Two charges $Q$ are placed at opposite corners of a square. Two charges $q$ are placed at the other two corners. If the net electric force on $Q$ is zero,then $Q/q$ is equal to:

Two identical charged spheres are suspended by strings of equal length. The strings make an angle of $30^{\circ}$ with each other. When they are immersed in a liquid of density $0.8 \, g \cdot cm^{-3}$,the angle remains the same. If the density of the material of the spheres is $1.6 \, g \cdot cm^{-3}$,find the dielectric constant of the liquid.

State the principle of superposition for electric forces.

$A$ charge $Q$ is placed at the midpoint of the line joining two identical point charges $q$. For what value of $q$ will the system be in equilibrium?

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

Two identical conducting spheres carrying different charges attract each other with a force $F$ when placed in air at a distance $d$ apart. The spheres are brought into contact and then returned to their original positions. Now,the two spheres repel each other with a force whose magnitude is equal to the initial attractive force. The ratio between the initial charges on the spheres is: