$A$ charge $Q$ is to be divided between two objects. The values of the charges on the objects so that the electrostatic force between them will be maximum is

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 $63.5$).

Three charges $q, Q$ and $+4q$ are placed in a straight line of length $d$ at positions $0, d/3$ and $2d/3$ respectively. In order to make the net force on $q$ be zero, the value of $Q$ should be

Two small balls,each having a negative charge $Q$,are suspended from a fixed point by two identical insulating strings of length $L$. This entire arrangement is placed inside a satellite in a region where there is no gravity (weightless condition). The angle between the strings is ......... and the tension in each string is ......... Newtons.

Three equal charges are placed on the three corners of a square. If the force between $q_1$ and $q_2$ is $F_{12}$ and that between $q_1$ and $q_3$ is $F_{13}$,the ratio of magnitudes $\frac{F_{12}}{F_{13}}$ is

The magnitude of the electric force on a $2\, \mu C$ charge placed at the centre $O$ of two equilateral triangles,each of side $10\, cm$,as shown in the figure,is $P$. If the charges at $A, B, C, D, E,$ and $F$ are $2\, \mu C, 2\, \mu C, 2\, \mu C, -2\, \mu C, -2\, \mu C,$ and $-2\, \mu C$ respectively,then $P$ is: ..... $N$