Definition
A physical law stating that the rate at which a gas diffuses through a porous barrier is inversely proportional to the square root of its density. Lighter gases diffuse faster than heavier gases under the same conditions.
Plain English
Lighter gases spread and mix faster than heavier gases. The heavier the gas, the slower it moves through small openings.
Context Anchor
Seen in aviation discussions of gas behavior, such as oxygen systems, fuel vapors, leaks, and basic aircraft-system physics.
Derivation
Named after Thomas Graham, a Scottish chemist who studied how gases move and mix in the 1830s and 1840s.
Why Pilots Care
Helps explain why lighter gases like helium or hydrogen leak out of containers faster than heavier gases like oxygen, which matters for aircraft systems that store or use compressed gases.
Grounding Statement
If you opened a bottle of helium and a bottle of carbon dioxide in the same room, you would smell or detect the helium first because its lighter molecules spread out faster.
Intuition Check
Do not read “law” here as an FAA rule. Graham's Law is a science law about gas movement, not a regulation pilots comply with.
Example Sentence 1
Graham's Law explains why a small leak in a hydrogen-filled balloon empties it faster than the same leak in an air-filled one.
Example Sentence 2
Graham's Law helped explain the observed behavior of fuel vapors moving through the vent line.