How do electrostatic generators work?

The diagram shows a Van de Graff generator.

Which part of the Van de Graaff generator works like a rubbing cloth?
 

The diagram shows a Van de Graff generator.

What type of charge builds up on the dome when the Van de Graaff generator

The diagram shows a Van de Graff generator.

When someone standing on an insulator touches an uncharged dome and waits while the dome is charged up, their hair stands up on end. Describe and explain what is happening with the electric charges.

In a Van de Graaff type generator a spherical metal shell is to be a $15\times {10}^6 \mathrm{V}$ electrode. The dielectric strength of the gas surrounding the electrode is $5\times {10}^7 {\mathrm{Vm}}^{-1}$. What is the minimum radius (in centimetres)  of the spherical shell required?