Definition
A gas law stating that, at a constant temperature, the absolute pressure of a confined gas is inversely proportional to its volume. As the volume of the gas decreases, its pressure increases by the same proportion, and vice versa.
Plain English
If you squeeze a sealed amount of gas into a smaller space (without changing its temperature), its pressure goes up. If you let it expand into a bigger space, its pressure goes down. Pressure and volume move in opposite directions.
Context Anchor
Seen in aviation discussions of altitude changes, trapped air in the body, aircraft pressurization, and gas-filled systems.
Derivation
Named after Robert Boyle, a 17th-century Anglo-Irish scientist who published the relationship in 1662. Knowing the name is just a person's name helps avoid the assumption that 'Boyle' refers to boiling or heat — this law is specifically about pressure and volume at constant temperature.
Why Pilots Care
The law explains why air trapped in an aircraft expands or contracts with altitude changes, affecting instrument accuracy, engine performance, and cabin pressure.
Analogy
Think of a sealed syringe with the tip blocked. Push the plunger in, and the trapped air has less space, so its pressure rises. Pull the plunger back, and the trapped air has more space, so its pressure drops.
Grounding Statement
Picture a sealed syringe with the tip blocked. Push the plunger in — the gas inside has less room, so it pushes back harder. Pull the plunger out — the gas has more room, so it pushes back less. Same gas, same temperature, just different volume.
Intuition Check
Boyle's Law is not about heating air to raise its pressure. It applies when the temperature stays constant and pressure changes because volume changes.
Example Sentence 1
Boyle's Law explains why the pressure inside an engine cylinder rises sharply during the compression stroke as the piston reduces the volume of the trapped fuel-air mixture.
Example Sentence 2
During preflight, a mechanic uses Boyle's Law to predict how a tire's pressure reading will change after the airplane has been sitting in cooler night air.