This HL-only mini-lesson covers Theme B.4 — Thermodynamics: internal energy, the first law Q = ΔU + W, work done by a gas, the four processes, entropy and the second law, and heat-engine efficiency.
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Watch for the HL flag on higher-level extensions. Press Start when you're ready.
The internal energy U of an ideal gas depends only on its temperature (U = (3/2)nRT for a monatomic gas). The first law is conservation of energy for a gas:
Sign care: in this IB convention, W is the work the gas does on its surroundings. If the gas is compressed, W is negative (work is done on it).
When a gas expands at constant pressure it does work on its surroundings equal to the area under the p–V graph:
A gas at 2.0 × 10⁵ Pa expands by ΔV = 0.0030 m³ at constant pressure.
W = pΔV = 2.0 × 10⁵ × 0.0030 = 600 J
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Entropy S measures the disorder (number of accessible microstates) of a system. The second law states that the entropy of an isolated system never decreases — it stays the same for a reversible process and increases for any real (irreversible) one.
This gives time its direction: heat flows spontaneously from hot to cold, never the reverse, because that increases total entropy. No heat engine can be 100% efficient.
A heat engine's efficiency is the useful work out per unit heat in. The maximum possible (Carnot) efficiency depends only on the reservoir temperatures (in kelvin):
An engine works between T_h = 500 K and T_c = 300 K.
η = 1 − 300 ÷ 500 = 0.40 = 40%
Tap a statement on the left, then its match on the right.
First law: Q = ΔU + W (heat in = internal-energy rise + work done by gas)
Work: W = pΔV (area under a p–V graph)
Processes: isovolumetric (W=0) · isobaric · isothermal (ΔU=0) · adiabatic (Q=0)
Second law: entropy of an isolated system never decreases
Efficiency: η = W/Q_H; Carnot ceiling η = 1 − T_c/T_h
That completes Thermodynamics for IB Diploma Physics HL. Press Finish to see your score.
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