conservation of charge
Conservation of charge is the rule that you can never make electric charge appear out of nothing, nor make it vanish. You can only move it around or swap it between objects. When you rub a balloon and it becomes negative, your hair becomes exactly as positive: no charge was created, it just changed address.
The precise statement: the total electric charge of an isolated system stays constant in time. Add up all the positive and negative charges, and that net total never changes, no matter what processes go on inside. In any reaction, the charge before equals the charge after. For example, when a neutral atom loses an electron (charge -e) it becomes a positive ion (charge +e), and the freed electron carries the missing -e away, so the books still balance to zero.
This is one of the deepest laws in physics, holding in every experiment ever done, from everyday sparks to particle collisions where matter is created and destroyed. Even when a particle and antiparticle are born from pure energy, they appear with equal and opposite charges so the total does not change. It ranks alongside conservation of energy and conservation of momentum as a foundation of the whole subject.
In the beta decay of a neutron (charge 0), a proton (+e), an electron (-e), and a neutral antineutrino appear; the charges +e and -e add back to zero, exactly matching the neutron.
Charge is balanced before and after, even in nuclear reactions.
Conservation of charge does not mean charge stays put. Current flows freely; the law only says the grand total, positives minus negatives, never changes.