 # Zeroth Law of Thermodynamics

On this page learn about the zeroth law of thermodynamics for class 11 physics. This topic would also be helpful for students preparing for competitive exams like JEE Mains, JEE Advanced, and NEET.

After studying the content of this page you would be able to explain the zeroth law of thermodynamics, its significance, and its definition along with its applications and examples.

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## Zeroth Law of thermodynamics Explanation

It is one of the four laws of thermodynamics. Ralph H. Fowler is the one who came up with this law. The zeroth law of thermodynamics was proposed or formulated a long time after the first three laws of thermodynamics.

This new law defined the temperature in a way that was much clearer than what the other three laws stated. In other words, we can say that the zeroth law of thermodynamics defines temperature.

The concept of temperature as an indicator of thermal equilibrium is framed by the zeroth law of thermodynamics.

Let us first learn about the concept of thermal equilibrium before going into the details of this law of thermodynamics.

### What is thermal equilibrium?

When two bodies come into contact, they are said to be in thermal equilibrium if there is no heat transfer.

The temperature of two objects that are in contact with each other does not change when they are in thermal equilibrium.

For instance, if you put an apple from the fridge on the kitchen counter, these two things have different temperatures. Because of the temperature difference, the heat begins to flow between the apple and the kitchen countertop until they both reach the same temperature. When their temperatures are the same after some time, they are in thermal equilibrium.

### State Zeroth Law of thermodynamics

Zeroth law of thermodynamics states that

If two bodies A and B are in thermal equilibrium and A and C are also in thermal equilibrium then B and C are also in thermal equilibrium.

Now, Thermal equilibrium between two systems is the condition under which two substances in physical contact with each other exchange no heat energy.

### Zeroth law of thermodynamics significance

The significance of the Zeroth law of thermodynamics lies in its application of defining and conceptualizing temperature.

The concept of temperature is derived from the concept of thermal equilibrium or the Zeroth law. All bodies in thermal equilibrium share a common property that has the same value for all of them. This property is known as temperature.

Temperature is the property of a body that tells us whether it is in thermal equilibrium with another given body or not.

As a result of the preceding discussion, the temperature of a system can be defined as the property that determines whether the body is in thermal equilibrium with the neighboring systems.

So, if a group of systems is in thermal equilibrium, a common property of the group can be represented by a single numerical value called Temperature. If two systems are not in thermal equilibrium, their temperatures are different.

#### Verifying Zeroth law of thermodynamics:-

1. Suppose there are two bodies A and B and a third body C ( assume it to be a thermometer).
2. Consider two bodies A and B , isolated from from each other.
3. Now we want to know whether A and B are in thermal equilibrium with each other.
4. For this we would put body A in contact with body C (thermometer) till they both are in thermal equilibrium .
5. At that stage reading of thermometer will become constant.
6. Thermometer is now put in thermal contact with body B.
7. We will wait till it is in thermal equilibrium with body B and note the temperature.
8. If two readings are same, then A and B are thermal equilibrium with each other.

Let us now discuss a few examples where we can find the use of this law of thermodynamics.

### Zeroth law of thermodynamics examples

1. After a while, a cup of hot tea goes colder. This is due to the cooler air in the room surrounding the hot tea. This hot tea loses heat to the air in the room. Hot tea will continue to lose heat until it reaches the temperature of the surrounding air in the room. The tea will lose heat until it establishes thermal equilibrium with its surroundings. Thus, the zeroth law of thermodynamics explains the cooling of hot tea to room temperature in this example.
2. If you leave a glass of ice-cold water and a glass of boiling water on the kitchen counter for a few hours, all 3 will reach the same temperature after a few hours.
3. If you put vegetables in the refrigerator overnight, you assume it's reached the same temperature as the refrigerator and other items.

### Application of zeroth law of thermodynamics

The thermometer is where the Zeroth Law of Thermodynamics is used the most frequently. We can observe the Zeroth law in action by using a straightforward thermometer with mercury in a tube. As the temperature rises, the mercury expands because the area of the tube doesn't change. As a result of this development, the height has increased. The variation in the mercury label's height is now indicative of temperature differences, which facilitates measurement.