In Millikan's oil drop experiment for the determination of the charge on the electron,the reason for using oil is:

There are two metallic spheres of same radii but one is solid and the other is hollow, then

Write the law of conservation of charge. Give one example.

What is the meaning of the additive nature of electric charges?

$(a)$ $A$ comb run through one's dry hair attracts small bits of paper. Why? What happens if the hair is wet or if it is a rainy day? (Remember,a paper does not conduct electricity.)
$(b)$ Ordinary rubber is an insulator. But special rubber tyres of aircraft are made slightly conducting. Why is this necessary?
$(c)$ Vehicles carrying inflammable materials usually have metallic ropes touching the ground during motion. Why?
$(d)$ $A$ bird perches on a bare high power line,and nothing happens to the bird. $A$ man standing on the ground touches the same line and gets a fatal shock. Why?

For what type of charge distribution,electric field can be obtained by using Coulomb's law and superposition principle?