Equations · equation
Zeroth Law of Thermodynamics
Thermodynamics · 1935
- Equation
- A ⇄ C and B ⇄ C ⟹ A ⇄ B (⇄ : in thermal equilibrium)
- Field
- Thermodynamics
- Named After
- Ralph H. Fowler (naming)
- Year
- 1935
Symbols
A, B, C any three systems · ⇄ in thermal equilibrium: put them in contact and no net heat flows · the transitivity of that relation, not of equality, is what lets a single number stand for it
Significance
If two systems each sit in equilibrium with a third, they are in equilibrium with each other — the quiet assumption that makes 'temperature' a well-defined number and thermometers meaningful. Written with equals signs it is a triviality about numbers; the content is that thermal equilibrium is transitive, which is what earns the right to write a temperature down in the first place. So foundational it was named zeroth, after the first and second were taken.
Open on the Equations shelf — the interactive card, the filters, and everything alongside it.