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AP Physics 2: Algebra-Based · Unit 3 of 7
Electric Circuits
15–18% of the exam4 lessons · 53 min24 terms
What this unit covers
The topics below follow the published Physics 2 course framework for Unit 3. This unit is worth 15–18% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Current & resistanceSeries & parallelRC circuitsKirchhoff’s rules
Lessons in this unit
- Current, Resistance & Ohm’s Law13 min · 3 objectivesDefine electric current as the rate of charge flow and identify conventional current direction · Relate voltage, current, and resistance through Ohm’s law · Calculate electrical power dissipated in a resistor
- Series & Parallel Circuits14 min · 3 objectivesCombine resistors in series and in parallel into a single equivalent resistance · Determine how current and voltage distribute in series and parallel branches · Reason about what happens to a circuit when a component is added or removed
- Kirchhoff’s Rules13 min · 3 objectivesApply the junction rule as a statement of charge conservation · Apply the loop rule as a statement of energy conservation · Assign consistent signs to EMFs and resistor voltage drops around a loop
- Capacitors & RC Circuits13 min · 3 objectivesRelate the charge, voltage, and capacitance of a capacitor · Describe how a capacitor charges and discharges through a resistor over time · Calculate the time constant of an RC circuit and interpret it
Formulas in Unit 3
Current, Ohm’s law, and power
I = Q / t · V = I · R · P = I · V = I² · R = V² / R
Power P is the rate at which the resistor converts electrical energy to heat/light, in watts (W). All three power forms are equivalent — pick the one matching your known quantities.
Combining resistors
Series: R = R₁ + R₂ + … · Parallel: 1/R = 1/R₁ + 1/R₂ + …
Series total is bigger than any part; parallel total is smaller than any part. For two parallel resistors, a handy shortcut is R = (R₁R₂)/(R₁ + R₂).
Kirchhoff’s rules
Junction: ΣI_in = ΣI_out · Loop: ΣΔV = 0
Going through a resistor in the direction of current is a voltage drop (−IR); passing from − to + inside a battery is a rise (+EMF). Reverse the sign if you traverse against that direction.
Capacitor charge and RC time constant
Q = C · V · τ = R · C
The time constant τ (tau), in seconds, sets the pace of charging: after one τ the capacitor reaches about 63% of full charge, and after ~5τ it is essentially fully charged. Larger R or C means slower charging.
Every term in Unit 3
All 24 terms we publish for Electric Circuits, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Current
- I = ΔQ/Δt, the rate of charge flow in amperes. Conventional current is defined as positive charge flow, opposite to actual electron motion.
- Resistance and resistivity
- R = ρL/A. A longer wire has more resistance and a thicker one has less; resistivity is the material property behind both.
- Ohm's law
- V = IR. Holds for ohmic materials at constant temperature; a filament bulb is non-ohmic because resistance rises as it heats.
- Electrical power
- P = IV = I²R = V²/R. Which form to use depends on which two quantities you know.
- Series resistors
- R_total = ΣR. Same current through each; voltage divides in proportion to resistance.
- Parallel resistors
- 1/R_total = Σ(1/R). Same voltage across each; total resistance is always less than the smallest branch.
- Kirchhoff's junction rule
- Current into a junction equals current out — a statement of charge conservation.
- Kirchhoff's loop rule
- Potential changes around any closed loop sum to zero — a statement of energy conservation.
- Capacitance
- C = Q/V in farads. For parallel plates, C = ε₀A/d, so larger plates or a smaller gap store more charge per volt.
- Energy stored in a capacitor
- U = ½QV = ½CV² = Q²/2C. All three forms are equivalent; pick the one matching your known quantities.
- Capacitors in series and parallel
- The rules are the reverse of resistors: parallel capacitances add, series capacitances combine reciprocally.
- Dielectric
- An insulator between the plates that reduces the field for the same charge, raising capacitance by the dielectric constant.
- RC circuit charging
- Charge and voltage rise exponentially toward their final values with time constant τ = RC; current starts maximum and decays.
- Capacitor behavior at t = 0 and t = ∞
- Initially uncharged, a capacitor acts like a wire; fully charged, it acts like a break in the circuit.
- Internal resistance and terminal voltage
- A real battery has internal resistance, so terminal voltage V = ε − Ir falls as current increases.
- Reading a circuit diagram
- Identify which components share both nodes (parallel) and which carry the same current (series) before writing any equation.
- Brightness reasoning
- Brightness tracks dissipated power. In series the same current flows, so the larger resistance is brighter; in parallel the same voltage applies, so the smaller resistance is brighter.
- Effect of adding a parallel branch
- Lowers total resistance, raising total current drawn from the source and increasing the voltage lost to internal resistance.
- Measuring resistance experimentally
- Plot V against I; the slope is resistance. A curved plot means the component is non-ohmic.
- Why ammeters must have low resistance
- They carry the circuit current, so any appreciable resistance would change the very quantity being measured.
- Capacitor charging graph
- Voltage rises with a decreasing slope toward the source value while current decays exponentially from its initial maximum.
- Energy dissipated during capacitor charging
- Exactly half the energy supplied by the battery ends up stored; the other half is dissipated in the resistance, regardless of its value.
- Series vs parallel capacitor voltage
- In series each capacitor carries the same charge and the smallest capacitance takes the largest voltage share.
- Kirchhoff sign conventions
- A potential drop across a resistor traversed with the current, a rise crossing a battery from negative to positive terminal.
What examiners penalize here
- Know all three power formulas: P = IV, P = I²R, and P = V²/R. If a problem gives you resistance and voltage but not current, P = V²/R saves a step. They are algebraically identical via Ohm’s law.
- Anchor each rule with its shared quantity: series shares current, parallel shares voltage. From that, use V = IR branch by branch. Complex networks reduce to one equivalent resistor by collapsing series and parallel groups one step at a time.
- Pair the rules with their conservation law: junction rule = conservation of charge, loop rule = conservation of energy. On multi-loop problems, write one junction equation and one loop equation per unknown current, then solve the system.
- For RC problems, answer three questions: what happens at t = 0 (capacitor = wire), what happens as t → ∞ (capacitor = open, fully charged with Q = CV), and how fast (τ = RC). Those three checkpoints capture nearly every exam item.
Practice Physics 2
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Physics 2: Algebra-Based exam is Unit 3?
Unit 3, Electric Circuits, is worth 15–18% of the Physics 2 multiple-choice section according to the published course framework. Across all 7 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Physics 2 Unit 3?
Electric Circuits covers Current & resistance, Series & parallel, RC circuits and Kirchhoff’s rules. We publish 24 terms with definitions for this unit, all of them on this page.
How should I study Physics 2 Unit 3?
Read the 4 lessons below first — about 55 minutes — then drill the 24 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 7 units of AP Physics 2: Algebra-Based
Unit names, topics and exam weights follow the published College Board course framework for AP Physics 2: Algebra-Based. AP® is a trademark registered by the College Board, which does not endorse this site.