How does transfer of electric charges between two objects work? 
Questions
1.) Here, the negative charge from the wool is transferred to the plastic rod. I'm curious as to what determines which object is responsible of transferring their charge to the other object. Like why didn't the plastic rod give its negative charge instead?
2.) If we rub 19 more neutral wools to this already negatively charged plastic rod, would that result in more transfers of negative charges from the other 19 neutral wools? Or is there a limit as to how many negative charges an object obtains?
 A: 1) After coming into contact, a chemical bond is formed between parts of the two surfaces, called adhesion, and charges move from one material to the other. When the materials are separated, the charges may not have the opportunity to reverse their path. Each substance has a different tendancy to donate electric charge when forming chemical bonds, which can be measured as the electrochemical potential of its ions. This depends on the lattice, chemical and atomic structures of the substance - i.e. on many factors.
2)  There is a high theoretical limit. If the charge is allowed to build up on the non-conductor, it becomes less likely that future chemical bonds will be formed as there is now a repulsive electric force in place. In practical devices, such as Van de Graaff generators, as in nature such as clouds, other limits are reached first (the breakdown voltage of air, beyond which there will be lightening).
You don't need 19 more wools, you just need to replenish the electrons in the first wool before using it again - which can be done by contacting its surface to an earthed conductor.
