## Definition of the zeroth Law of Thermodynamics

### Definition :

The zeroth law of thermodynamics states that if a system A is in thermal equilibrium with system B, and A is also in thermal equilibrium with a third system C, then B and C are in thermal equilibrium with each other. This fundamental principle forms the basis of how we define and measure temperature.

There are several ways to express the zeroth Law more simply:

- It tells us that any systems in thermal equilibrium share the same temperature.

- It confirms that temperature is a meaningful property, since it determines whether or not heat will transfer between systems.

- Heat can be exchanged even without direct contact, such as through radiation.

- However, if two systems are in thermal equilibrium, no net heat transfer occurs between them.

Zeroth law of thermodynamics

### Example :

A common real-life demonstration of the zeroth Law is the use of a thermometer. For instance, in a traditional mercury thermometer, the mercury expands as the temperature rises. Because the cross-sectional area of the glass tube is constant, the expanding mercury moves upward. This increase in height corresponds to the rise in temperature, allowing us to measure how hot or cold something is thanks to thermal equilibrium between the thermometer and the object being measured.