Zeroth Law of Thermodynamics

Definition

The Zeroth Law of Thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This fundamental law provides the physical basis for the concept of temperature and the construction of thermometers.

Main Content

1. Thermal Equilibrium

• Two systems are said to be in thermal equilibrium if they are in contact through a diathermal wall (a wall that allows heat transfer) and there is no net flow of heat between them.
• When two bodies are in thermal equilibrium, they possess the same temperature.

2. The Concept of Transitivity

• The law functions on the principle of transitivity: if A = C and B = C, then A = B.
• In thermodynamic terms, if system A is at the same temperature as system C, and system B is also at the same temperature as system C, then A and B must have the same temperature.

3. Basis for Temperature Measurement

• Because of this law, we can use a "third system" (a thermometer) to compare the temperatures of two other systems without bringing them into direct contact.
• It validates the use of a standard scale to measure the thermal state of matter.

      [System A]
          |
    (Thermal Contact)
          |
      [System C] <---- (The Thermometer)
          |
    (Thermal Contact)
          |
      [System B]

If A is in equilibrium with C, and B is in equilibrium with C, 
then A and B are in equilibrium with each other.

Working / Process

1. Establishing Reference

• A standard thermometer (the third system) is chosen as a reference.
• The thermometer contains a substance whose physical properties (like volume or pressure) change predictably with temperature.

2. Interaction with System A

• The thermometer is brought into contact with System A. Heat flows until the reading on the thermometer stabilizes.
• This establishes the thermal state (temperature) of System A as equal to the thermometer.

3. Interaction with System B

• The thermometer is then brought into contact with System B.
• If the thermometer shows the same reading as it did for System A, we conclude that System A and System B are in thermal equilibrium.

Advantages / Applications

• It allows for the calibration of temperature measuring instruments like mercury thermometers, thermocouples, and resistance thermometers.
• It enables scientists to compare the temperatures of two objects that are far apart or chemically reactive, preventing the need for direct contact.
• It provides a logical foundation for the first and second laws of thermodynamics, as temperature must be well-defined before energy and entropy can be calculated.

Summary

• The Zeroth Law establishes that thermal equilibrium is a transitive property between systems.
• It serves as the scientific justification for the existence of temperature as a measurable physical property.
• It enables the practical use of thermometers to verify if two objects share the same thermal energy level.
• Important terms to remember: Thermal Equilibrium, Diathermal Wall, Transitivity, and Temperature.