$A$ charge $q$ is placed at the centre of the line joining two equal charges $Q$. The system of the three charges will be in equilibrium if $q$ is equal to:

When air is replaced by a dielectric medium of constant $k$,the maximum force of attraction between two charges separated by a distance $r$:

Two charges $2 C$ and $6 C$ are separated by a finite distance. If a charge of $-4 C$ is added to each of them, the initial force of $12 \times 10^3 \,N$ will change to

Write some important points for the vector form of Coulomb's law.

Three charges of each magnitude $100 \mu C$ are placed at the corners $A, B$ and $C$ of an equilateral triangle of side $4 \text{ m}$. If the charges at points $A$ and $C$ are positive and the charge at point $B$ is negative,then the magnitude of the total force acting on the charge at $C$ and the angle made by it with $AC$ are:

Two copper balls,each weighing $10\,g$,are kept in air $10\,cm$ apart. If one electron from every $10^6$ atoms is transferred from one ball to the other,the Coulomb force between them is (atomic weight of copper is $63.5$).