The net charge of an isolated system remains constant. The only way to change the net charge of a system is to bring in charge from elsewhere, or remove charge from the system.

Charge can be created and destroyed, but only in positive-negative pairs.

Example

Two identical metal spheres are charged. Sphere A has a net charge of +7Q. Sphere B has a net charge of -3Q. The spheres are brought together, allowed to touch, and then separated. What is the net charge on each sphere now?

1. Each sphere has a net charge of +4Q
2. Each sphere has a net charge of +2Q
3. Sphere A has +4Q, Sphere B has no net charge
4. Sphere A has +7Q, Sphere B has -3Q

When the spheres are touched the net +4Q charge will spread out evenly over the two spheres. Each sphere will have a net charge of +2Q.