Conservation of electric charge is the principle that the total electric charge of a closed system stays constant; charge can be transferred between objects or carried by current, but it is never created or destroyed. In AP Physics 2, it underlies charging processes (Topic 3.3) and Kirchhoff's junction rule (Topic 4.5).
Conservation of electric charge says the universe keeps a perfect ledger. Charge can move from one object to another, but the total amount of charge in a closed system never changes. Rub a balloon on your hair and the balloon picks up exactly the negative charge your hair loses. Nothing appeared, nothing vanished. The books always balance.
In AP Physics 2, this idea shows up in two different costumes. In electrostatics (Topic 3.3), it governs charging by friction, conduction, and induction. When two identical conducting spheres touch, the total charge gets shared, so a +6 μC sphere touching a -2 μC sphere leaves each with +2 μC, because (+6 - 2)/2 = +2. In circuits (Topics 4.1 and 4.5), the same principle becomes Kirchhoff's junction rule. Charge flowing into a junction can't pile up or disappear, so the current in must equal the current out. One law, two units, same accounting.
Conservation of electric charge is one of the few ideas that anchors two whole units of AP Physics 2. It's the core of Topic 3.3 (Conservation of Electric Charge) in the electrostatics unit, and it returns in Topic 4.1 (Definition and Conservation of Electric Charge) and Topic 4.5 (Kirchhoff's Junction Rule) in circuits. The exam loves conservation laws because they let you reason about a system without knowing every detail of what's happening inside it. If you can say "total charge before equals total charge after," you can solve sphere-touching problems, explain induction, and justify why currents split the way they do at a junction. On FRQs, citing conservation of charge by name is often exactly the justification the rubric is looking for.
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Kirchhoff's Junction Rule (Unit 4)
The junction rule is conservation of charge applied to a circuit. Charge flowing into a node has nowhere to hide, so the sum of currents in equals the sum of currents out. When you write I₁ = I₂ + I₃ at a branch point, you're really writing a charge-conservation equation.
Conductors and Insulators (Unit 3)
Conservation of charge tells you the total never changes, but conductors and insulators tell you where the charge can go. In a conductor, charge redistributes freely (which is why touching spheres share charge), while in an insulator it stays put. Charging by induction only makes sense once you combine both ideas.
Conventional Current (Unit 4)
Current is just charge in motion, measured as charge per second passing a point. Because charge is conserved, current can't simply vanish partway down a wire. That's why the current is the same everywhere in a series circuit.
Closed System (Units 3-4)
Conservation laws only apply to closed systems. If charge can leak in or out (say, through a grounding wire), the total charge of your object can change. Spotting whether the system is closed is step one of every charge-conservation problem.
No released FRQ uses the phrase "conservation of electric charge" as a question stem, but the principle is baked into problems across Units 3 and 4. In multiple choice, expect sphere-touching problems where you compute the final charge on identical conductors after contact, and circuit questions where you find an unknown current at a junction. On FRQs, this term is your justification language. When asked why currents at a node add up, or why an object charged by induction has the charge it does, the credited reasoning is "because electric charge is conserved." You're also expected to know charge is quantized (it comes in multiples of e, about 1.6 × 10⁻¹⁹ C), so a conserved total is always a whole number of elementary charges.
Kirchhoff has two rules and they come from two different conservation laws. The junction rule (currents in = currents out) comes from conservation of charge. The loop rule (voltage changes around a closed loop sum to zero) comes from conservation of energy. If an FRQ asks you to justify a current equation, cite charge conservation; if it asks about a voltage equation, cite energy conservation. Mixing these up is a classic way to lose a justification point.
The total electric charge of a closed system never changes; charge can only be transferred, not created or destroyed.
When two identical conducting spheres touch, they share the total charge equally, so add the charges and divide by two.
Kirchhoff's junction rule is conservation of charge in circuit form, meaning the current into a junction equals the current out.
The junction rule comes from conservation of charge, while the loop rule comes from conservation of energy, and the exam expects you to keep those justifications straight.
Charge is quantized, so any conserved total is a whole-number multiple of the elementary charge e (about 1.6 × 10⁻¹⁹ C).
Charging by friction, conduction, or induction never creates charge; it just rearranges charge between objects or within one object.
It's the principle that the total charge of a closed system stays constant. Charge can transfer between objects or flow as current, but it is never created or destroyed. It appears in Topics 3.3, 4.1, and 4.5 of the AP Physics 2 CED.
No. Friction only transfers electrons from your hair to the balloon. The balloon gains exactly the negative charge your hair loses, so the total charge of the hair-plus-balloon system is unchanged.
The loop rule is conservation of energy, not charge. Conservation of charge gives you the junction rule (currents in equal currents out), while energy conservation gives you the loop rule (voltages around a closed loop sum to zero). On FRQs, cite the right law for the right equation.
If the spheres are identical conductors, the total charge spreads evenly between them. A +6 μC sphere touching a -2 μC sphere leaves each with +2 μC, because the +4 μC total splits in half. The total is conserved the whole time.
In a series path, yes, because charge can't pile up or vanish in a wire. At a junction, the current splits among branches, but the total flowing in still equals the total flowing out. That's exactly what Kirchhoff's junction rule says.