What is Zeroth Law of Thermodynamics?

The Zeroth Law of Thermodynamics is a fundamental principle of thermal equilibrium. It states:

If two systems (A and B) are each in thermal equilibrium with a third system (C), then A and B are also in thermal equilibrium with each other.

In simple terms, it defines the concept of temperature and allows us to measure it consistently. Without this law, the idea of temperature as a measurable quantity wouldn’t exist.

Why is Zeroth Law of Thermodynamics Important?

• Establishes temperature as a measurable property.
• Forms the basis of thermometry (use of thermometers).
• Ensures consistency in comparing heat states of different systems.

This law is called the “Zeroth” law because it was formulated after the First and Second Laws, but is more fundamental—hence given priority as the starting point.

Also Check: Is Internal Energy a State Function?

Example of Zeroth Law

Imagine:

• System A = Cup of hot coffee
• System B = Glass of water
• System C = Thermometer
• When the thermometer is placed in coffee (A), after some time, it reaches the same temperature → A and C are in equilibrium.
• When the same thermometer is placed in water (B), it also shows equilibrium.
• By the Zeroth Law → Coffee (A) and Water (B) must also share a defined relationship in terms of temperature.

Thus, the thermometer acts as a medium to compare temperatures, which is why it works.

The Zeroth Law is crucial in both classical thermodynamics and modern AI-driven thermal simulations. In fields like climate modeling, HVAC optimization, and material science, AI uses the Zeroth Law as a foundation for predicting how systems reach equilibrium.