Thermodynamics

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Thermodynamics->> ZEROTH LAW

The vocabulary of empirical sciences is often borrowed from daily language. Thus, although the term temperature appeals to common sense, its meaning suffers from the imprecision of nonmathematical language. A precise, though empirical, definition of temperature is provided by the so-called zeroth law of thermodynamics as explained below.

When two systems are in equilibrium, they share a certain property. This property can be measured and a definite numerical value ascribed to it. A consequence of this fact is the zeroth law of thermodynamics, which states that when each of two systems is in equilibrium with a third, the first two systems must be in equilibrium with each other. This shared property of equilibrium is the temperature.

If any such system is placed in contact with an infinite environment that exists at some certain temperature, the system will eventually come into equilibrium with the environment—that is, reach the same temperature. (The so-called infinite environment is a mathematical abstraction called a thermal reservoir; in reality the environment need only be large relative to the system being studied.)

Temperatures are measured with devices called thermometers (see Thermometer). A thermometer contains a substance with conveniently identifiable and reproducible states, such as the normal boiling and freezing points of pure water. If a graduated scale is marked between two such states, the temperature of any system can be determined by having that system brought into thermal contact with the thermometer, provided that the system is large relative to the thermometer.

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