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LearnAQA7408First teaching 2015Paper 3B · Option: Engineering physicsThermodynamics and engines

Paper 3B · Module

Thermodynamics and engines

2 subtopics≈ 90 min in total

Start the module: The first law of thermodynamics Open graphing calculator

The first law of thermodynamics
The first law is conservation of energy for a system: $\Delta U=q+W$ , where $q$ is the energy transferred to the system by heating and $W$ is the work done on the system. When a gas expands at constant pressure, it does work $p\Delta V$ on its surroundings, so $W=-p\Delta V$ . On a $p$ – $V$ graph, work is the area under the curve.
≈ 40 min
Heat engines and heat pumps
The first law $Q=\Delta U+W$ tracks energy in a gas. Isothermal changes keep $pV$ constant; adiabatic changes ( $Q=0$ ) keep $pV^\gamma$ constant. Work done is the area under a p–V curve, so the area enclosed by a cycle is the net work per cycle. No engine can turn all heat into work: maximum efficiency is $\frac{T_H-T_C}{T_H}$ . Heat pumps and refrigerators run engines backwards.
≈ 50 min