Figure shows three concentric metallic spherical shells. The outermost shell has charge $q_{2}$, the inner most shell has charge $q_{1}$, and the middle shell is uncharged. The charge appearing on the inner surface of outermost shell is

The magnitude of electric field intensity $E$ is such that, an electron placed in it would experience an electrical force equal to its weight, is given by

The figure shown is a plot of electric field lines due to two charges $Q_1$ and $Q_2.$ The sign of charges is

The figure shows electric field lines. If $E_A$ and $E_B$ are electric fields at $A$ and $B$ and distance $AB$ is $r,$ then