### Zeroth Law of Thermodynamics

• State the zeroth law of thermodynamics.

It states that if two bodies are in thermal equilibrium with a third body, then they are in thermal equilibrium with each other. Recognition of this law was done after the formulation of first and second law of thermodynamics, so it was termed zeroth law of thermodynamics.
Let us consider the temperature equality concept to three systems say A, B and C. The system A consists of mass of gas enclosed in a vessel fitted with a thermometer and the system B is a cold iron body. When A and B are brought in contact, after some time they attain a common temperature and are then said to exist in thermal equilibrium.
Now the system is brought into contact with a third system C, again A and C attain thermal equilibrium, then system B and C will show no further change in properties when brought into contact. That is system A is in thermal equilibrium with system B and also separately with system C. Then B and C will be in thermal equilibrium with each other. This law provides the basis for temperature measurement.

### Solved Example:

#### 71-2-01

When two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. This statement is called:

### Solved Example:

#### 71-2-02

Zeroth law of thermodynamics: