AP Physics 2 Unit 9: Thermodynamics
Thermodynamics connects microscopic particle motion to macroscopic pressure, temperature, energy transfer, work and entropy. Master the six topics as one chain: collisions → gas state → thermal transfer → first-law energy accounting → material response → second-law direction.
What is AP Physics 2 Unit 9 about?
Unit 9 studies how microscopic particle motion produces pressure and temperature, how ideal-gas state variables are related, how energy transfers between thermal systems, and how the first and second laws constrain thermodynamic processes. It contains six topics and combines particle models, P–V/P–T/V–T graphs, energy accounting, proportional reasoning and experimental evidence.
Six topics that build one thermodynamics model
The order matters on a first pass because each topic reuses language and representations introduced earlier.
Kinetic Theory of Temperature and Pressure
Microscopic collisions → pressure & temperature
Topic 9.2The Ideal Gas Law
State variables → ideal-gas relationships
Topic 9.3Thermal Energy Transfer and Equilibrium
Temperature difference → thermal transfer
Topic 9.4The First Law of Thermodynamics
Heating + work → internal-energy change
Topic 9.5Specific Heat and Thermal Conductivity
Material properties → amount/rate of transfer
Topic 9.6Entropy and the Second Law of Thermodynamics
Energy spreading → spontaneous direction
Eight thermodynamics equations worth recognizing
Equations are useful only after the physical model and sign convention are identified. Topics 9.3 and 9.6 are primarily qualitative.
P = F⊥/APressure from perpendicular force per area.
Kavg = 3/2 kBT = 1/2 mvᵣₘₛ²Temperature, kinetic energy and rms speed.
PV = nRT = NkBTIdeal-gas state equation.
U = 3/2 nRT = 3/2 NkBTInternal energy of ideal monatomic gas.
Wₒₙ = −PΔVConstant/average-pressure boundary work.
ΔU = Q + WₒₙFirst-law energy accounting.
Q = mcΔTEnergy for temperature change.
Q/Δt = kAΔT/LSteady conduction rate.
Know what stays constant before doing algebra
These special processes appear repeatedly in P–V reasoning.
| Process | Held constant / defining condition | Immediate reasoning consequence | P–V clue |
|---|---|---|---|
| Isovolumetric | V constant | ΔV=0 → boundary work = 0 | vertical path |
| Isobaric | P constant | W_on=−PΔV | horizontal path |
| Isothermal | T constant | For ideal monatomic gas, ΔU=0 | follows an isotherm |
| Adiabatic | Q=0 | ΔU=W_on | temperature changes as work changes U |
Four graphs and diagrams that make Unit 9 easier
Do not treat graphs as decoration. Read slope, area, curvature, direction and controlled variables.
Pressure vs volume
Use path shape to identify process type and area to reason about work.
Pressure vs temperature
At fixed amount and volume, P is linear in absolute temperature for an ideal gas.
Volume vs temperature
At fixed amount and pressure, V is linear in absolute temperature.
Molecular-speed graph
Higher temperature broadens the distribution and shifts characteristic speeds upward.
Find the first topic that actually needs repair
Rate your confidence from 1 (weak) to 4 (strong). The tool recommends the lowest-confidence starting point and remembers your choices in this browser.
Use the lowest-confidence topic first; weighting alone should not override repeated evidence from your mistakes.
Use data to test the Unit 9 models
Labs 1 and 2 are full interactive experiments; Lab 3 currently provides a guided P–V path-work analyzer.
Repair these before taking a mixed exam
These are model errors, not merely arithmetic errors.
Unit 9 Thermodynamics FAQs
How many topics are in Unit 9?
Six topics: kinetic theory, ideal gas law, thermal transfer/equilibrium, first law, specific heat/conductivity, and entropy/second law.
What are the most important Unit 9 graphs?
P–V, P–T, V–T and molecular-speed distributions. Each graph tests a different controlled-variable or representation skill.
Which Unit 9 topic has the most energy accounting?
Topic 9.4. It combines internal energy, heating/cooling, work, P–V diagrams and the first law.
Do I calculate entropy in Topic 9.6?
No. Use qualitative second-law reasoning: isolated versus closed systems, reversible versus irreversible processes, energy spreading and equilibrium.
What sign convention does this site use for the first law?
ΔU = Q + W_on. Positive Q means energy enters by heating; positive W_on means work is done on the system.
How should I practice Unit 9?
Learn one topic, complete its 42 MCQs and four FRQs, repair errors, then use a mixed model exam to verify transfer.