A unit positive charge has to be brought from infinity to a midpoint between two charges $20 \mu C$ and $10 \mu C$ separated by a distance of $50 \mathrm{m}.$ How much work will be required?

You are to bring two charges ${\mathrm{q}}_{1 }\mathrm{and} {\mathrm{q}}_2$from infinity to the point represented by the potential ${\mathrm{V}}_1\mathrm{and} {\mathrm{V}}_2$ in an electrical field $\mathrm{E}.$. If the distance between ${\mathrm{q}}_1 \mathrm{and} {\mathrm{q}}_2$ within the field is $r$. Find the total work done in assembling the configuration.

 Imagine an electric field  $\mathrm{E}=\left(20\widehat{ \mathrm{i}}+30\widehat{ \mathrm{j}}\right) {\mathrm{NC}}^{-1}$ in a space. The potential at the origin is zero Find the potential at the $\left(2,2\right)\mathrm{m}$,

Four point charge (with equal magnitude of charge of $5 C;$ but with different signs) are placed at four corners of a square of side $10 \mathrm{m}.$ Assuming that the square is centered at the origin and the configuration of the charges are as given in the figure, the potential and the magnitude of electric field at the origin, respectively are

[Note $\left.k=\frac{1}{4\pi {\epsilon }_0}\right]$

A charge Q is distributed over two concentric conducting thin spherical shells radii $\mathrm{r}$ and $\mathrm{R} \left(\mathrm{R}>\mathrm{r}\right)$. If the surface charge densities on the two shells are equal, the electric potential at the common centre is : 

Van de Graaff generator consists of a hollow metal sphere of diameter $2 \mathrm{m}$. If the potential on the surface of the sphere is $6$ million volt, the charge accumulated over the surface of the sphere is: