AP Physics 2: Algebra-Based, AP exams, 15–18%

Unit 9: Thermodynamics

Unit 9 opens the course by treating a gas as a mechanical system. Pressure, temperature, and the ideal gas law connect to forces and free-body diagrams, and the laws of thermodynamics connect to conservation of energy. Entropy is introduced through probability and the tendency toward the most likely arrangement.

Position: 1 of 7 units
Published weighting: 15–18% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Physics 2: Algebra-Based. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Thermodynamic systems: Defining a system and its surroundings, internal energy, and the ways energy crosses the boundary.
Pressure, thermal equilibrium, and the ideal gas law: Relating pressure, volume, temperature, and moles, and describing thermal equilibrium at the particle level.
Thermodynamics and forces: Pressure as force per area applied to pistons and containers, analyzed with free-body diagrams.
Heat, work, and the first law: Conservation of energy applied to gases, including work as the area under a pressure-volume curve.
Thermal processes on PV diagrams: Isothermal, isobaric, isochoric, and adiabatic processes and what each looks like graphically.
Probability and entropy: Microstates, the statistical basis of the second law, and the direction of spontaneous change.

Study tips

How to practice Thermodynamics.

Section-specific guidance that follows from how this material is usually tested.

Practice reading work directly off pressure-volume diagrams, including the sign for compression versus expansion.
Fix the first-law sign convention once and test it on a case where the gas does work on the surroundings.
Explain temperature in terms of average molecular kinetic energy rather than as a primitive quantity.

Where this fits

Unit 9: Thermodynamics is 1 of 7 units in the AP Physics 2: Algebra-Based guide, and it carries a published weighting of 15–18% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Physics 2: Algebra-Based guide Official exam information

Sample Thermodynamics questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
A sealed rigid container holds an ideal gas at 300 K and a pressure of 1.0 x 10^5 Pa. The gas is heated until its temperature reaches 600 K. What is the new pressure of the gas?
1. A5.0 x 10^4 Pa
2. B1.0 x 10^5 Pa
3. C2.0 x 10^5 Pa
4. D4.0 x 10^5 Pa
Show answer
Answer: C. 2.0 x 10^5 Pa
The container is rigid, so volume and the number of moles are fixed and PV = nRT reduces to P proportional to absolute temperature. Doubling T from 300 K to 600 K doubles the pressure to 2.0 x 10^5 Pa; the 4.0 x 10^5 Pa choice would require the pressure to scale with the square of temperature.
Key insight: Gas law ratios only work with absolute temperature in kelvins, never with Celsius readings.
Question 2Foundational
In the kinetic theory model of an ideal gas, the average translational kinetic energy of the gas molecules is directly proportional to which quantity?
1. AThe absolute temperature of the gas
2. BThe pressure of the gas
3. CThe volume of the container
4. DThe number of moles of gas present
Show answer
Answer: A. The absolute temperature of the gas
Kinetic theory gives the average translational kinetic energy per molecule as (3/2)kT, so it depends only on absolute temperature. Adding more molecules at the same temperature raises the pressure and the total internal energy but leaves the average energy per molecule unchanged.
Key insight: Temperature is a measure of average molecular kinetic energy, which is why two gases in thermal equilibrium share that average.
Question 3Intermediate
A gas expands from a volume of 0.010 m^3 to 0.030 m^3 while its pressure is held constant at 2.0 x 10^5 Pa. How much work does the gas do on its surroundings?
1. A2000 J
2. B4000 J
3. C6000 J
4. D8000 J
Show answer
Answer: B. 4000 J
At constant pressure the work is the area under the pressure-volume curve: W = P(change in V) = (2.0 x 10^5)(0.020) = 4000 J. The 6000 J choice uses the final volume rather than the change in volume.
Key insight: Work in a thermodynamic process is the area under the curve on a pressure-volume diagram, and it is positive when the gas expands.
Question 4Intermediate
True or false: the second law of thermodynamics requires that the total entropy of an isolated system never decreases.
1. ATrue
2. BFalse
Show answer
Answer: A. True
Entropy measures the number of microscopic arrangements available to a system, and an isolated system evolves toward more probable arrangements, so its total entropy increases or, for a reversible process, stays the same. A single part of a system can lose entropy, as when water freezes, but only if the surroundings gain at least as much.
Key insight: Entropy sets the direction of spontaneous change: energy spreads out rather than spontaneously concentrating.
Question 5Advanced
During one process, 400 J of heat flows into a gas and the gas does 600 J of work on its surroundings. What is the change in the internal energy of the gas?
1. A-1000 J
2. B-200 J
3. C+200 J
4. D+1000 J
Show answer
Answer: B. -200 J
The first law gives the change in internal energy as heat added minus work done by the gas: 400 J - 600 J = -200 J. The +1000 J choice adds the two contributions, ignoring that work done by the gas drains energy from it.
Key insight: Energy entering as heat raises internal energy; energy leaving as work done by the gas lowers it.

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Unit 9: Thermodynamics