Unit Overview

Thermodynamics Overview

AP Physics 2· 5 min read 📊 12-18% of total AP Physics 2 exam score

1. Unit at a Glance

We build this unit from foundational definitions to advanced applications. You will start by defining what a thermodynamic system is, learn core state variables and ideal gas behavior, then explore energy transfer, the two laws of thermodynamics, and end with entropy and the direction of thermal processes. Every concept connects to answering two key questions: how much energy is transferred, and why processes proceed the way they do.

Below are all sub-topics in this unit, ordered by learning sequence:

AP Physics 2 Thermodynamic Systems

Learn how to define systems, surroundings, and core thermal state variables for analysis.

★⏱ 6 min

AP Physics 2 Pressure, Thermal Equilibrium and Ideal Gas Law

Understand thermal equilibrium and apply the ideal gas law to relate gas state variables.

★★⏱ 9 min

AP Physics 2 Heat and Energy Transfer

Explore the three primary modes of heat transfer between systems and their properties.

★★⏱ 7 min

AP Physics 2 First Law of Thermodynamics

Master the first law, which relates internal energy change to heat and work.

★★⏱ 8 min

AP Physics 2 Thermal Processes

Analyze common thermal processes (isothermal, adiabatic, etc.) on PV diagrams.

★★★⏱ 10 min

AP Physics 2 Second Law of Thermodynamics

Learn formulations of the second law and its implications for spontaneous processes.

★★★⏱ 9 min

AP Physics 2 Entropy

Understand entropy, how it changes in processes, and its connection to the second law.

★★★★⏱ 10 min

2. Common Pitfalls

Wrong move:

Confusing work done on the system vs. work done by the system in the first law.

Why:

Inconsistent sign conventions across sources lead to frequent calculation errors on the AP exam.

Correct move:

Use the standard AP convention: $e x t F i r s t L a w = D e l t a U = Q + W$ , where $W$ is work done on the system.

Wrong move:

Treating entropy as only 'disorder' without connecting it to energy dispersal.

Why:

The colloquial 'disorder' definition often leads to incorrect reasoning about entropy change in processes.

Correct move:

Always frame entropy change in terms of how much energy is dispersed into or out of a system.

Wrong move:

Applying the ideal gas law to non-equilibrium states.

Why:

The ideal gas law only holds for systems at thermal equilibrium, not during rapid non-equilibrium changes.

Correct move:

Only use $P V = n R T$ for points on PV diagrams that correspond to equilibrium states.

3. Quick Reference Cheatsheet

Concept / Key Formula	Key AP Notes
First Law of Thermodynamics	$oxed{\\Delta U = Q + W}$ , $W$ = work done on system
Ideal Gas Law	$P V = n R T = N k_{B} T$
Entropy change (reversible process)	$Δ S = \frac{Q}{T}$
Second Law of Thermodynamics	$\boxed{\
Second Law of Thermodynamics (entropy form)	$\boxed{\
Adiabatic process	$Q = 0$ , so $\
Isothermal process (ideal gas)	$\
Thermal equilibrium	Equal temperature across systems, no net heat flow