Two cells ${\epsilon }_1$ and ${\epsilon }_2$ connected in opposite to each other as shown in figure. The cell ${\epsilon }_1$ is of emf $9\mathrm{ }\mathrm{V}$ and internal resistance $3\mathrm{ }\mathrm{\Omega }$, the cell ${\epsilon }_2$ is of emf $7\mathrm{ }\mathrm{V}$ and internal resistance $7\mathrm{ }\mathrm{\Omega }$. The potential difference between the points $A$ and $B$ is 

A thin metallic spherical shell carries a charge $Q$ on it. A point charge $q$ is placed at the centre of the shell and another charge $q_1$ is placed outside it, as shown in figure. All the three charges are positive.

The force on the charge at the centre is