Two charges are $+10 \mu \mathrm{C}$ and $-10 \mu \mathrm{C}$ are separated by 10 cm . The magnitude of force acting on another charge $5 \mu \mathrm{C}$ placed at the mid-point of the line joining the two charges will be (use, $\frac{1}{4 \pi \varepsilon_0}=9 \times 10^9$ in SI unit)

A sphere-1 with redius $R$ has charge $q$. Sphere-2 with radius $3 R$ is far from sphere-1 and is initially uncharged. If the two spheres are now connected with a thin conducting wire, then the ratio $\frac{\sigma_1}{\sigma_2}$ of the surface charge densities is

$6 \mu \mathrm{C}$ charge is placed at the centre of a cube. What will be the electric flux at each face of the cube?

$$ \left[\text { Take, } \frac{1}{4 \pi \varepsilon_0}=9 \times 10^9 \mathrm{~N}-\mathrm{m}^2 \mathrm{C}^2\right] $$

There are two thin wire rings, each of radius $R$, whose axes coincide. The charges of the rings are $q$ and $-q$. The magnitude of potential difference between the centres of the rings separated by a distance $\sqrt{3} R$ is