Physics 2 · Unit 2: Thermodynamics · 14 min read · Updated 2026-05-11
First Law of Thermodynamics — AP Physics 2
AP Physics 2 · Unit 2: Thermodynamics · 14 min read
1. Core Definition of the First Law★★☆☆☆⏱ 3 min
The first law of thermodynamics is an application of the conservation of energy principle to thermal systems, explicitly accounting for two primary energy transfer methods: heat transfer between the system and surroundings, and macroscopic work done by or on the system. This law is foundational for all thermodynamics questions on the AP Physics 2 exam, making up 1-2% of total score directly and required for nearly all thermodynamics problems.
2. AP-Specific Sign Conventions★★★☆☆⏱ 3 min
Consistent sign conventions are the most common source of lost points on the AP exam. The College Board explicitly requires the convention outlined below, which differs from some introductory textbooks.
$\Delta U$: Positive when system internal energy increases, negative when it decreases
$Q$: Positive when net heat flows *into* the system from surroundings, negative when heat flows out
$W$: Positive when net work is done *by* the system on surroundings, negative when work is done *on* the system by surroundings
Exam tip: Always write down each variable with its sign before substituting. AP FRQ graders award points for correct sign reasoning, and MCQ distractors are designed for wrong convention errors.
3. Work Calculations and PV Diagrams★★★☆☆⏱ 4 min
For any quasi-static (slow, equilibrium) process (the only type tested on AP Physics 2), work done by the gas equals the area under the process path on a pressure-volume (PV) diagram. The general formula for work is:
W = \int_{V_1}^{V_2} P dV
If volume increases (expansion, moving right on the diagram), work is positive. If volume decreases (compression, moving left), work is negative. For constant pressure processes, the integral simplifies to $W = P\Delta V$. For a cyclic process (closed loop on a PV diagram that starts and ends at the same state), net work equals the area enclosed by the loop: clockwise loops give positive net work, counterclockwise loops give negative net work.
Exam tip: For any cyclic process, immediately note the direction of the loop to get the sign of work right. You do not need to recalculate the sign from scratch once you know direction.
4. Simplifications for Common Ideal Gas Processes★★★★☆⏱ 4 min
AP Physics 2 regularly tests application of the first law to four standard ideal gas processes, each with a defining constraint that simplifies the general formula, using the fact that ideal gas internal energy depends only on temperature.
Exam tip: Memorize the simplified forms, but always derive them from the full first law $\Delta U = Q - W$ on FRQs to earn full credit even if you misremember the simplification.
Common Pitfalls
Why: Many introductory textbooks use this alternate convention, leading to flipped signs that lose points.
Why: Students remember work equals loop area but ignore the effect of direction on sign.
Why: $P\Delta V$ only works for constant pressure processes, and students overgeneralize the formula.
Why: Students rush through problems and forget that sign depends on the direction of heat transfer.
Why: Students only associate $\Delta U = 0$ with isothermal processes.
Why: Students confuse state functions (internal energy) with path functions (work and heat).