Theme B · Particulate Nature of Matter · HL only · 8 hours
B.4 Thermodynamics
In what way is entropy fundamental to the evolution of the universe?
Higher Level only — no Standard Level content in this topic
The first law of thermodynamics is conservation of energy: Q = ΔU + W. Every engine, every refrigerator, every living cell obeys it. But it doesn't tell you which direction a process goes. That is the job of the second law, and of entropy.
Entropy S measures disorder — or more precisely, the number of microstates available to the system (S = kB ln Ω). The second law says entropy in an isolated system never decreases. This single asymmetry — the arrow of time — explains why ice never spontaneously forms in a warm room, why engines always waste some heat, and why the universe moves towards greater disorder. The Carnot cycle establishes the absolute maximum efficiency any heat engine can achieve: η = 1 − Tc/Th, regardless of what working fluid or mechanism is used.
A heat engine operates between reservoirs at 500 K and 300 K. It absorbs 800 J of heat per cycle. What is the maximum work it can do per cycle, and how much heat must it reject? What process does the Carnot cycle use to achieve this maximum?
Key equations
First law: Q = ΔU + W (Q into system; W done by system)
Physics 27 First Law of Thermodynamics (1 of 22) What is the First Law of Thermodynamics?Physics 27 First Law of Thermodynamics (21 of 22) Summary of the 4 Thermodynamic Processes
WNY Tutor — worked problems
The figure shows a reversible cycleOne mole of neon gas is heated from 300 K to 420 K at constant pressure