The electrostatic force between two charges kept in air is $F$. If $30 \%$ of the space between the charges is filled with a medium, then the electrostatic force between the charges becomes $\frac{F}{2.56}$. The dielectric constant of the medium is

729 small identical spheres each charged to an electric potential 3V combine to form a bigger sphere. The electric potential of the bigger sphere is

The electric field due to an infinitely long thin straight wire with uniform linear charge density of $2.5 \times 10^{-7} \mathrm{~cm}^{-1}$ at a radial distance of $x$ from the wire is $7.5 \times 10^4 \mathrm{NC}^{-1}$. Then, $x=$

An alpha particle and a proton are accelerated from rest in a uniform electric field. The ratio of the times taken by proton and alpha particle to attain equal displacements is