Four point charges are placed at the vertices of a square of side $2.8 \,m$ as shown in the figure. Find the electric potential at the centre of the square. (in $\,V$)

Define an electron Volt $(eV)$ and express its value in Joule $(J)$ units.

Three concentric spherical shells have radii $a, b$ and $c$ $(a < b < c)$ and have surface charge densities $\sigma, -\sigma$ and $\sigma$ respectively. If $V_A, V_B$ and $V_C$ denote the potentials of the three shells,then,for $c = a + b$,we have

As shown in the figure, charges $+q, +q, -q$ and $-q$ are placed on the vertices of a square, each side length is $2l$. The electric potential at the mid-point '$A$' of the charges $+q$ and $+q$ is . . . . . . .

When a positive $q$ charge is taken from a lower potential point to a higher potential point,its potential energy will

Three concentric spherical metallic shells $X$,$Y$,and $Z$ of radii $a$,$b$,and $c$ respectively $[a < b < c]$ have surface charge densities $\sigma$,$-\sigma$,and $\sigma$,respectively. The shells $X$ and $Z$ are at the same potential. If the radii of $X$ and $Y$ are $2\,cm$ and $3\,cm$,respectively,then the radius of shell $Z$ is $......\,cm$.