$A$ charge $Q$ is divided into two parts $q$ and $Q-q$ and separated by a distance $R$. The force of repulsion between them will be maximum when:

Four charges equal to $-Q$ are placed at the four corners of a square and a charge $q$ is at its centre. If the system is in equilibrium, 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 $^{\circ}$)?

Two electric charges $+q$ and $+q$ are placed at a distance $r$. The force between them is $F$. If one charge is fixed and the other moves in a circle of radius $r$,the work done will be .......

Two particles of equal mass $m$ and equal charge $q$ are separated by a distance of $16 \text{ cm}$. They do not experience any net force. The value of $\frac{q}{m}$ is . . . . . . (where $G$ is the universal gravitational constant and $\epsilon_0$ is the permittivity of free space).

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: