For the system shown in the figure,find $Q$ such that the resultant force on $q$ is zero.

Three point charges are placed at the corners of an equilateral triangle with sides of $10 \ cm$ as shown in the figure. The resultant force on the charge at $B$ will be:

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 $^{\circ}$)?

$A$ charge of $1 \mu C$ is divided into two parts such that their charges are in the ratio of $2: 3$. These two charges are kept at a distance of $1 \ m$ apart in vacuum. Then,the electric force between them (in $N$) is

In the figure,the force on the charge at $A$ in the direction perpendicular to $BC$ is ...... .

$A$ negatively charged oil drop is prevented from falling under gravity by applying a vertical electric field of $100 \ V m^{-1}$. If the mass of the drop is $1.6 \times 10^{-3} \ g$,the number of electrons contained in the drop is: