A charged metallic sphere $A$ is suspended by a riylon thread. Another charged metallic sphere $B$ held by an insulating handle is brought close to $A$ such that the distance between their centres is $10 \,cm ,$ as shown in Figure $(a) .$ The resulting repulsion of $A$ is noted (for example, by shining a beam of light and measuring the deflection of its shadow on a screen). Spheres $A$ and $B$ are touched by uncharged spheres $C$ and $D$ respectively. as shown in Figure $(b)$ $C$ and $D$ are then removed and $B$ is brought closer to $A$ to a distance of $5.0 \,cm$ between their centres, as shown in Figure $(c)$ What is the expected repulsion of A on the basis of Coulomb's law? Spheres $A$ and $C$ and spheres $B$ and $D$ have identical sizes. Ignore the sizes of $A$ and $B$ in comparison to the separation between their centres.

A glass rod rubbed with silk is used to charge a gold leaf electroscope and the leaves are observed to diverge. The electroscope thus charged is exposed to $X$-rays for a short period. Then

What type of charges are obtained on silk cloth and glass rod ?

It is now believed that protons and neutrons (which constitute nuclei of ordinary matter) are themselves built out of more elementary units called quarks. A proton and a neutron consist of three quarks each. Two types of quarks, the so called 'up' quark (denoted by $u$) of charge $+(2 / 3) e,$ and the 'down' quark (denoted by $d$) of charge $(-1 / 3) e$ together with electrons build up ordinary matter. (Quarks of other types have also been found which give rise to different unusual varieties of matter.) Suggest a possible quark composition of a proton and neutron.

Write the activity to make simple electroscope.