The potential at a point,due to a positive charge of $100\,\mu C$ at a distance of $9\,m$,is

Along the $X$-axis,three charges $\frac{q}{2}, -q$ and $\frac{q}{2}$ are placed at $x=0, x=a$ and $x=2a$ respectively. The resultant electric potential at $x=a+r$ (if $a << r$) is: ($\varepsilon_0$ is the permittivity of free space)

Charges of $1 \mu C$ are placed at each of the four corners of a square of side $2 \sqrt{2} \text{ m}$. The potential at the point of intersection of the diagonals is . . . . . . .

Two point charges of $1 \text{ nC}$ and $2 \text{ nC}$ are placed at two corners of an equilateral triangle of side $3 \text{ cm}$. The work done in bringing a charge of $3 \text{ nC}$ from infinity to the third corner of the triangle is . . . . . . $\mu\text{J}$. $( \frac{1}{4\pi\epsilon_{0}} = 9 \times 10^{9} \text{ N.m}^{2}/\text{C}^{2} )$

$A$ charge of $10\, e.s.u.$ is placed at a distance of $2\, cm$ from a charge of $40\, e.s.u.$ and $4\, cm$ from another charge of $20\, e.s.u.$ The potential energy of the charge $10\, e.s.u.$ is (in $ergs$)

Concentric metallic hollow spheres of radii $R$ and $4R$ hold charges $Q_1$ and $Q_2$ respectively. Given that surface charge densities of the concentric spheres are equal,the potential difference $V(R) - V(4R)$ is