If two charges of $1 \,C$ each are placed $1 \,km$ apart,then the force between them will be ........... $N$.

Two similar tiny balls of mass $m$,each carrying charge $q$,are hung from silk threads of length $l$ as shown in the figure. These are separated by a distance $x$ and the angle $2\theta$ is small. Then,for equilibrium:

For the regular pentagon system shown in the figure,find the net electrostatic force on the charge $q_0$ placed at the center.

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:

Two charges are placed as shown in the figure. Where should a third charge be placed so that it remains at rest?

Two charges $q_1 = +6q$ and $q_2 = -3q$ are placed as shown in the figure. $A$ proton is placed on the $x$-axis away from $q_2$. To keep the proton in equilibrium,the distance between $q_1$ and the proton is: