Thermal Processes — AP Physics 2
1. First Law of Thermodynamics and PV Work ★★☆☆☆ ⏱ 4 min
A thermal (thermodynamic) process is any change in the macroscopic state of a system, defined by changes in pressure $P$, volume $V$, and temperature $T$, driven by energy transfer as heat or work. For AP Physics 2, we analyze closed systems with a fixed number of moles of gas, so all processes follow the ideal gas law and first law of thermodynamics. This topic makes up ~40% of Unit 2, which accounts for 12-18% of your total AP Physics 2 exam score.
A core AP skill is calculating work from a pressure-volume (PV) diagram. The magnitude of work done on the gas equals the area under the process path on the diagram: - If the gas expands ($\Delta V > 0$), the gas does work on the surroundings, so $W$ is negative. - If the gas is compressed ($\Delta V < 0$), $W$ is positive. For constant pressure processes, the area forms a rectangle, so the formula simplifies to $W = -P\Delta V$, where $\Delta V = V_{final} - V_{initial}$.
Exam tip: Always confirm the sign of $W$ by checking the volume change first: expansion = negative $W$, compression = positive $W$. This rule works for any process, not just constant pressure, and prevents sign errors on the first law.
2. Four Core Thermal Processes ★★★☆☆ ⏱ 5 min
Every common thermal process tested on AP Physics 2 is defined by a constraint that keeps one state variable constant, simplifying calculations for ideal gases. The four core processes are:
- **Isochoric (isovolumetric):** Volume $V$ is constant ($\Delta V = 0$). This means $W = 0$, so the first law reduces to $\Delta U = Q$: all heat transfer changes the internal energy (and thus temperature) of the gas.
- **Isobaric:** Pressure $P$ is constant. Work is calculated directly as $W = -P\Delta V$, as shown in the previous section.
- **Isothermal:** Temperature $T$ is constant ($\Delta T = 0$). For ideal gases, internal energy depends only on temperature, so $\Delta U = 0$. The first law reduces to $Q = -W$: all heat added to the gas is converted to work done by the gas (for expansion).
- **Adiabatic:** No heat transfer between the system and surroundings ($Q = 0$). The first law reduces to $\Delta U = W$: all work done on the gas changes the internal energy (and temperature) of the gas. Expansion cools the gas, compression heats it.
Exam tip: When comparing work between two processes from the same initial to final state on a PV diagram, the process with the larger area under its path has a larger magnitude of work. For expansion, this means a more negative $W$.
3. Entropy Change for Thermal Processes ★★★☆☆ ⏱ 3 min
Entropy $S$ is a state function that measures the disorder of a thermodynamic system. The second law of thermodynamics states that the total entropy of an isolated system never decreases for any real process. For AP Physics 2, you only need to calculate entropy change for reversible processes occurring at constant temperature, such as phase changes or isothermal processes.
Because entropy is a state function, its change depends only on the initial and final states, not the path taken between them. For any full cycle that returns a system to its initial state, the total entropy change of the system is zero.
Exam tip: Always convert temperature to Kelvin before plugging into entropy formulas. Using Celsius will give an incorrect magnitude and can lead to nonsensical results.
4. Concept Check ★★☆☆☆ ⏱ 2 min
Common Pitfalls
Why: Students mix up the 'constant X' definitions of the two processes, remembering the $\Delta U = 0$ rule and applying it to the wrong process.
Why: Students learn the alternate convention where $W$ is work done by the system and forget AP uses work done on the system.
Why: Students associate internal energy with heat transfer, so if $Q$ is non-zero they assume $\Delta U$ must be non-zero.
Why: Students confuse area for a single process with area enclosed by a full cycle.
Why: Students forget that entropy, like internal energy, temperature, pressure, and volume, is a state function that only depends on the current state.