3.2 Conservation of Electric Charge

Charge is conserved

The law of conservation of charge states that electric charge can neither be created nor destroyed, only transferred between objects. In any isolated system, the total net charge before and after any process (rubbing, conduction, circuit operation) stays constant. Charge doesn't disappear at circuit junctions — it's redistributed.

Current in and out of a node

At any junction (node) in a circuit, the total current flowing in must equal the total current flowing out — this is Kirchhoff's Current Law (KCL), a direct consequence of charge conservation. Charge can't pile up or vanish at a junction, so charge carriers entering must equal those leaving per unit time.

Charge conservation in circuit elements

Even through resistors, capacitors, and batteries, the *rate* of charge flow (current) into a component equals the rate out — charge isn't consumed by resistors (energy is dissipated, not charge). Capacitor plates store equal and opposite charge; charging one plate positively requires an equal negative charge on the other, keeping total charge conserved.

Key terms

Conservation of charge

Electric charge cannot be created or destroyed in an isolated system, only transferred between objects.

Kirchhoff's Current Law (KCL)

At any junction, the total current entering equals the total current leaving — a consequence of charge conservation.

Current through a resistor

Current entering equals current leaving; resistors dissipate energy, not charge.

Capacitor plate charges

The two plates always hold equal and opposite charges (+Q and −Q), conserving total charge.

Why charge can't 'pile up' at a node

Charge conservation forbids accumulation or loss at a junction, forcing current in to equal current out.

Try an AP-style question

In a series circuit, how does the current compare at different points?

1. It is largest near the battery
2. It is the same everywhere
3. It is zero after the last resistor
4. It decreases after each resistor