Conservation of charge:  The net charge of an isolated system of objects remains constant.

Electric charge is conserved. The net charge of an isolated system may be positive, negative or neutral. Charge can move between objects in the system, but the net charge of the system remains unchanged.

To illustrate this principle, we again use the example of a silk cloth and a glass rod to demonstrate how two different objects can become charged while, at the same time, overall charge is conserved. Let’s assume that the cloth and the rod are both neutral to begin with. They each become charged when rubbed together, but their combined charge is unchanged: It remains zero, or neutral. It is true that electrons have moved between the rod and the cloth, but to the extent that one object is negative, the other is positive. The cloth and the rod constitute an isolated system because all the charge moves solely between them and no charge leaves them. If charge flowed to a person holding these objects, that person would become part of the system as well, and charge would still be conserved.

Early on, scientists such as Benjamin Franklin (yes, that Ben Franklin) suggested this conservation principle based on experimental data and intuition, but he and his colleagues were unable to show why it was so. The discovery of electrons showed why the conjecture was true: It was the movement of electrons that created the charged objects that Franklin observed. Under ordinary circumstances, these particles are neither created nor destroyed, and Franklin observed the results of electrons flowing from one object to another.

An object is charged when it has an imbalance of electrons and protons. Charge is said to be quantized: It is always observed as an integer multiple of e, the magnitude of the charge of an electron or a proton.

In extreme circumstances, charged particles can be destroyed. For example, when a positron (an exotic particle that is the mirror image of an electron, identical in mass but opposite in charge) collides with an electron, the two will annihilate each other and produce gamma rays, a kind of high-energy radiation. Does this scenario violate the conservation of charge? No, because gamma rays have no net charge. Before the collision, the system of one electron and one positron has no net charge. After the collision, the system consists of neutral gamma radiation, so charge is conserved.