Review AP Physics 2 Unit 9 to build a complete picture of how energy moves, transforms, and spreads in thermal systems. From atomic collisions driving gas pressure to entropy limiting what heat engines can do, this unit connects microscopic particle behavior to macroscopic thermodynamic laws.
Use the topic guides, key terms, and practice questions available for every topic in this unit to work through each concept before exam day.
Thermodynamics in AP Physics 2 spans from the microscopic world of colliding gas atoms to the macroscopic constraints that govern every heat engine and energy transfer process. The unit builds in a logical sequence: kinetic theory explains what pressure and temperature mean at the atomic level, the ideal gas law ties those quantities together mathematically, and the first and second laws of thermodynamics set the rules for how energy can change and where it ultimately goes.
Gas pressure comes from atoms colliding with container walls. Temperature measures average kinetic energy: K_avg = (3/2)k_B T. The ideal gas law PV = nRT = Nk_B T ties pressure, volume, temperature, and amount together, and PV, PT, and VT graphs each reveal a different gas relationship.
Thermal energy moves by conduction, convection, and radiation, always spontaneously from hot to cold. The first law, ΔU = Q + W, tracks internal energy changes across four key process types: isothermal, isobaric, isochoric, and adiabatic. PV diagrams let you read work as the area under the curve.
Entropy measures the tendency of energy to spread out. The second law states that the total entropy of an isolated system never decreases. Spontaneous processes increase entropy, and maximum entropy corresponds to thermodynamic equilibrium. A closed system's entropy can decrease only if energy leaves it, but the surroundings gain at least as much entropy.
The first law says energy is always conserved, but the second law says not all of that energy is available to do useful work. Every real process increases the total entropy of the universe, which is why heat engines cannot be perfectly efficient and why energy spontaneously disperses rather than concentrating. These two laws together define what is physically possible in any thermal system.
Gas pressure comes from atomic collisions with container walls. Temperature equals average kinetic energy: K_avg = (3/2)k_B T. Root-mean-square speed v_rms = sqrt(3k_B T / m). The Maxwell-Boltzmann distribution shows how speeds vary with temperature.
PV = nRT = Nk_B T connects pressure, volume, temperature, and amount for an ideal gas. Boyle's, Charles's, and Gay-Lussac's laws are special cases. PT graphs extrapolate to absolute zero. PV graphs show isothermal hyperbolas.
Energy transfers spontaneously from hot to cold by conduction, convection, or radiation. Thermal equilibrium is reached when both systems share the same temperature and net energy transfer stops.
ΔU = Q + W. Internal energy of an ideal monatomic gas is U = (3/2)nRT. Work done on a gas is W = -PΔV. PV diagrams track isothermal, isobaric, isochoric, and adiabatic processes; area under the curve equals work.
Q = mcΔT calculates energy needed to change an object's temperature. Fourier's law Q/Δt = kA(ΔT)/L gives the rate of conductive heat flow. Both specific heat and thermal conductivity are intrinsic material properties.
Entropy measures energy dispersal. The second law: total entropy of an isolated system never decreases. Entropy is a state function. Isolated systems move spontaneously toward thermodynamic equilibrium, the state of maximum entropy.
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Review Thermal Energy Transfer and Equilibrium with attention to how the concept appears in AP-style source and evidence questions.
Review Kinetic Theory of Temperature and Pressure with attention to how the concept appears in AP-style source and evidence questions.
Review Specific Heat and Thermal Conductivity with attention to how the concept appears in AP-style source and evidence questions.
Gas pressure arises from atoms colliding with container walls and transferring momentum. Each collision exerts a perpendicular force on the surface, and pressure is the total perpendicular force divided by area: P = F_perp / A. Pressure exists throughout the gas, not only at the walls. Temperature is a measure of the average translational kinetic energy of the atoms, given by K_avg = (3/2)k_B T. The root-mean-square speed v_rms = sqrt(3k_B T / m) tells you the speed corresponding to that average kinetic energy. The Maxwell-Boltzmann distribution shows how speeds are spread across atoms at a given temperature; as temperature rises, the peak shifts right and the distribution broadens.
| Quantity | Symbol | Depends on |
|---|---|---|
| Pressure | P | Force per area from collisions |
| Average kinetic energy | K_avg | Temperature T only |
| Root-mean-square speed | v_rms | Temperature T and atomic mass m |
An ideal gas is modeled with four assumptions: random instantaneous velocities, negligible atomic volume, elastic collisions, and no forces between atoms except during collisions. These assumptions allow the equation PV = nRT = Nk_B T to relate pressure, volume, moles (or number of atoms), and absolute temperature. Graphs of gas behavior reveal specific relationships: a PV graph at constant temperature is a hyperbola (Boyle's law), a VT graph at constant pressure is linear through the origin (Charles's law), and a PT graph at constant volume is linear and can be extrapolated to absolute zero where pressure would reach zero.
| Gas Law | Constant quantity | Relationship |
|---|---|---|
| Boyle's law | Temperature | P inversely proportional to V |
| Charles's law | Pressure | V directly proportional to T |
| Gay-Lussac's law | Volume | P directly proportional to T |
Two systems in thermal contact can exchange energy by thermal processes. Energy flows spontaneously from the higher-temperature system to the lower-temperature system through three mechanisms: conduction (direct atomic collisions through a material), convection (bulk movement of a fluid carrying thermal energy), and radiation (electromagnetic energy transfer requiring no medium). At the atomic level, higher-energy atoms are more likely to transfer energy to lower-energy atoms during collisions. This continues until both systems reach the same temperature, which is thermal equilibrium. At equilibrium, no net energy is transferred by thermal processes.
| Mechanism | Medium required | Atomic-level description |
|---|---|---|
| Conduction | Yes (solid or fluid) | Collisions pass energy atom to atom through material |
| Convection | Yes (fluid) | Bulk fluid motion carries higher-energy atoms to new regions |
| Radiation | No | Electromagnetic waves carry energy through space |
The internal energy U of an ideal monatomic gas is the sum of the kinetic energies of all its atoms: U = (3/2)nRT = (3/2)Nk_B T. Because ideal gas atoms have no potential energy from interactions, U depends only on temperature. The first law, ΔU = Q + W, states that the change in internal energy equals the heat added to the system plus the work done on the system. Work done on a gas by a constant external pressure is W = -PΔV. PV diagrams represent thermodynamic processes visually; the area under the curve equals the magnitude of work done. Four standard process types each have a defining constraint that simplifies the first law.
| Process | Constraint | First law simplification |
|---|---|---|
| Isothermal | T constant | ΔU = 0, so Q = -W |
| Isobaric | P constant | W = -PΔV; ΔU = Q + W |
| Isochoric | V constant | W = 0, so ΔU = Q |
| Adiabatic | Q = 0 | ΔU = W |
Specific heat c is an intrinsic material property that tells you how much energy is needed to raise one kilogram of a material by one kelvin. The equation Q = mcΔT lets you calculate the energy transferred when an object's temperature changes. In calorimetry problems, energy lost by a hot object equals energy gained by a cold object until they reach the same temperature. Thermal conductivity k describes how quickly energy flows through a material by conduction. Fourier's law gives the rate: Q/Δt = kA(ΔT)/L, where A is cross-sectional area, ΔT is the temperature difference across the material, and L is its thickness. Materials with high k (metals) conduct heat rapidly; materials with low k (wood, air) are insulators.
| Equation | What it calculates | Key variables |
|---|---|---|
| Q = mcΔT | Total energy to change temperature | Mass, specific heat, temperature change |
| Q/Δt = kA(ΔT)/L | Rate of conductive energy flow | Conductivity, area, temperature difference, thickness |
Entropy is a state function that describes the tendency of energy to spread out and the unavailability of some energy to do useful work. The second law of thermodynamics states that the total entropy of an isolated system can never decrease; it increases for irreversible processes and stays constant only for reversible ones. Localized energy spontaneously disperses, and isolated systems move toward thermodynamic equilibrium, which is the state of maximum entropy. A closed system's entropy can decrease if energy leaves it, but the entropy gained by the surroundings is always at least as large, so the total entropy of the universe does not decrease. Entropy is a state function, meaning it depends only on the current state of the system, not on the path taken to reach it.
| System type | Energy exchange | Entropy rule |
|---|---|---|
| Isolated | No energy in or out | Entropy never decreases |
| Closed | Energy can enter or leave | Entropy can decrease if energy leaves, but universe total does not decrease |
| Reversible process | Any | Total entropy constant |
| Irreversible process | Any | Total entropy increases |
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| Term | Definition |
|---|---|
| root-mean-square speed | The speed corresponding to the average kinetic energy of atoms in an ideal gas: v_rms = sqrt(3k_B T / m). Increases with temperature and decreases with atomic mass. |
| Maxwell-Boltzmann distribution | A graph showing the spread of atomic speeds in an ideal gas at a given temperature. Higher temperatures shift the peak to higher speeds and broaden the curve. |
| Boyle's Law | At constant temperature, pressure and volume are inversely proportional: P1V1 = P2V2. |
| absolute zero | The temperature at which an ideal gas would have zero pressure, extrapolated from a PT graph; equal to 0 K or -273.15 degrees C. |
| P-V diagram | A graph of pressure versus volume used to represent thermodynamic processes. The area under the curve equals the magnitude of work done on or by the gas. |
| ΔU = Q + W | The first law of thermodynamics: change in internal energy equals heat added to the system plus work done on the system. |
| work done on a system | For a gas under constant or average external pressure: W = -PΔV. Positive when the gas is compressed, negative when it expands. |
| constant volume process | A thermodynamic process where volume does not change, so W = 0 and ΔU = Q entirely. |
| reversible process | A theoretical process in which total entropy remains constant; all real processes are irreversible and increase total entropy. |
| state function | A property that depends only on the current state of the system, not on the path taken to reach it. Entropy and internal energy are both state functions. |
| Q=mcΔT | Energy transferred to or from an object equals its mass times its specific heat times its temperature change. Specific heat is treated as temperature-independent in AP Physics 2. |
| Fourier's Law of Heat Conduction | Q/Δt = kA(ΔT)/L; the rate of conductive heat flow is proportional to thermal conductivity, cross-sectional area, and temperature difference, and inversely proportional to thickness. |
| degrees of freedom | The number of independent ways a particle can store energy. A monatomic ideal gas has 3 translational degrees of freedom, giving U = (3/2)nRT. |
| thermodynamic cycle | A sequence of thermodynamic processes that returns a gas to its initial state, represented as a closed loop on a PV diagram. Net work equals the area enclosed by the loop. |
PV = nRT, K_avg = (3/2)k_B T, and v_rms = sqrt(3k_B T / m) all require absolute temperature in kelvins. Always convert: T(K) = T(°C) + 273.15 before substituting.
In AP Physics 2, W = -PΔV is work done on the gas. When a gas expands, ΔV is positive, so W is negative (the gas does work on surroundings, losing internal energy). When compressed, W is positive. Keep track of the sign convention consistently.
Isothermal means constant temperature, so ΔU = 0 for an ideal gas. But heat Q and work W are both nonzero; they are equal and opposite. Students often incorrectly set Q = 0 for an isothermal process, which applies to adiabatic processes instead.
The entropy of a closed system can decrease if energy leaves it. The second law says the total entropy of an isolated system never decreases, meaning you must account for the surroundings when analyzing a closed system's entropy change.
Pressure exists throughout the gas, not just where atoms hit the walls. This is a common conceptual error when explaining gas pressure using kinetic theory.
AP Physics 2 free-response questions frequently ask you to explain macroscopic phenomena using atomic or molecular reasoning. For Unit 9, this means describing why pressure changes when temperature rises, why energy flows from hot to cold, or why entropy increases in a given process. Practice writing explanations that explicitly reference atomic collisions, kinetic energy, and energy dispersal rather than just citing equations.
Expect questions that present a PV diagram and ask you to identify the process type, determine the sign of Q, W, and ΔU, or compare work done in different processes by comparing areas under curves. You may also need to apply ΔU = Q + W quantitatively across a multi-step process or cycle.
Questions often combine Q = mcΔT, Fourier's law, and thermal equilibrium in a single scenario, such as two objects exchanging heat through a conducting material until equilibrium is reached. You may need to calculate final temperatures, energy transferred, or conduction rates, and then explain the result using the second law or entropy reasoning.
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open calculatorAP Physics 2 Unit 9 covers 6 topics in thermodynamics: 9.1 Kinetic Theory of Temperature and Pressure, 9.2 The Ideal Gas Law, 9.3 Thermal Energy Transfer and Equilibrium, 9.4 The First Law of Thermodynamics, 9.5 Specific Heat and Thermal Conductivity, and 9.6 Entropy and the Second Law of Thermodynamics. Together these topics connect microscopic particle behavior to large-scale energy rules. You'll work through how temperature relates to molecular motion, how ideal gases behave under changing conditions, how energy moves between systems, and why entropy sets limits on what any heat engine can do. See AP Physics 2 Unit 9 for practice on all six topics.
AP Physics 2 Unit 9 makes up 15-18% of the AP exam, making thermodynamics one of the heavier-weighted units on the test. That means roughly 1 in 6 points traces back to topics like entropy, the ideal gas law, the first and second laws of thermodynamics, and thermal energy transfer. It's worth putting real time into this unit.
The AP Physics 2 Unit 9 progress check in AP Classroom includes both MCQ and FRQ parts drawn from all six thermodynamics topics. The MCQ section tests conceptual understanding of kinetic theory, the ideal gas law, thermal equilibrium, and entropy. The FRQ part asks you to apply the first and second laws of thermodynamics, often requiring you to explain energy conservation or justify why entropy increases in a given process. For matched practice before you take the progress check, head to AP Physics 2 Unit 9 to review each topic and work through similar question types.
AP Physics 2 Unit 9 FRQs most often pull from entropy and the second law of thermodynamics, the first law of thermodynamics, and the ideal gas law. These questions typically ask you to analyze a thermodynamic process, calculate work or internal energy changes, or argue why a proposed process violates the second law. To practice, write out full explanations rather than just numbers, because AP Physics 2 FRQs reward clear reasoning. Start by reviewing each topic at AP Physics 2 Unit 9, then try past College Board FRQs on heat engines and gas processes. Check your answers against the scoring guidelines to see exactly where reasoning points are awarded.
The best starting point for AP Physics 2 Unit 9 practice questions, including multiple-choice and practice test sets, is AP Physics 2 Unit 9. That page organizes practice by topic, so you can target weak spots like kinetic theory, the ideal gas law, or entropy separately before doing a full mixed set. For MCQ practice, focus on questions that ask you to compare thermodynamic processes or predict how changing pressure, volume, or temperature affects an ideal gas. For a practice test feel, work through a timed set covering all six topics in one sitting.
Start AP Physics 2 Unit 9 by building a solid picture of entropy and the second law of thermodynamics, since that concept ties the whole unit together. Then work forward through the six topics in order: kinetic theory of temperature and pressure, the ideal gas law, thermal energy transfer and equilibrium, the first law of thermodynamics, and specific heat and thermal conductivity. Here's a concrete plan: - Sketch PV diagrams for isothermal, adiabatic, and isochoric processes until the shapes feel automatic. - Practice applying the ideal gas law with changing variables, not just plugging in numbers. - For entropy questions, always ask: does disorder increase? That single check handles most second-law FRQs. - After each topic, do a short MCQ set to catch gaps before moving on. All six topics with practice are at AP Physics 2 Unit 9. Since this unit is 15-18% of the exam, a few focused study sessions here pay off more than spreading that time across lighter units.