2.21 Explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons

Charge by friction
As explained in previous post(s), insulators are charged by friction when they rub against another insulator (eg a plastic rod against cloth)

When the two things rub together, one gains electrons from the other. Electrons can move. Protons remain still (hence the name "electrostatic"). This leaves one with a negative charge, and one with a positive charge. 

The charge is proportional to the amount of electrons gained / lost.
Eg. the rod loses 5 electrons to the cloth, so its charge is +5.
the cloth gains 5 electrons from the rod, so its charge is -5.

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