If two particles $A$ and $B$ of charges $1.6 \times 10^{-19} \mathrm{C}$ and $3.2 \times 10^{-19} \mathrm{C}$ respectively are separated by a distance of 3 cm in air, then the magnitude of electrostatic force on particle $A$ due to particle $B$ is

If four charges $+12 \mathrm{nC},-20 \mathrm{nC},+32 \mathrm{nC}$ and -15 nC are arranged at the four vertices of a square of side $\sqrt{2} \mathrm{~m}$, then the net electric potential at the centre of the square due to these four charges is

The force between two conducting spheres of same radius having charges $+8 \mu \mathrm{C}$ and $-4 \mu \mathrm{C}$ separated by some distance in air is $F$. If the spheres are connected by a conducting wire and after some time the wire is removed, then the magnitude of the force between the two conducting spheres is

In space the electric potential varies as $V=20|\mathbf{r}|$ volt. where $\mathbf{r}=x \hat{\mathbf{i}}+y \hat{\mathbf{j}}+z \hat{\mathbf{k}}$ is the position vector. Then, electric field in $\left(\mathrm{NC}^{-1}\right)$ at the point $(4 \mathrm{~m}, 3 \mathrm{~m},-5 \mathrm{~m})$ is