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Unit 3 · Electric Circuits Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Physics 2 Unit 3 Essentials

The must-know terms and big ideas for Unit 3: Electric Circuits. Every vocabulary word and concept you need to master.

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Big Idea 1
Series and parallel rules are mirror images of each other
Resistors in series add directly (R_eq = R1 + R2 + ...) while resistors in parallel combine reciprocally (1/R_eq = 1/R1 + 1/R2 + ...). Capacitors do the exact opposite — they combine reciprocally in series and add directly in parallel. Once you internalize this mirror-image relationship, you can derive any combination rule on the spot instead of memorizing four separate formulas.
Series Circuits Parallel Circuits Equivalent Resistance
Big Idea 2
Kirchhoff's rules are just conservation laws applied to circuits
The junction rule isn't a new piece of circuit theory — it's conservation of charge, applied at a node where wires meet. The loop rule isn't new either — it's conservation of energy, applied around any closed path in a circuit. Recognizing these as familiar conservation principles (rather than circuit-specific tricks) makes multi-loop circuit problems far less intimidating: you're just writing down "charge in = charge out" and "energy gained = energy lost" in algebraic form.
Kirchhoff's Rules Conservation of Charge Conservation of Energy
Big Idea 3
Capacitors resist change in voltage, which creates time-dependent behavior
Unlike resistors, which respond instantly to changes in voltage or current, capacitors take time to charge or discharge — this is the source of every RC circuit's exponential behavior. Right after a switch closes, an uncharged capacitor briefly acts like a wire (maximum current flows, since there's no charge built up yet to oppose it); after a long time, a charged capacitor acts like an open circuit (no more current flows, since the capacitor's voltage now opposes the source). Everything about RC circuits follows from this single idea.
Capacitors RC Circuits Time Constant
Electric current (I)
The rate of charge flow through a conductor, I = ΔQ/Δt, measured in amperes (A).
Current
Conventional current
The convention that current direction is defined as the direction positive charge flows, even in metals where electrons (negative) are the actual moving carriers.
Current
Drift velocity
The average net velocity of charge carriers due to an applied field — much slower than their random thermal motion between collisions.
Current
Ohm's law
V = IR — relates voltage across a resistor to the current through it and its resistance.
Resistance
Resistance (R)
A measure of opposition to current flow, measured in ohms (Ω).
Resistance
Resistivity (ρ)
An intrinsic material property (independent of shape) describing how strongly a material opposes current; used in R = ρL/A.
Resistance
Ohmic vs. non-ohmic
An ohmic component has constant resistance regardless of voltage or current (V = IR holds with fixed R); a non-ohmic component's resistance changes with conditions.
Resistance
Electric power (P)
The rate of electrical energy conversion: P = IV = I²R = V²/R, measured in watts.
Power
Series circuit
A circuit configuration where components are connected end-to-end along a single path; current is the same through all components, voltage divides.
Circuit Configurations
Parallel circuit
A circuit configuration where components are connected across the same two points, providing multiple paths; voltage is the same across all branches, current divides.
Circuit Configurations
Equivalent resistance
A single resistance value that could replace a network of resistors and produce the same overall circuit behavior.
Circuit Configurations
Kirchhoff's junction (current) rule
The total current entering a junction equals the total current leaving it — conservation of charge applied to circuits.
Kirchhoff's Rules
Kirchhoff's loop (voltage) rule
The sum of voltage changes around any closed loop in a circuit equals zero — conservation of energy applied to circuits.
Kirchhoff's Rules
Electromotive force (EMF)
The energy per unit charge supplied by a source like a battery, measured in volts; distinct from terminal voltage when internal resistance is present.
Kirchhoff's Rules
Internal resistance
Resistance within a real battery that causes terminal voltage to drop below EMF whenever current flows.
Kirchhoff's Rules
Capacitor in steady-state DC
A capacitor that has been connected long enough to be fully charged behaves like an open circuit — no current flows through that branch.
Capacitors in Circuits
Capacitor immediately after connection
An initially uncharged capacitor briefly behaves like a wire (zero resistance), allowing maximum current to flow.
Capacitors in Circuits
Capacitors in series
Combine reciprocally: 1/C_eq = 1/C1 + 1/C2 + ... — opposite of how resistors combine in series.
Capacitors in Circuits
Capacitors in parallel
Combine directly: C_eq = C1 + C2 + ... — opposite of how resistors combine in parallel.
Capacitors in Circuits
RC circuit
A circuit with a resistor and capacitor in series, showing time-dependent (exponential) charging or discharging behavior.
RC Circuits
Time constant (τ = RC)
The characteristic time for an RC circuit's charge, current, or voltage to change by about 63% toward its final value.
RC Circuits
Exponential charging/discharging
The mathematical pattern (involving e^(-t/τ)) that describes how current, charge, and voltage evolve over time in an RC circuit.
RC Circuits