Conservation of charge is the principle that the total electric charge of a closed system stays constant; charge can be transferred between objects or moved around a circuit, but it can never be created or destroyed.
Conservation of charge says the net charge of a closed system is fixed forever. You can move charge from one object to another, separate positive from negative, or push it around a circuit, but if you add up all the charge before and after any process, you get the same number. Charge is never created out of nothing and never vanishes.
In AP Physics C: E&M, this shows up first in Unit 1 electrostatics. When two identical conducting spheres touch, the total charge gets shared (split equally for identical spheres) because the sum can't change. When you charge a rod by rubbing it, the rod gains exactly the charge the cloth loses. Later in the course, the same principle becomes Kirchhoff's junction rule. Current into a node equals current out, because charge can't pile up out of nowhere or disappear at a junction. One principle, two costumes.
Conservation of charge is one of the bedrock conservation laws of the course, sitting alongside conservation of energy and momentum from Mechanics. It anchors Topic 1.1 (Electrostatics), where it explains every charging process you'll analyze, including contact charging and induction. But its real exam payoff comes later. Kirchhoff's junction rule, the equation you'll write in nearly every multi-loop circuit problem, is literally conservation of charge restated for steady currents. Capacitor problems lean on it too. When capacitors share charge or sit in series, the charge bookkeeping only works because total charge is conserved. If you ever get a weird answer where charge appeared from nowhere, conservation of charge is the sanity check that catches it.
Keep studying AP Physics C: E&M Unit 1
Contact Charging (Unit 1)
Contact charging is conservation of charge in action. Two identical conducting spheres that touch end up with equal charges because the total has to stay the same, so the only fair split is half and half. The exam loves asking for the final charge on each sphere, and the answer is always (q₁ + q₂)/2 for identical spheres.
Kirchhoff's Junction Rule in Circuits (Unit 3)
The junction rule (current in equals current out at every node) is just conservation of charge applied to steady currents. Charge flowing into a junction can't vanish or accumulate from nowhere, so the rates must balance. When you write node equations on a multi-loop circuit FRQ, you're invoking this principle.
Conservation of Energy (Mechanics & Unit 3)
These are sibling principles, and circuits use both. The junction rule is conservation of charge; the loop rule is conservation of energy. Keeping straight which rule conserves which quantity is a classic point of confusion, and the exam tests whether you know the difference.
Capacitors and Charge Sharing (Unit 2)
Series capacitors carry the same charge, and capacitors connected together redistribute charge until potentials match, all because total charge is conserved. The 2023 FRQ Q3 circuit with capacitors C and 2C is exactly this kind of charge bookkeeping problem.
You'll rarely see a question that just asks you to state conservation of charge. Instead, it's the hidden engine behind problems you have to set up correctly. In multiple choice, expect contact-charging questions (find the final charge on each sphere after touching) and circuit questions where the junction rule is the key equation. On FRQs, it shows up in circuit analysis with capacitors, like the 2023 FRQ Q3, where a battery, resistors, and capacitors C and 2C require you to track where charge sits before and after a switch closes. The skill being tested is bookkeeping. Account for every coulomb, make sure charge into a node equals charge out, and confirm that capacitors in series carry equal charge. If your final answers create or destroy net charge, you've made an error somewhere.
Both are conservation laws, but they conserve different things and produce different circuit equations. Conservation of charge gives you Kirchhoff's junction rule (currents at a node balance). Conservation of energy gives you Kirchhoff's loop rule (voltage changes around a closed loop sum to zero). A quick check that helps on the exam is that charge is a stuff you count, while energy is something charge gains or loses as it moves through circuit elements.
The total electric charge of a closed system never changes; charge can only be transferred, never created or destroyed.
When two identical conducting spheres touch, they split the total charge equally, so each ends up with (q₁ + q₂)/2.
Kirchhoff's junction rule is conservation of charge restated for circuits, meaning current into any node equals current out.
Capacitors in series carry the same charge because the charge separated on connected plates must sum to what was there before, which is often zero.
Conservation of charge handles the junction rule while conservation of energy handles the loop rule, and mixing these up is a common exam mistake.
Use conservation of charge as a sanity check on FRQs; if your answers imply net charge appeared or vanished, something went wrong.
It's the principle that the total electric charge in a closed system stays constant over time. Charge can move between objects or through circuits, but it can't be created or destroyed. It anchors Topic 1.1 electrostatics and reappears as Kirchhoff's junction rule in circuits.
No. Rubbing separates charge that already exists by transferring electrons from one surface to the other. The balloon gains exactly the negative charge your hair loses, so the total charge of the balloon-plus-hair system is unchanged.
Conservation of charge gives you the junction rule, where currents into a node equal currents out. Conservation of energy gives you the loop rule, where voltage changes around a closed loop sum to zero. They're separate laws that produce separate equations, and you usually need both to solve a multi-loop circuit.
The total charge is conserved and redistributes. For identical conducting spheres, it splits equally, so each ends up with (q₁ + q₂)/2. So a sphere with +6 μC touching one with -2 μC leaves each sphere with +2 μC.
The plates connected between two series capacitors form an isolated section of conductor that started neutral. Conservation of charge means any +Q on one plate must be matched by -Q on the connected plate, so every capacitor in the series chain carries the same charge Q.