The Zeroth Law of Thermodynamics is about the equilibrium.
The zeroth law of thermodynamics states that if two bodies are each in thermal equilibrium with some third body, then they are also in equilibrium with each other.
Thermal equilibrium means that when two bodies are brought into contact with each other and separated by a barrier that is permeable to heat, there will be no transfer of heat from one to the other.
The Zeroth Law of Thermodynamics states that systems in thermal equilibrium are at the same temperature.
so if A ↔ B and B ↔ C, then A ↔ C.
Following this the systems are in thermal equilibrium if they do not transfer heat, even though they are in a position to do so, based on other factors.
- food that’s been in the refrigerator overnight is in thermal equilibrium with the air in the refrigerator: heat no longer flows from one source (the food) to the other source (the air) or back.
- a person with fever, that measures his temperature of the body by thermometer. Thermometer is kept in contact with person, so heat from person’s body transfer into the mercury in the thermometer and the volume of mercury increases. When the temperature of the body and that of the mercury in thermometer becomes equal, the increase in volume of mercury stops and scale on thermometer is measure which shows the value to fever on person’s body.
What the zeroth law of thermodynamics means is that temperature is something worth measuring, because it indicates whether heat will move between objects. This will be true regardless of how the objects interact. Even if two objects don’t touch, heat may still flow between them, such as by radiation (as from a heat lamp). However, according to the zeroth law of thermodynamics, if the systems are in thermal equilibrium, no heat flow will take place.
If there are three systems A, B, C, and if A and C are in thermal equilibrium, and A and B are in thermal equilibrium, then B and C are in thermal equilibrium (according to the zeroth law).
Two systems are in thermal equilibrium if they could transfer heat between each other, but do not.
The Zeroth Law of Thermodynamics implies that temperature is a quantity worth measuring.
Some examples of equilibrium:
- Mechanical equilibrium – mechanical systems have reached equilibrium when there is no movement between the systems
- Thermal equilibrium – thermodynamic systems have reached equilibrium when there is no heat transfer between the systems. This state of equilibrium requires the systems to have the same temperature.
- Electrostatic equilibrium – electrostatic systems have reached equilibrium when there is no net charge flow between the systems
- Phase equilibrium – material substance systems have reached equilibrium when there is no net phase transformation, like melting, between the systems.
- Chemical Equilibrium – Chemical systems have reached equilibrium when there is no net chemical reaction between the system