1. What Is the Zeroth Law of Thermodynamics?
The Zeroth Law of Thermodynamics states that if two bodies are each in thermal equilibrium with a third body, then they are in thermal equilibrium with each other.
In simple words: if A is in equilibrium with C, and B is also in equilibrium with C, then A and B must be at the same temperature.
2. Meaning of Thermal Equilibrium
Two bodies are in thermal equilibrium when there is no net flow of heat between them. This happens when they are at the same temperature.
If two objects are touching or exchanging heat through radiation but their temperatures do not change, they are in thermal equilibrium.
2.1. Examples of Thermal Equilibrium
- A thermometer in contact with your skin eventually shows a steady reading—it has reached the same temperature.
- A cup of tea left on a table cools until it reaches room temperature; at this point, both are in equilibrium.
3. Why the Zeroth Law Is Important
The Zeroth Law gives a clear definition of temperature. Without it, temperature would not be a measurable or meaningful concept. The law tells us that temperature is the property that determines whether two systems are in thermal equilibrium.
3.1. Foundation for Temperature Measurement
Because of the Zeroth Law, we can use a thermometer to measure temperature. If the thermometer is in equilibrium with a body, it has the same temperature as that body.
3.2. Transitive Nature of Temperature
The law shows that thermal equilibrium is a transitive relationship. If A = C and B = C in temperature, then A = B. This makes temperature a well-defined physical quantity.
4. Practical Interpretation of the Zeroth Law
The Zeroth Law explains how we can compare temperatures of different bodies without placing them in direct contact. We only need a reliable intermediate body (a thermometer).
4.1. Daily-Life Connections
- Electronic thermometers use sensors that come into thermal equilibrium with your body.
- Room thermostats sense air temperature using the Zeroth Law principle.
- Cooking relies on temperature comparison—food is “done” when its temperature matches what is needed.
5. Zeroth Law and Thermodynamic Consistency
Thermodynamics needs consistent rules. The Zeroth Law ensures we can talk about temperature without contradictions. It fills the logical gap between the First and Second laws, even though it was added later.
5.1. Why It Is Called the 'Zeroth' Law
The concept was introduced after the First and Second laws, but it was so fundamental that scientists realised it should come before them. So it was named the “Zeroth” Law.