A small block of mass 5 g and charge $5 \mu \mathrm{C}$ is placed on insulated, frictionless, inclined plane of angle $60^{\circ}$. An electric field is applied parallel to the inclined plane. If the block remains at rest, then the magnitude of electric field is (take, $g=10 \mathrm{~m} / \mathrm{s}^2$ )

Two metal spheres have their radii in the ratio of $4: 7$. They are put in contact and a charge $8.8 \times 10^{-7} \mathrm{C}$ is given to the system. Then they are separated, so that each can exert no influence on the other. The potential due to the smaller sphere at 60 m from it in V is

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