Zeroth law of thermodynamics
Zeroth law of thermodynamics was established by Ralph H. Fowler in 1930.
This law was formulated long after the first, second, and third laws of thermodynamics had been widely recognized. But since this law was considered to be of primary importance. So it was named as zeroth law of thermodynamics.
This law states as follows:
“If a body A is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in thermal equilibrium with each other“.
Alternatively, when two objects or bodies are separately in thermodynamic equilibrium with a third object, they are in equilibrium with each other.
Examples of zeroth law of thermodynamics
1. Thermostate in our room
If the temperature of room and thermostate is same (say 25°C) then according to zeroth law of thermodynamics, the temperature of all the things in room (e.g. bed, sofa, table etc.) will be same i.e., 25°C
2. If we place thermometer in two cups (say A and B) of boiling water. Let’s suppose the thermometer reads 100°C in both cups. So we can say that, thermometer is in equilibrium with Cup A and also with cup B.
Then according to zeroth law of thermodynamics, cup A and cup B are in thermal equilibrium with each other.
3. Brewing tea
The temperature of a system consists of cup, water and tea leaves becomes constant. Finally they reach thermal equilibrium.
4. If we have two glasses of water. One of them containing hot water and other containing cold water. Few hours later, they will attain thermal equilibrium with the temperature of the room.
It is important to understand thermal equilibrium to understand the zeroth law of thermodynamics. Let us have a look at the topic “thermal equilibrium”.
Thermal equilibrium ?
A system is said to be in thermal equilibrium if there is no flow of heat from one portion of the system to another.
This is possible if the temperature remains the same throughout in all parts of the system.
We can attain thermal equilibrium in following two ways :
1. Consider two systems A and B separated by an insulated wall as shown in the figure.
(Let the systems A and B are also insulated from the rest of surrounding by similar adiabatic / insulated walls.)
Due to this insulated wall, there is no heat transfer between system A and B.
i.e., system A and B are in thermal equilibrium.
In this case, we can define thermal equilibrium as :
“A thermodynamic system is in thermal equilibrium when its parameters like V, T, P etc. do not change with the passage of time.”
2. Consider same systems A and B separated by a conducting wall as shown in the figure.
Let T1 > T2 (initially)
Due to this conducting wall, there will be transfer of heat till both systems attain equal temperature (T1 = T2) . And finally, the system A and B attain thermal equilibrium.
In this case, we can define thermal equilibrium as :
“Two thermodynamics systems are said to be in thermal equilibrium when both the systems have same temperature.”
Other Important Points :-
1. zeroth law of thermodynamics is an important observation which allows us to create thermometers. If we bring the metric thermometer in contact with any other system (our tongue) then we can determine then we can determine the temperature of that system by noting the change in the thermal property i.e., by noting the length of the mercury column.
2. Diathermic Walls and Adiabatic Walls
Diathetmic walls – The wall that allows only the flow of heat energy not the matter is called as diathermic wall. By using diathermic wall, two systems attain their thermal equilibrium by transfer of heat energy.
Adiabatic Wall – The wall that does not allows the flow of heat energy and matter between the system and surrounding is called as adiabatic wall. By use of adiabatic wall, themal equilibrium between the two systems is maintained without interfering with each other.
Also Read –Â Other topics of thermodynamics
