Physics 2 · Unit 2: Thermodynamics · 14 min read · Updated 2026-05-11
AP Physics 2 Thermodynamic Systems — AP Physics 2
AP Physics 2 · Unit 2: Thermodynamics · 14 min read
1. What Is a Thermodynamic System?★★☆☆☆⏱ 2 min
A thermodynamic system is any clearly defined region of matter or space that we choose to analyze for energy transfer and property changes; everything outside the system that can exchange energy or matter with it is called the surroundings. This topic is the foundational framework for all thermodynamics problems on the AP Physics 2 exam, and Unit 2 Thermodynamics accounts for 18-22% of the total exam score, with questions appearing in both multiple-choice and free-response sections.
Standard notation defines system internal properties as unprimed variables, and surrounding properties as primed. Systems are classified by whether they can exchange energy and/or matter with the surroundings, a classification that dictates which thermodynamic laws apply to a given problem. Unlike many other topics in physics, thermodynamics analysis always starts with explicitly defining your system—this choice simplifies calculations and avoids sign errors in energy transfer problems.
2. Classification of Thermodynamic Systems★★☆☆☆⏱ 3 min
All thermodynamic systems are grouped into three categories based on what can cross the system boundary, the edge separating the system from the surroundings.
**Open system**: Can exchange both energy and matter with the surroundings. Common examples include an open cup of hot tea (water vapor leaves as matter, heat leaves as energy) and a living plant exchanging gases with the air.
**Closed system**: Can exchange energy, but *not* matter, with the surroundings. All calculation-based thermodynamics problems on the AP exam use closed systems, such as a sealed piston containing gas or an unopened soda can.
**Isolated system**: Cannot exchange either energy or matter with the surroundings. Perfect isolation is an idealization, but highly insulated sealed containers (like a well-made thermos with a sealed lid) are approximated as isolated for problem-solving.
When you first approach any thermodynamics problem, drawing the system boundary and classifying the system is the critical first step, as energy accounting rules change based on the type. AP Physics 2 regularly tests classification in multiple-choice questions, so memorizing the rules for each category is essential.
Exam tip: When classifying systems, always check for matter exchange first—if any matter crosses the system boundary, it cannot be closed or isolated, so it is automatically open.
3. State vs Path Functions★★★☆☆⏱ 4 min
A core distinction in thermodynamics that is heavily tested on the AP exam is between state functions and path functions.
This distinction is the basis for many common AP exam questions that test conceptual understanding of thermodynamics, beyond just calculation.
Exam tip: Any question asking about change in internal energy between two states for an ideal gas only depends on the initial and final temperatures, not the work or heat transferred along the path.
4. First Law of Thermodynamics and P-V Work★★★☆☆⏱ 5 min
For closed thermodynamic systems (the only type AP Physics 2 asks you to calculate for), energy conservation is written as the first law of thermodynamics. The AP exam uses the following standard sign convention:
$Q$ is positive when heat is added *to* the system, negative when heat leaves the system.
$W$ is the work done *on* the system by the surroundings, positive when work is done on the system, negative when the system does work on the surroundings.
ext{Delta} U = Q + W
For a system that expands or compresses at constant pressure, the work done on the system is given by the P-V work formula:
W = -P\Delta V
where $\Delta V = V_{\text{final}} - V_{\text{initial}}$. The negative sign accounts for the sign convention: if the system expands, $\Delta V > 0$, so $W$ is negative (the system does work on the surroundings), which matches our convention. On a pressure-volume (P-V) diagram, the work done *by* the system is equal to the area under the process curve between the initial and final volume.
Exam tip: Always confirm the sign convention for work in the problem—if the problem explicitly states it uses $W$ as work done *by* the system, adjust the first law to $\Delta U = Q - W$ to avoid sign errors.
5. P-V Cycle Analysis★★★★☆⏱ 3 min
Common Pitfalls
Why: Students confuse no matter exchange with no energy exchange—insulation prevents heat transfer, but you can still do work on the gas by compressing the piston, so energy can cross the boundary
Why: Students confuse path functions (Q, W) with the state function (U), and assume the change in U depends on work or heat
Why: Students forget the negative sign in $W = -P\Delta V$, and just multiply P and the magnitude of volume change
Why: Students forget that water vapor is matter that leaves the system, so matter exchange does occur
Why: Students mix up work for a single process and net work for a cycle