Pistone

Meet Pistone, a piston who gets lifted by hot gas and shows how a gas does work.

Gas in a cylinder pushes on every surface it touches, including the piston on top. If the gas expands and the piston moves outward, that push acts through a distance, and the gas does work on whatever the piston is holding up. If the pressure stays constant while this happens, the process is called isobaric.

The work is then W = pΔV. Gas at 200 kPa that expands by 0.5 L, or 5 × 10⁻⁴ m³, does W = 200,000 × 0.0005 = 100 J of work. That is enough to lift a 10 kg weight about 1 m.

If the gas is squeezed instead, ΔV is negative and work is done on the gas. When the pressure changes during the stroke, the work is the area under the curve on a pressure–volume graph.

Car engines, steam engines and even a popping cork all turn hot expanding gas into useful work.