$A$ $1$ paisa coin is made up of an $Al-Mg$ alloy and weighs $0.75\, g$. It has a square shape and its diagonal measures $17\, mm$. It is electrically neutral and contains equal amounts of positive and negative charges. Calculate the total amount of positive or negative charge in the coin.

What is a point charge?

How can an object be charged by contact?

$A$ positively charged glass rod is brought near an uncharged metal sphere,which is mounted on an insulated stand. If the glass rod is removed,what is the net charge on the metal sphere?

$(a)$ $A$ 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?

While doing his experiment,Millikan one day observed the following charges on a single drop:
$(i)$ $6.563 \times 10^{-19} \ C$
$(ii)$ $8.204 \times 10^{-19} \ C$
$(iii)$ $11.50 \times 10^{-19} \ C$
$(iv)$ $13.13 \times 10^{-19} \ C$
$(v)$ $16.48 \times 10^{-19} \ C$
$(vi)$ $18.09 \times 10^{-19} \ C$
From this data,the value of the elementary charge $(e)$ was found to be: