The concept of temperature is so familiar in everyday life that its scientific foundation is often taken for granted. In thermodynamics, however, temperature must be defined in a precise and logical manner before laws involving energy and work can be formulated. The Zeroth Law of Thermodynamics provides this foundation and establishes temperature as a fundamental thermodynamic property.
Although the Zeroth Law was formulated after the First and Second Laws of Thermodynamics, it is placed before them because of its logical priority. Without the Zeroth Law, the measurement and comparison of temperature would have no theoretical basis, and the entire structure of thermodynamics would lack coherence.
Thermal Equilibrium
When two bodies at different temperatures are brought into contact, heat flows from the body at higher temperature to the body at lower temperature. This flow of heat continues until a stage is reached where no further heat transfer occurs. At this stage, the two bodies are said to be in thermal equilibrium.
Thermal equilibrium depends only on temperature and not on the nature, size, or shape of the bodies involved. Two systems in thermal equilibrium have the same temperature even though other physical properties such as mass, volume, or composition may be different.
Statement of the Zeroth Law of Thermodynamics
If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.
This statement expresses the transitive nature of thermal equilibrium. It allows temperature to be treated as a property that can be compared between systems without requiring direct contact between every pair of systems.
Explanation of the Zeroth Law
Consider three systems A, B, and C. If system A is in thermal equilibrium with system C and system B is also in thermal equilibrium with system C, then experimental observations show that systems A and B will be in thermal equilibrium with each other when brought into contact. This implies that systems A and B have the same temperature.
The Zeroth Law therefore permits the comparison of temperatures indirectly and forms the conceptual basis for all thermometric measurements.
Definition of Temperature
Temperature may be defined as the thermodynamic property that determines the direction of heat flow between two systems placed in thermal contact. Heat flows spontaneously from a system at higher temperature to a system at lower temperature, and no heat flows when both systems are at the same temperature.
This definition is objective and independent of human sensation. Temperature is thus established as a measurable and universal property based on thermal equilibrium.
Thermometric Property
To measure temperature, a physical property is required that changes uniformly with temperature and can be measured accurately. Such a property is known as a thermometric property. Examples include the volume of a liquid, the pressure of a gas at constant volume, and the electrical resistance of a conductor.
When a thermometer is placed in contact with a system, it attains thermal equilibrium with the system. The value of the thermometric property at equilibrium is then used to assign a temperature to the system, a procedure justified entirely by the Zeroth Law of Thermodynamics.
Importance of the Zeroth Law
The Zeroth Law provides the theoretical foundation for temperature measurement and the construction of temperature scales. It allows different thermometers based on different physical principles to be calibrated consistently and ensures that temperature has the same meaning for all systems.
Although the Zeroth Law does not involve energy equations or mathematical expressions, it is indispensable. It prepares the ground for the First Law of Thermodynamics, which deals with energy conservation, and the Second Law, which introduces the concept of direction and spontaneity of processes.