Kelvin

The kelvin (symbol K) is the unit of thermodynamic temperature in the International System of Units (SI). It is one of the seven base units of that system. The unit is named after the British physicist William Thomson, known as Lord Kelvin.

The kelvin is an absolute scale, meaning its zero point is at absolute zero, the lowest possible temperature. For this reason, kelvin measurements can never be negative. When written, only the symbol K is used without a degree sign; the notation °K is an outdated form no longer used.

Relationship to Celsius

The size of one kelvin and one Celsius degree are equal: a difference of one K is the same as a difference of one °C. The only difference is where the zero point is set: absolute zero is 0 K, which corresponds to approximately −273.15 °C. Therefore, a temperature given in Celsius can be converted to kelvin by adding 273.15.

Definition

For a long time, the kelvin was defined by reference to the triple point of water, set at 273.16 K. In 2019, a revision of the SI units shifted the definition to a physical constant: the kelvin is now defined by assigning a fixed value to the Boltzmann constant. The purpose of this change was to base the definition on a universal constant rather than on a particular substance like water.

Use

The kelvin is used in physics and chemistry research, particularly in gas calculations, thermodynamics, and astrophysics, where stellar temperatures are expressed in kelvins. It is also used to describe 'colour temperature' in lighting and cameras: a light source at 2700 K produces a warm, yellowish light, while one near 6000 K approximates daylight.