Two metal plates are separated by $2\mathrm{cm}.$ The potentials of the plates are $-10 \mathrm{V}$ and $+30 \mathrm{V}$. The electric field between the two plates is:

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 :