Kelvin

Kelvin
Unit system	SI base unit
Unit of	Temperature
Symbol	K
Named after	William Thomson, 1st Baron Kelvin

The Kelvin scale (symbol: K) is the SI unit of temperature. It is named in honour of the physicist William Thomson, the first Lord Kelvin (1824–1907). 0 degree Celsius = 273 kelvin

Definition

Lord Kelvin

The Kelvin scale is defined by a specific relationship between the pressure of a gas and the temperature. This says that "the pressure of the gas is directly proportional to the temperature in Kelvin". This means that Kelvin is an absolute temperature scale, and scientists use this scale more than any other.

The kelvin is a base SI unit of measurement, since 2018 defined by setting the fixed numerical value of the Boltzmann constant k to 1.380649×10⁻²³ J⋅K⁻¹.

The temperature of the triple point of water is a hundredth of a degree Celsius above the freezing point, or 0.01 °C. The coldest possible temperature is called absolute zero and is equal to -273.15 degrees Celsius, or zero kelvin (0 K). By writing temperatures in kelvins one does not need to use negative numbers.

The absolute temperature scale was designed so that a change in temperature of 1 kelvin is equal to a change of 1 degree Celsius. This means that it is easy to convert a temperature from degrees Celsius to kelvin.

• To change a temperature in degrees Celsius into Kelvins you must add 273.15 units. For example, 0 degrees Celsius (0 °C), which is the temperature at which water freezes, is 273.15 kelvins (273.15 K).
• To change a temperature in Kelvins into degrees Celsius you must subtract 273.15 units. For example, 310 kelvins is 36.85 degrees Celsius, which is roughly the normal temperature of a human body.

It is important to notice that the name of this unit is simply kelvin (with a lowercase initial), not "degree Kelvin". In English, it undergoes normal plural inflection as kelvins. For example, the boiling point of liquid nitrogen is 77 kelvins.

In everyday use, the kelvin is most commonly used to measure very low or very high temperatures, such as the temperature of liquid nitrogen or the temperature of a light bulb filament.

The rankine (or degree Rankine) is a related absolute temperature scale, but based on the degree Fahrenheit rather than the degree Celsius.

Related pages

• Temperature
• Celsius