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Zeroth law
Beginner
Bodies in equilibrium with a third body are in equilibrium with each other.
If A is in thermal equilibrium with C and B is too, then A and B are in equilibrium with each other. This makes temperature a well-defined property and thermometers possible.
Two cups both matching a thermometer have the same temperature.
Equilibrium means no net heat flow.
In more detail
The zeroth law states that if two systems are each in thermal equilibrium with a third, they are in equilibrium with each other. It justifies the use of thermometers.
Real-world uses
- Thermometers rely on the zeroth law to indicate temperature reliably.
- It explains why a thermometer read by one person matches another.
- A thermometer can be used to check whether two objects are at the same temperature, because each is in equilibrium with the thermometer.
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Frequently asked questions
Why is it called the zeroth law?
It was recognised after the first and second laws, but logically it comes before them.
What does thermal equilibrium mean?
Two bodies in contact no longer exchange net heat.
Why is the zeroth law needed?
It is the logical basis for defining temperature, since it says that thermal equilibrium is transitive.
Related topics in Heat & Thermodynamics
Temperature · Temperature scales · Thermometers · Thermal expansion · Area and volume expansion · Anomalous expansion of water