The Zeroth Law of Thermodynamics states that if two systems are each in thermal equilibrium with a third
system, then they are in thermal equilibrium with each other.
This law essentially defines the concept of temperature. If body A is in thermal
equilibrium with body C, and body B is also in thermal equilibrium with body C, then body A and body B are
in thermal equilibrium with each other.
This law is called the "Zeroth" law because it was formulated after the First and Second Laws of
Thermodynamics, but it logically precedes them in establishing the foundation of temperature comparison.
Problem:
Object A is in thermal equilibrium with object C. Object B is also in thermal equilibrium with object C.
What can you say about objects A and B?
Solution:
According to the Zeroth Law of Thermodynamics, since A is in thermal equilibrium with C and B is also in
thermal equilibrium with C, then A and B must also be in thermal equilibrium with each other.
This means that:
- There is no net heat transfer between A and B.
- Objects A and B are at the same temperature.
This is the foundation of using thermometers: if a thermometer (object C) is in equilibrium with a system
(object A or B), then the temperature can be accurately measured.
Problem:
A thermometer is placed in a cup of water until it reaches a constant reading. The thermometer is then
placed in an oil bath, and the reading remains unchanged. What can be concluded?
Solution:
Since the thermometer showed the same temperature in both the water and the oil bath, this means that
both the water and the oil are in thermal equilibrium with the thermometer.
By the Zeroth Law, this implies:
- Water and oil are in thermal equilibrium with each other.
- Therefore, the temperature of the water equals the temperature of the oil.
This illustrates how thermometers work in practice by using the Zeroth Law to determine if multiple systems
share the same temperature.