Question

Sir I want to ask that on rubbing a glass with fur or rubbing ani two such materials, why is charge generated on it and why that charge only.

Answer 1

Electric Charge is the result of excess or insufficiency of electrons (negatively charged particles) as compared to protons (positively charged particles). If there are more electrons than protons, the object will be negatively charged and if there are less electrons than positively charged protons then they are positively charged .
By the process of rubbing two materials against each other, electrons migrate from one material to another.
When we rub two materials, it causes the capturing of certain number of electrons by one material from another. As a consequence of there is migration of electrons, the material that captures them becomes negatively charged (because of excess of electrons over protons), while another material simultaneously becomes positively charged (because of insufficient number of electrons relatively to protons).

Now coming to your question, depending on the two materials rubbed the charge attained is different.
For instance when glass rod is rubbed with fur, glass rod will capture electrons from fur and become negatively charged . This is because electrons are more loosely held in the fur than in glass rod.
But when we rub silk and glass rod together, silk will become negatively charged. This is because in glass rod electrons are held more loosely than in silk. So silk can capture the loose electrons of glass while rubbing leaving glass positively charged and silk negative.