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

Six charges of equal magnitude $3$ positive and $3$ negative are to be placed on $\mathrm{PQRSTU}$ corners of a regular hexagon Fig. $2.107$ such that field at the centre is double that of what it would have been if only one $+$ ve charge is placed $\mathrm{R}$

situation is there a point to the left of the charges where an electron would be in equilibrium?

Two small spherical balls each carrying charge $q=10\mu \mathrm{C}$ are suspended by two insulated threads of equal length $1 \mathrm{m}$ each, from a point fixed in the ceiling. It is found that in equilibrium, threads are separated by an angle ${60}^{°}$ between them as shown in figure Fig. $2.109$. The tension in the thread is