The space between the plates of a parallel plate capacitor is halved and a dielectric medium of relative permittivity 10 is introduced between the plates. The ratio of the final and initial capacitances of the capacitor is

Eight capacitors each of capacity $2 \mu \mathrm{~F}$ are arranged as shown in figure. The effective capacitance between $A$ and $B$ is

When a parallel plate capacitor is charged up to 95 V , its capacitance is $C$. If a dielectrtic slab of thickness 2 mm is inserted between plates and distance between the plates is increased by 1.6 mm such that the same potential difference is maintained. The dielectric constant of the material (slab) is

The capacitance of an isolated sphere of radius $r_1$ is increased by 5 times, when it is enclosed by an earthed concentric sphere of radius $r_2$. The ratio of their radii is