IB Physics HL: Entropy & the Second Law of Thermodynamics FAQ

Entropy and the second law appear in Topic B.4 (Thermodynamics), HL only, and are examined on Paper 1 (multiple choice), Paper 2 (data/calculation questions, often on Carnot cycles or entropy change), and occasionally Paper 3. According to the IB, first exams for the current Physics guide were 2025, replacing the old Topic 3/10 split.

Entropy is a measure of the number of microscopic arrangements (microstates) consistent with a system's macroscopic state — more disorder means more microstates and higher entropy. In IB Physics HL you use both the statistical definition, $S=k\ln \Omega$, and the thermodynamic one, $\Delta S=\frac{Q}{T}$.

The second law states that the total entropy of an isolated system never decreases over time — it stays constant for a reversible process and increases for any irreversible one. In IB Physics HL this explains why heat flows from hot to cold spontaneously and why no engine can be 100% efficient.

Use $\Delta S=\frac{Q}{T}$ for a reversible process at constant temperature, where Q is heat transferred in joules and T is absolute temperature in kelvin. For processes with changing temperature, examiners usually give you enough data to treat it as several constant-T steps rather than requiring calculus.

Carnot efficiency is $\eta =1-\frac{T_C}{T_H}$, where both temperatures are in kelvin and $T_C$ is the cold reservoir, $T_H$ the hot reservoir. This is the theoretical maximum efficiency for any heat engine operating between those two temperatures — no real engine beats it.

A heat engine takes heat from a hot reservoir, converts some to work, and rejects the rest to a cold reservoir — efficiency is work output over heat input. A refrigerator runs the cycle backwards, using work input to move heat from cold to hot, and is assessed using coefficient of performance rather than efficiency.

Yes, relatively — it's consistently one of the lower-scoring HL topics I see on mock scripts, mainly because it's conceptual rather than purely mathematical. Students who can plug numbers into $\Delta S=Q/T$ often can't explain why entropy increasing means a process is irreversible, and that explanation is exactly what long-answer questions ask for.

The biggest one: forgetting to convert Celsius to kelvin before using $\Delta S=Q/T$ or the Carnot formula — this alone costs marks on almost every cohort I've marked. The second biggest: describing entropy as 'disorder' without connecting it to microstates or the second law, which loses marks on explain-type questions.

It varies by exam session, but a typical HL Paper 2 question on thermodynamics runs 6-10 marks, often split between a calculation (entropy change or Carnot efficiency) and a short explanation of the second law's implication. It rarely appears as a standalone essay-length question — usually it's one part of a larger energy question.

Master the two core formulas first — $\Delta S=Q/T$ and $\eta =1-T_C/T_H$ — then practise past-paper questions that ask you to explain, not just calculate, why a process is irreversible or why an engine can't hit 100%. Examiners reward explanations that name the second law explicitly rather than vague talk of 'energy loss'.

No — entropy and the second law of thermodynamics are HL-only content in Topic B.4. SL students study gas laws and thermal energy transfers in Topics B.1-B.3 but aren't examined on entropy, Carnot cycles, or heat engine efficiency.

Past-paper Paper 2 questions are the single best predictor of what you'll actually face, since thermodynamics questions follow recognisable patterns year to year. Pair that with concise Revision Notes covering the second law's three forms and a set of Topical Worksheets isolating entropy and Carnot calculations before mixing them with harder energy questions.