Everyday Examples of Zeroth Law of Thermodynamics

Thermometers, fridges, and air conditioners all work because of a simple rule called the Zeroth Law of Thermodynamics. This law explains how heat moves between objects until they are the same temperature. When fruits get cold in the fridge, thermometers show your body temperature, or machines control hot water, the law is working in the background. Scientists and engineers use it in tools, pipes, and labs. This rule helps us understand temperature and how to measure it.

Learn about 1st law of Thermodynamics and 2nd law of Thermodynamics

Zeroth Law of Thermodynamics Examples

Zeroth Law of Thermodynamics explains that if two objects are each the same temperature as a third object, they are also the same temperature as each other. This helps us measure and manage heat in everyday life and science. Here are Everyday Examples of the Zeroth Law of Thermodynamics:

1.    Thermometers

A thermometer tells us how hot or cold something is. But how does it know?

When you place a mercury thermometer under your tongue or on your forehead, the mercury inside touches your skin. It reaches the same temperature as your body. This is called thermal equilibrium.

As the mercury heats up, it expands. As it cools, it shrinks. The height of the mercury tells us your temperature.

The thermometer only works because it shares heat with your body until both are the same temperature. That’s the Zeroth Law in action.

2.    Food in Refrigerators

When you put fruits in the fridge, they feel warm at first. The air inside the refrigerator is much cooler.

Slowly, the fruits lose heat to the cold air. Over time, the temperature of the fruit and the air become the same.

When the fruit is no longer warmer than the air, no more heat moves between them. This is thermal equilibrium.

The Zeroth Law explains why the fruit stays cold — it has reached the same temperature as the fridge air.

3.    Air Conditioners

Air conditioners help keep rooms cool, even on hot days. They do this by moving hot air out and letting cool air in.

Inside the air conditioner is a thermometer or sensor. It measures the room’s temperature.

The air conditioner keeps cooling the room until the air inside reaches the set temperature. When the room’s air and the sensor are at the same temperature, the cooling stops.

That’s thermal equilibrium. The Zeroth Law explains how the machine knows when to stop or start to keep things just right.

4.    Heat Exchangers (Industrial)

Factories often need to move heat from one place to another. They use tools called heat exchangers.

A heat exchanger allows hot fluid and cold fluid to move past each other, separated by metal walls.

The hot fluid gives off heat. The cold fluid takes in heat. Over time, their temperatures balance out — but they never mix.

The Zeroth Law ensures that each fluid reaches thermal equilibrium with the metal wall. That’s how heat is transferred safely and efficiently.

5.    Temperature Sensors in Pipes

In factories, hot liquids often flow through pipes. To check their temperature, workers use thermometers or sensors attached to the outside of the pipe.

These sensors wait until they match the liquid’s temperature. Then they give a reading.

If the sensor and liquid are not in thermal equilibrium, the temperature reading will be wrong.

Thanks to the Zeroth Law, we know that once they are the same temperature, the sensor gives an accurate measurement.

6.    Three-System Equilibrium (Scientific)

Let’s say you have three objects: A, B, and C.

If A is the same temperature as B, and A is also the same temperature as C, then B and C must be the same temperature too.

This idea is simple, but it’s very important in science. It helps us build temperature scales like Celsius and Kelvin.

It also helps calibrate thermometers, which means setting them correctly. This way, all scientists can measure heat the same way.

The Zeroth Law makes all of this possible by showing how heat balance works between different systems.