Temperature, thermometer and temperature scales

Temperature is a scalar quantity that measures the heat content (cold or warm) of a thermodynamic system (an object, a substance, etc…). Although both concepts are related, temperature is not the same as heat. While temperature is a quantity that describes a thermodynamic system, heat is a form of energy transfer.

Without going into details, temperature is associated with the kinetic energy of molecules at a microscopic level. The microscopic structure of matter was not understood when classical thermodynamic was developed, so we can use this quantity without having to consider the atoms and molecules that make up a thermodynamic system.

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Temperature is measured with a device called thermometer. A thermometer has two elements: a temperature sensor and a scale. The temperature sensor generally measures the variation of a thermometric property of some substance (a physical property that varies continuously with temperature, such as density, electric resistance, etc). The scale is used to convert the variation into a numerical value.

The Zeroth law of thermodynamics makes possible to define temperature:

If two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other.

A scale must have fixed points easily reproducible. The fixed points are arbitrary but they should always be at the same temperature. This ensures that all thermometers are calibrated in the same way.

The most widely used temperature scale worldwide is the Celsius scale. Historically this scale had two fixed points, which are written as 00C and 1000C, divided into one hundred degrees. Formerly they were respectively the fusion point and boiling point of water at a pressure of one atmosphere. Since May 20, 2019, the Celsius scale is defined in terms of the Boltzmann constant.

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The International System of Units (SI) of temperature is kelvin (K). The Kelvin scale is also know as the absolute temperature scale, because its null point is the absolute zero, when all molecular motion ceases. It is therefore the lowest theoretical temperature and no temperature can be lower than 0 K. The relation between the absolute temperature and the Celsius scale is:

For practical purposes the 0,15 are usually omitted and the relation is therefore:

As you can see, the degree symbol is not used when the temperature is expressed in kelvin. The unit symbol (K) is written in uppercase because it is based on a person’s name.

You can obtain more information about the kelvin scale in the International Bureau of Weights and Measures (BIPM) website.

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