Zeroth law of thermodynamics with examples and Applications

Thermodynamics refers to two words thermo + dynamics, means heat + motion. We can also say it is a mixture of heat, work and internal energy. Thermodynamics is quite confusing topics. There are some differences between chemistry’s thermodynamics and physic’s thermodynamics. But this is not our topics.

There are total four laws of thermodynamics. The fourth one is Zeroth law of thermodynamics. But this is not easy to keep name of zeroth law. It is fourth law of thermodynamics but know as zeroth law. After the discovery of 1st, 2nd and third laws, Zeroth was discovered. But our point is not that to know the discovery of zeroth law. We come here to know the meaning of zeroth law of thermodynamics. But before we discuss the 0th law of thermodynamics, we have to know some common definition used to define zeroth law of thermodynamics.

 There are four fundamental laws of thermodynamics. Each law has its own identity with quit similar applications. 1st law express energy conservation, 2nd law express heat and zeroth law express heat, work and internal energy.

Thermal equilibrium means bodies with same criteria or two bodies are said to be in thermal equilibrium if their temperature is same. I know it is quite confusing even now. Let’s understand it with examples.

Note that we use two conditions for better understanding of the thermal equilibrium. 

Case 1:- 

Let two thermodynamics bodies A and B connected with an insulating wall as shown in figure. Suppose the temperature, pressure, and volume of body A is T1, P1, and V1 and the temperature, pressure and volume of body B is T2, P2 and V2. As we know there is an insulating wall between body A and body B. So there is no possibility of flow of heat from body A and body B. There is also no chance in volume and pressure with time passes.

Hence, we can say that this thermodynamic system has no flow of heat. Therefore it is not in thermal equilibrium.

Let two thermodynamics bodies A and B. Now it is connected with a conducting wall as shown in fig. Suppose as first case the temperature, pressure, and volume of these two bodies are T1, P1, V1 and T2, P2, V2 respectively. Now, just because of conducting wall between body A and body B, there is possibility of transfer of heat and volume. So after time passes there will be a time when heat transfer stops. And at that time the the temperature of the body A and body B will be same equal to T. That means T1 will be equal to T2.

hence we can say that this thermodynamics system is in thermal equilibrium because of heat transfer from body A to body B.

Therefore we can define thermal equilibrium as when two bodies with different temperature comes in contact and there is conducting wall present then the heat transfer from hotter body to colder body. So that the thermodynamics system becomes thermal equilibrium.

After discussing about thermal equilibrium. We have an idea of thermodynamics system. So we can now define zeroth law of thermodynamics.

Zeroth law of thermodynamics deals with the temperature. That means zeroth law of thermodynamics represent the flow of heat from one thermal equilibrium body to another thermal equilibrium body. Don’t be confused let’s see another definition.

In other words if two thermodynamics system which is in thermal equilibrium with each other, connected with unknown third thermodynamics system then all the three thermodynamics systems will be in thermal equilibrium. This happens as follows:

If three body A, B and C are connected with a heat conducting material and two of them are in thermal equilibrium or we can say all of them have different temperature then after time passes all the bodies A, B and C will have the same temperature. This happens because of heat transfer. so that we can say all the three bodies attend thermal equilibrium state. Hence the system will be equilibrium with constant heat.

Note that zeroth law of thermodynamics was formulated by Ralph H. Fowler. he named the zeroth law with the replacement of name fourth law.

Now, we try to understand 0th law of thermodynamics with the help of some examples.

Suppose there is a three bodies A, B and C. It is in thermal equilibrium. Now let’s suppose the body A have temperature T1, body B have T2 and body C have T3. As we know for heat transfer there must be the difference in temperature. So for the work of 0th law, we also take temperature difference. Therefore, T1>T2> T3.

Hence, the temperature T1 is greater than temperature T2. And T2 is greater than T3. So that the temperature flow from T1 to T2 and then T3.

After time passes we noticed that all the bodies A, B and C will stop transferring of heat and there is no difference in temperature. So the system is in thermal equilibrium.

These are the application of zeroth law of thermodynamics.