Three charges $+Q, q, +Q$ are placed respectively at distances $0, \frac{d}{2},$ and $d$ from the origin on the $x$-axis. If the net force experienced by the charge $+Q$ placed at $x = 0$ is zero,then the value of $q$ is:

Two pith balls,each carrying charge $+q$,are hung from a hook by two strings of length $L$. It is found that when each charge is tripled,the angle between the strings doubles. What was the initial angle between the strings (in $^{\circ}$)?

State the principle of superposition for electric forces.

Two small spheres,each having an equal positive charge $Q$ (Coulomb),are suspended by two insulating strings of equal length $L$ (metre) from a rigid hook. The whole setup is taken into a satellite where there is no gravity. The two balls are now held by electrostatic forces in a horizontal position. The tension in each string is then:

Two balls with charges $+7 \ \mu C$ and $-5 \ \mu C$ attract each other with a force $F$. If $-2 \ \mu C$ charge is added to both,what will be the force between them?

Three equal charges are placed on the three corners of a square. If the force between $q_1$ and $q_2$ is $F_{12}$ and that between $q_1$ and $q_3$ is $F_{13}$,the ratio of magnitudes $\frac{F_{12}}{F_{13}}$ is