If two systems are each in thermal equilibrium with a third, they are in equilibrium with each other.
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The zeroth law of thermodynamics states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This statement may seem obvious, but it establishes thermal equilibrium as a transitive relation, which is necessary for temperature to be a meaningful property. It justifies the use of thermometers, since a thermometer can be calibrated against a standard and then used to compare any two objects. The law gives thermodynamics a formal basis for defining temperature independently of the properties of any particular substance. Its name was assigned later, because it logically precedes the first and second laws.
Key ideas: The law means that equality of temperature is an equivalence relation, so a thermometer reading is meaningful. It lets us define temperature without needing a particular material. Two bodies in contact with no net heat flow share the same temperature.
Common mistakes and connections: It sounds trivial, but without it thermometers could not compare bodies indirectly. It is numbered zero because it was formulated after the other laws but logically comes before them. It is the starting point for the temperature concept used throughout thermodynamics.
Two bodies in contact converge to a common temperature. If both match a third body (a thermometer) they are in equilibrium with each other. The interactive lesson shows this as a live animation or chart.
A thermometer placed in a beaker reads the same temperature as the water once the two have reached thermal equilibrium.
If two systems are each in thermal equilibrium with a third, they are in equilibrium with each other.
Zeroth Law of Thermodynamics is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Thermodynamics & Energetics first: Heat Capacity Ratio, Carnot Cycle, Heat Engine Efficiency.
The next topics in Thermodynamics & Energetics are Thermal Equilibrium and Temperature Scales.
Yes. The interactive lesson includes a self-check question on Zeroth Law of Thermodynamics, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Zeroth Law of Thermodynamics is topic 56 of 91 in Thermodynamics & Energetics.