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

Four electric charges $2 \mu \mathrm{C}, Q, 4 \mu \mathrm{C}$ and $12 \mu \mathrm{C}$ are placed on $X$-axis at distance $x=0,1 \mathrm{~cm}, 2 \mathrm{~cm}$ and 4 cm respectively. If the net force acting on the charge at origin is zero, then $Q=$