An isolated capacitor has charge $Q$ and plate area $A$. Between the plates of a parallel plate condenser, a plate of thickness $t_1$ and dielectric constant $k_1$ is placed. In the rest of the space, there is another plate of thickness $t_2$ and dielectric constant $k_2.$ The potential difference across the condenser will be

Force of attraction between the plates of area $A$ of a parallel plate capacitor is

$A$and $C$are concentric conducting spherical shells of radius $a$and $c$, respectively. $A$is surrounded by a concentric-dielectric medium of inner radius $a,$outer radius $b$and dielectric constant $k.$If the sphere $A$is given a charge $Q,$the potential at the outer surface of the dielectric is