Why does an ebonite rod get negatively charged on rubbing with fur?

An ebonite rod held in hand can be charged by rubbing with flannel but a copper rod cannot be charged like this.why?

When a body becomes negatively charged. Its mass.

Why do not the negatively charged electrons fall into the positively charged nucleus?

Why does leather get hardened after tanning ?

Out of MgCl_2 and AlCl_3 , which one is more effective in causing coagulation of negatively charged sol and why?

Which charges area transferred on rubbing?

What are positive and negative charges? How are these produced?