Transformation of energy

Kelvin. Scale and temperature unit of the International System

Kelvin. Scale and temperature unit of the International System

Kelvin is the SI unit of thermodynamic temperature. It is one of the seven basic temperature units. Its symbol in the international system of units is K.

This base unit was created on the basis of the degree Celsius (formerly degrees Celsius). 

This is the proper temperature unit to use in derived units, such as W / (m · K) or to have a prefix, such as milli in mK.

This unit is named after the British physicist, mathematician, and engineer William Thomson Kelvin. Later he was appointed Lord Kelvin.

The Boltzmann constant is a physical constant which both indicates the relationship between the temperature and energy. The value is:

k = 1,38064852 (± 0.00000079) 10^−23 J/K

Definition of Kelvin

The thermodynamic temperature kelvin Kelvin (old degree Kelvin) is defined based on 2 factors,

Zero corresponds to absolute zero, which is the lowest temperature that can be achieved. At 0K there is no movement at the molecular level. 

A K is exactly 1 / 273.16 of the thermodynamic temperature of the triple point of water. A difference of one kelvin is equivalent to that of one degree Celsius.

The triple point temperature of water is the temperature at which water can be in a solid, liquid, and gaseous state at the same time.

The normal boiling point of water ins 372.97 K (211.9 °F) at a pressure of 1 atm.

Kelvin scale

The Kelvin thermodynamic temperature scale is an absolute scale to measure temperature with zero at absolute zero.

This scale is a thermodynamic (absolute) temperature scale where absolute zero is zero (0 K). The theoretical absence of energy.

Being an absolute scale, it has no degrees. This is a difference from the Celsius and Fahrenheit scales that do have degrees Celsius and degrees Fahrenheit, respectively.


Absolute scale

Its measurement system refers to absolute zero. Absolute zero corresponds to the lowest temperature that, in theory, can be obtained in any macroscopic system. For this reason, it is an absolute scale.

For the same reason, the degree symbol (°) is not placed before the K symbol.

Sometimes it can be seen written as a degree, or with the symbol ° K for basically two reasons:

  1. By analogy with degree Celsius

  2. Why in the old days it was called a Kelvin degree.

Practical uses of kelvin

The Kelvin degree is used in many aspects of physics. Below we present two applications in which this unit is of special importance.

  • Color temperature

  • Noise measurement

Color temperature

Kelvin is often used in measuring the color scale temperature of light sources.

Color temperature is based on the principle that a blackbody radiator emits light. The color of this light depends on the temperature of the radiator.

Black bodies with temperatures below about 4000K appear reddish, while those above 7500K appear bluish.

Kelvin as a measure of noise

In electronics, the kelvin is used as an indicator of how noisy a circuit is relative to the final noise floor, i.e. the noise temperature.

The so-called Johnson-Nyquist noise of discrete resistors and capacitors can be used to determine the noise temperature of a circuit using the Friis formulas for noise.


Kelvin is the unit of the international system of measurements for expressing temperature.

The Kelvin temperature scale is an absolute scale. It has a zero point at the absolute zero point. The value of this scale is directly proportional to the expected value of the energy of the thermal movement of a substance.

The degree range is the same as that of the Celsius scale, that is, when temperature differences are expressed 1 K = 1 ° C.

Publication Date: August 28, 2017
Last Revision: June 6, 2020