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Thermodynamics
Introduction
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Concept Version 9
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A Review of the Zeroth Law

Zeroth law justifies the use of thermodynamic temperature, defined as the shared temperature of three designated systems at equilibrium.

Learning Objective

  • Discuss how the Zeroth Law of Thermodynamics justifies the use of thermodynamic temperature


Key Points

    • The zeroth law of thermodynamics states that when systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system, C, then A is also in thermal equilibrium with C.
    • Two systems are in thermal equilibrium if they could transfer heat between each other, but don't.
    • If enough time is allowed for heat transfer to occur between a thermometer and a system, the temperature the thermometer registers does represent the system with which it achieves thermal equilibrium.

Terms

  • thermal equilibrium

    Two systems are in thermal equilibrium if they could transfer heat between each other, but don't.

  • thermodynamic temperature

    Temperature defined in terms of the laws of thermodynamics rather than the properties of a real material: expressed in kelvins.


Full Text

The Zeroth Law of Thermodynamics states: If two systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system, C, then A is also in thermal equilibrium with C.

This law was postulated in the 1930s, after the first and second laws of thermodynamics had been developed and named. It is called the "zeroth" law because it comes logically before the first and second laws (discussed in Atoms on the 1st and 2nd laws).

Thermodynamics and PV Diagrams

A brief introduction to the zeroth and 1st laws of thermodynamics as well as PV diagrams for students.

Two systems are in thermal equilibrium if they could transfer heat between each other, but don't. Indeed, experiments have shown that if two systems, A and B, are in thermal equilibrium with each other, and B is in thermal equilibrium with a third system C, then A is also in thermal equilibrium with C. This conclusion may seem obvious, because all three have the same temperature, but zeroth law is basic to thermodynamics. Zeroth law justifies the use of thermodynamic temperature : the common "label" that the three systems in the definition above share is defined as the temperature of the systems.

Thermometer

A thermometer calibrated in degrees Celsius

Temperature

Thermometers actually take their own temperature, not the temperature of the object they are measuring. This raises the question of how we can be certain that a thermometer measures the temperature of the object with which it is in contact. The answer lies in the fact that any two systems placed in thermal contact (meaning heat transfer can occur between them) will reach the same temperature. That is, heat will flow from the hotter object to the cooler one until they reach exactly the same temperature. The objects are then in thermal equilibrium, and no further changes will occur. The systems interact and change because their temperatures differ, and the changes stop once their temperatures are the same. Thus, if enough time is allowed for this transfer of heat to run its course, the temperature a thermometer registers does represent the system with which it achieves thermal equilibrium.

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