at the same distance as before,the new force of repulsion is:

Find the value of $q$ so that the net electrostatic force on the $10 \mu C$ charge placed at a vertex of the square is zero. The charges are arranged as shown in the figure.

Two charges each equal to $2\,\mu C$ are $0.5\,m$ apart. If both of them exist inside vacuum,then the force between them is.......$N$

The distance between charges $5 \times 10^{-11} \, C$ and $-2.7 \times 10^{-11} \, C$ is $0.2 \, m$. The distance at which a third charge should be placed in order that it will not experience any force along the line joining the two charges is . . . . . . $m$.

Three charges of equal magnitude are placed at three corners of a square of side $a$. If $F_{12}$ is the force between the charges at corners $1$ and $2$,and $F_{13}$ is the force between the charges at corners $1$ and $3$,what is the ratio $F_{12}/F_{13}$?

Two identical conducting spheres carry identical charges. If the spheres are set at a certain distance apart,they repel each other with a force $F$. $A$ third conducting sphere identical to the other two,but initially uncharged,is touched to one sphere and then to the other before being removed. The force between the original two spheres is now