AP PHYSICS 2 — physicsMedium3 min read

Thermodynamics

AP Physics 2 thermodynamics: laws of thermodynamics, heat transfer, ideal gas law, PV diagrams, heat engines, and entropy.

Tutor AI Team2026-02-26T00:10:28.828Z

# Thermodynamics — AP Physics 2

Thermodynamics connects heat, work, and energy at the macroscopic level. AP Physics 2 covers the ideal gas law, the laws of thermodynamics, PV diagrams, and heat engines. Mastering these topics requires understanding both the mathematical relationships and the conceptual meaning of each process.

Key Concepts

Temperature and Thermal Energy

Temperature: measure of average kinetic energy of particles.
Average KE per molecule: $\overline{KE} = \frac{3}{2}k_B T$ (for ideal monatomic gas).
$k_B = 1.38 \times 10^{-23}\ \text{J/K}$ .

Ideal Gas Law

$PV = nRT = Nk_B T$ where $n$ = moles, $R = 8.314\ \text{J/(mol·K)}$ , $N$ = number of molecules.

First Law of Thermodynamics

$\Delta U = Q - W$

$\Delta U$ : change in internal energy
$Q$ : heat added to the system (positive = heat in)
$W$ : work done by the system (positive = expansion)

Thermodynamic Processes

Process	Constant	Consequence
Isothermal	$T$	$\Delta U = 0$ , $Q = W$
Isobaric	$P$	$W = P\Delta V$
Isochoric (Isovolumetric)	$V$	$W = 0$ , $\Delta U = Q$
Adiabatic	$Q = 0$	$\Delta U = -W$

PV Diagrams

Work done by gas = area under the curve on a PV diagram.
Clockwise cycle: net positive work (heat engine).
Counterclockwise cycle: net negative work (refrigerator).

Second Law of Thermodynamics

Heat flows spontaneously from hot to cold.
Entropy of an isolated system never decreases.
No heat engine can be 100% efficient.

Heat Engine Efficiency

$e = \frac{W_{\text{net}}}{Q_H} = 1 - \frac{Q_C}{Q_H}$

Carnot (maximum) efficiency: $e_{\text{Carnot}} = 1 - \frac{T_C}{T_H}$

Worked Example

Problem: An ideal gas in a sealed container ( $V = 0.01\ \text{m}^3$ ) at $300\ \text{K}$ and $1\ \text{atm}$ is heated to $600\ \text{K}$ at constant volume. What is the final pressure?

Solution:

At constant volume: $P_1/T_1 = P_2/T_2$ $P_2 = P_1 \cdot \frac{T_2}{T_1} = 1\ \text{atm} \cdot \frac{600}{300} = 2\ \text{atm}$

Practice Questions

1. How much work does a gas do when it expands from $0.02\ \text{m}^3$ to $0.05\ \text{m}^3$ at a constant pressure of $200\ \text{kPa}$ ?

$W = P\Delta V = 200{,}000 \times 0.03 = 6000\ \text{J}$ .

2. A gas absorbs $500\ \text{J}$ of heat and does $200\ \text{J}$ of work. What is $\Delta U$ ?

$\Delta U = Q - W = 500 - 200 = 300\ \text{J}$ .

3. A heat engine operates between $600\ \text{K}$ and $300\ \text{K}$ . What is its maximum efficiency?

$e_{\text{Carnot}} = 1 - 300/600 = 0.5 = 50\%$ .

4. During an adiabatic expansion, what happens to the temperature of an ideal gas?

It decreases. Since $Q = 0$ , the gas does work at the expense of internal energy, so $\Delta U < 0$ and temperature drops.

Want to check your answers and get step-by-step solutions?

Summary

The ideal gas law ( $PV = nRT$ ) describes gas behavior.
First law: $\Delta U = Q - W$ — energy is conserved.
PV diagram area = work done by the gas.
Carnot efficiency sets the upper limit for heat engines.

Tags:

#ap#physics#thermodynamics

Ready to Ace Your AP PHYSICS 2 physics?

Get instant step-by-step solutions to any problem. Snap a photo and learn with Tutor AI — your personal exam prep companion.