$A$ point charge '$Q$' is placed at the centre of the line joining two equal charges '$+q$' and '$+q$'. The value of '$Q$' when the system is in equilibrium is

$A$ negatively charged plate has a surface charge density of $2 \times 10^{-6} \, C/m^2$. Find the minimum initial distance of an electron moving toward the plate such that it does not strike the plate,given that its initial kinetic energy is $200 \, eV$.

Two identical small spheres carry charges $Q_1$ and $Q_2$ $(Q_1 >> Q_2)$. The force between them is $F_1$. The spheres are brought into contact and then placed at the same distance. The new force between them is $F_2$. Then $F_1/F_2$ will be:

Two charged identical metal spheres $A$ and $B$ repel each other with a force of $3 \times 10^{-5} \,N$. Another identical uncharged sphere $C$ is touched with sphere $A$ and then it is placed mid-way between $A$ and $B$. Then, the magnitude of net force on $C$ is

Charges are placed on the vertices of a square as shown in the figure. Let $\vec{E}$ be the electric field and $V$ be the electric potential at the center. If the charges at $A$ and $B$ are interchanged with those at $D$ and $C$ respectively,then:

Consider the points lying on a straight line joining two fixed opposite charges. Between the charges,there is: