Law of Conservation of Charge

title: Conservation of Electric Charge
The net electric charge in an isolated system must remain constant. 

One very important thing about charges is that charge is neither created nor destroyed. Charge can be transferred, but the total amount of charge remains the same. This is called the law of conservation of charge.

When we rub a glass rod with wool, the glass rod gains electrons and becomes negatively charged. However, because of the law of conservation of charge, the wool has to be positively charged, and indeed is, so the net charge remains zero as it was originally as both objects were neutral.