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Hot and cold are measured by a numerical scale called temperature. Temperature scales are how we communicate about the weather, measure safety and comfort, and explain the physical world. Using baselines chosen by scientists to create relative measurements, temperature scales measure the intensity of heat, or the amount of thermal energy contained in a material or substance (such as air, a pot of water, or the surface of the sun). There are three commonly used measurement systems: Fahrenheit, Celsius, and Kelvin.
What is temperature?
Temperature is an energy that is measured with an instrument called a thermometer, which comes from the Greek words thermos (hot) and metron (measure), according to the online etymological dictionary. Another definition of temperature is that it is a measure of the average kinetic energy (the energy of a moving mass) of the molecules of a substance, according to Georgia State University.
The ancient Greek physician Hippocrates apparently taught that the human hand could be used to judge the presence of a fever in a person as early as 400 BC, according to a 2019 report published in the journal Nature Public Health Emergency Collection. However, precise instruments for measuring human body temperature were not developed until the 16th and 17th centuries.
Kelvin: an absolute scale for scientists
In 1848, the British mathematician and scientist William Thomson (also known as Lord Kelvin) proposed an absolute temperature scale, independent of the properties of a substance such as ice or the human body. He suggested that the range of possible temperatures in the universe far exceeded those proposed by Celsius and Fahrenheit. The concept of absolute minimum temperature wasn’t new, according to NIST, but Kelvin gave you an exact number: 0 kelvins equals -273.15 C.
Thermodynamic temperature is absolute, not relative to fixed points. This describes the amount of kinetic energy contained in the particles that make up a blob of matter, wiggling and fluttering at submicroscopic levels, Thomson explained. As the temperature drops, the particles slow down until at some point all movement ceases. It is about absolute zero, which is the reference of the Kelvin scale.
absolute zero
Absolute zero occurs at -273.15 C or -459.67 F. Until recently, scientists thought that humans could not recreate that temperature since, to get that cold, energy would have to be added to the system to cool it, which would mean that the system would be hotter than absolute zero. But in 2013, German physicists managed to bring the particles to paradoxical temperatures below absolute zero.
In Kelvin’s mind, absolute zero was the starting point for a temperature scale, but for convenience he used the markers and intervals of the widely known Celsius scale as the basis for his own. Thus, on the Kelvin scale, water freezes at 273.15 K (0°C) and boils at 373.15 K, that is, at 100°C. A kelvin is designated as a unit, rather than a degree, and equals a single degree on the Celsius scale. The Kelvin scale is mainly used by scientists.
In 2018, the kelvin was redefined to make it more precise, according to an article in Metrologia magazine, and its definition is now linked to the Boltzmann constant. This constant relates temperature to the kinetic energy of matter. The new definition, according to the General Conference on Weights and Measures, is as follows: “The kelvin, symbol K, is the SI unit of thermodynamic temperature; its magnitude is fixed by setting the numerical value of Boltzmann’s constant to exactly 1.380649 × 10 -23 J K-1 [joules per kelvin]’.
Sources
- Britannica Encyclopedia. (nd). Boltzmann constant . Physics.
- Metrology. (2019). The revision of the SI —the result of three decades of progress in metrology.
- International Bureau of Weights and Measures. (2006). The International System of Units (SI) Booklet. International Committee of Weights and Measures.
- YES Brochure. (2019). Mise en pratique for the definition of the kelvin in the SI. Consultative Committee for Thermometry
