A parallel place capacitor is charged by connecting a $$2 \mathrm{~V}$$ battery across it. It is then disconnected from the battery and a glass slab is introduced between plates. Which of the following pairs of quantities decrease?

If a slab of insulating material (conceptual). $$4 \times 10^{-3} \mathrm{~m}$$ thick is introduced between the plates of a parallel plate capacitor, the separation between the plates has to be increased by $$3.5 \times 10^{-3} \mathrm{~m}$$ to restore the capacity to original value. The dielectric constant of the material will be

Eight drops of mercury of equal radii combine to form a big drop. The capacitance of a bigger drop as compared to each smaller drop is

An electrician requires a capacitance of $$6 \mu \mathrm{F}$$ in a circuit across a potential difference of $$1.5 \mathrm{~kV}$$. A large number of $$2 \mu \mathrm{F}$$ capacitors which can withstand a potential difference of not more than $$500 \mathrm{~V}$$ are available. The minimum number of capacitors required for the purpose is