Four point charges $q_{A}=2\; \mu C, q_{B}=-5\; \mu C, q_{C}=2\; \mu C,$ and $q_{D}=-5\; \mu C$ are located at the corners of a square $ABCD$ of side $10\; cm$. What is the force on a charge of $1\; \mu C$ placed at the centre of the square (in $; N$)?

Two small spherical balls,each carrying a charge $Q = 10\,\mu C$ ($10$ micro-coulomb),are suspended by two insulating threads of equal lengths $1\,m$ each,from a point fixed in the ceiling. It is found that in equilibrium,the threads are separated by an angle of $60^o$ between them,as shown in the figure. What is the tension in the threads in $N$? (Given: $\frac{1}{4\pi \varepsilon_0} = 9 \times 10^9\,Nm^2/C^2$)

Two electrons are separated by a distance of $1\,\mathring{A}$. What is the Coulomb force between them?

The ratio of the forces between two small spheres with constant charge $(a)$ in air and $(b)$ in a medium of dielectric constant $K$ is:

Two identical spheres have charges $+q$ and $-q$ and are placed at a certain distance. The force between them is $F$. If a third identical sphere with charge $+q$ is placed exactly between the two spheres, what is the magnitude and direction of the force experienced by it?

The charges on two spheres are $+7\,\mu C$ and $-5\,\mu C$ respectively. They experience a force $F$. If each of them is given an additional charge of $-2\,\mu C$,the new force of attraction will be