Three charges $4q, Q$ and $q$ are placed at positions $0, \frac{l}{2}$ and $l$ respectively along a straight line. If the resultant force on $q$ is zero,then $Q$ is equal to

Electrostatic force between two identical charges placed in vacuum at a distance of $r$ is $F$. $A$ slab of width $\frac{r}{5}$ and dielectric constant $9$ is inserted between these two charges. The new force between the charges is:

Consider the charges $q, q$,and $-q$ placed at the vertices of an equilateral triangle of side $l$,as shown in the figure. What is the force on each charge?

Two charges $2 C$ and $6 C$ are separated by a finite distance. If a charge of $-4 C$ is added to each of them, the initial force of $12 \times 10^3 \,N$ will change to

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

Three equal charges are placed on the three corners of a square as shown below. If the magnitude of force between $q_1$ and $q_2$ is $F_{12}$ and that between $q_1$ and $q_3$ is $F_{13}$,then the ratio of $F_{13}$ to $F_{12}$ is