The electric charge of an $\alpha$-particle is ..........

How does a simple paper electroscope work?

How much positive and negative charge is there in a cup of water $(250 \; g)$?

$(a)$ $A$ 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?

Two identical spherical conductors $B$ and $C$ have equal radii and carry equal charges. They repel each other with a force $F$ when placed at a certain distance apart. $A$ third identical spherical conductor $A$,which is uncharged,is brought in contact with $B$ and then removed. It is then brought in contact with $C$ and removed. What is the new force of repulsion between $B$ and $C$?

$A$ body can be negatively charged by which of the following methods?