In hydrogen atom an electron revolves around a proton (in nucleus) at a distance 'r' m. the intensity of electric field due to the proton at distance 'r' is $$5 \times 10^{11} \mathrm{NC}^{-1}$$, the magnitude of force between the electron and proton is [charge on electron $$=1.6 \times 10^{-19} \mathrm{C}$$]

Two charged metallic spheres are joined by a very thin metal wire. If the radius of the larger sphere is four times that of the smaller sphere, the electric field near the larger sphere is

A charged spherical conductor has radius '$$r$$'. The potential difference between its surface and 3 point at a distance '$$3 r$$' from the centre is '$$v$$' The electric intensity at a distance '$$3 r$$' from the centre of the conductor is

Two spherical conductors of radii $$4 \mathrm{~cm}$$ and $$5 \mathrm{~cm}$$ are charged to the same potential. If '$$\sigma_1$$' and '$$\sigma_2$$' be the respective values of the surface density of charge on the two conductors then the ratio $$\sigma_1: \sigma_2$$ is