Physics 2 · Unit 4: Electric Circuits · 14 min read · Updated 2026-05-11
Definition of a Circuit — AP Physics 2
AP Physics 2 · Unit 4: Electric Circuits · 14 min read
1. Core Definition of an Electric Circuit★☆☆☆☆⏱ 3 min
An electric circuit is a network of conducting components connected to allow continuous flow of electric charge (current) driven by a potential difference. This topic is the foundation of Unit 4, which accounts for 15-19% of your total AP Physics 2 exam score, and is tested in both MCQ and FRQ sections.
2. Open vs Closed Circuits★★☆☆☆⏱ 4 min
Every functional steady-current circuit requires three core components: (1) a source of potential difference (emf) to drive charge flow; (2) a continuous closed conducting path connecting one source terminal back to the other; (3) a load that converts electrical energy to other forms (the source's internal resistance can act as a load if no external load is present).
A closed circuit has no breaks in the path, so steady current can flow. An open circuit has at least one break, so no steady current can flow. Even in an open circuit, potential difference exists across the break: with no current flowing, there is no voltage drop across connected components, so the full emf of the source appears across the open gap.
Exam tip: On AP MCQ questions asking for voltage across a single open switch in a series circuit, the answer is almost always equal to the source emf, as long as there are no other breaks in the circuit.
3. Emf vs Terminal Potential Difference★★★☆☆⏱ 5 min
All real sources of electrical energy (batteries, generators) have internal resistance $r$, which comes from resistance to charge flow within the source material itself.
The relationship between emf and terminal potential difference comes from Kirchhoff's loop rule:
V = \varepsilon - Ir
Exam tip: Always check if the problem gives an internal resistance for the source. If it does, never use emf directly to calculate power delivered to the external load — always use terminal voltage first.
4. Charge Conservation and Steady Current★★☆☆☆⏱ 3 min
For a steady-state circuit (current does not change with time), charge cannot be created, destroyed, or accumulated at any point in the circuit. This fundamental principle leads to Kirchhoff's Current Law (Junction Rule):
\sum I_{\text{in}} = \sum I_{\text{out}}
A common misconception is that current is 'used up' by resistors: in reality, electrical energy is converted to other forms (heat, light), but charge is conserved. The same amount of charge that enters a resistor must leave it, so current is constant at all points in a series circuit with no junctions.
Exam tip: When asked to explain why current is constant in a series circuit, always explicitly cite charge conservation for steady current — generic statements about 'charge flowing through' will not earn full points on FRQ.
5. Concept Check★★☆☆☆⏱ 3 min
Common Pitfalls
Why: Students confuse zero current with zero voltage, recalling $V=IR$ and incorrectly concluding $I=0$ implies $V=0$.
Why: Students default to ideal battery assumptions even when internal resistance is explicitly given.
Why: Students confuse energy consumption with charge consumption.
Why: Students memorize that circuits require loads, so they incorrectly exclude short circuits.
Why: Students associate batteries with working circuits, so they rush past checks for continuity.