The potential on the plates of capacitor are $$+20 \mathrm{~V}$$ and $$-20 \mathrm{~V}$$. The charge on the plate is $$40 \mathrm{C}$$. The capacitance of the capacitor is

Two spherical conductors of capacities $$3 \mu \mathrm{F}$$ and $$2 \mu \mathrm{F}$$ are charged to same potential having radii $$3 \mathrm{~cm}$$ and $$2 \mathrm{~cm}$$ respectively. If '$$\sigma_1$$' and '$$\sigma_2$$' represent surface density of charge on respective conductors then $$\frac{\sigma_1}{\sigma_2}$$ is

Three identical capacitors of capacitance '$$\mathrm{C}$$' each are connected in series and this connection is connected in parallel with one more such identical capacitor. Then the capacitance of whole combination is

A parallel plate capacitor is charged by a battery and battery remains connected. The dielectric slab of constant '$$\mathrm{K}$$' is inserted between the plates and then taken out. Then electric field between the plates