Heat, entropy, and the arrow of time.
Thermodynamics
Thermodynamics governs energy transfer in systems made of countless particles. Its four laws explain why heat flows, engines run, and time moves forward.
Chapter outline
Lessons in this chapter
Short, focused lessons. Each one ends with a worked example and a quick quiz.
Key concepts at a glance
The essential ideas, paired with their equations.
First Law
01Energy is conserved: internal energy change equals heat in minus work out.
Second Law
02Entropy of an isolated system never decreases.
Ideal Gas Law
03Pressure, volume, and temperature for an idealized gas.
Carnot Efficiency
04The maximum efficiency any heat engine can achieve.
Heat Capacity
05Energy needed to raise a substance's temperature.
Boltzmann Entropy
06Entropy measures the number of microscopic arrangements.
Frequently asked
What is entropy in simple terms?+
Entropy measures disorder — the number of microscopic ways a system can be arranged for the same macroscopic state.
Why can't engines be 100% efficient?+
The second law forbids it: some energy must always be exhausted as waste heat.