Zeroth Law and Temperature
The zeroth law of thermodynamics establishes that thermal equilibrium is transitive, making temperature a well-defined, measurable property shared by systems in equilibrium.
Definition
The zeroth law states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other, which allows temperature to be defined as the property that is equal among systems in mutual thermal equilibrium.
Scope
This topic covers the statement of the zeroth law, the notion of thermal equilibrium and the diathermal wall, the transitivity that lets temperature be defined, and the construction of empirical and absolute temperature scales. The operation of thermometers and the ideal-gas scale are included as practical consequences.
Core questions
- What does it mean for two systems to be in thermal equilibrium?
- Why does transitivity of equilibrium permit a consistent definition of temperature?
- How are empirical temperature scales constructed and related to the absolute scale?
- Why must the zeroth law precede the first and second laws logically?
Key concepts
- Thermal equilibrium and diathermal walls
- Transitivity of equilibrium
- Empirical temperature and isotherms
- Ideal-gas and absolute temperature scales
- Thermometry
Clinical relevance
The zeroth law underlies every act of temperature measurement, from laboratory thermometry to industrial process control and meteorology, by guaranteeing that a thermometer reads a property genuinely shared with the system it touches.
History
Although thermometry predates formal thermodynamics by centuries, the principle was articulated as a distinct law and named the 'zeroth law' by Fowler in the 1930s, formalizing the long-implicit basis for measuring temperature.
Key figures
- Ralph H. Fowler
- James Clerk Maxwell
Related topics
Seminal works
- callen1985
- fowler1939
Frequently asked questions
- Why is the zeroth law necessary if temperature seems obvious?
- Without transitivity of thermal equilibrium there would be no guarantee that a single number could label a system's hotness consistently; the zeroth law supplies exactly that guarantee, turning temperature from an intuition into a defined state variable.