Three charges $q, \sqrt 2q, 2q$ are placed at the corners $A, B$ and $C$ respectively of the square $ABCD$ of side $'a'$ then potential at point $'D'$

Four charges $ + Q,\, - Q,\, + Q,\, - Q$ are placed at the corners of a square taken in order. At the centre of the square

There are four concentric shells $A, B, C $ and $D $ of radii $ a, 2a, 3a$  and $4a$ respectively. Shells $B$ and $D$ are given charges $+q$ and $-q$ respectively. Shell $C$ is now earthed. The potential difference $V_A - V_C $ is : 

Three concentric metallic spherical shell $A, B$ and $C$ or radii $a, b$ and $c$ $(a < b < c)$ have surface charge densities $- \sigma , + \sigma ,$ and $- \sigma $ respectively. The potential of shell $A$ is :

An infinitely long thin wire, having a uniform charge density per unit length of $5 nC / m$, is passing through a spherical shell of radius $1 m$, as shown in the figure. A $10 nC$ charge is distributed uniformly over the spherical shell. If the configuration of the charges remains static, the magnitude of the potential difference between points $P$ and $R$, in Volt, is. . . .

[Given: In SI units $\frac{1}{4 \pi \epsilon_0}=9 \times 10^9, \ln 2=0.7$. Ignore the area pierced by the wire.]

Two insulated charged spheres of radii $20\,cm$ and $25\,cm$ respectively and having an equal charge $Q$ are connected by a copper wire, then they are separated