Kelvin

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Thermometer with Celsius and kelvin degrees

The kelvin (symbol: K), formerly called the degree Kelvin, is the unit of temperature on the scale created in 1848 by William Thomson, 1st Baron de Kelvin, based on degrees Celsius, setting the zero point at absolute zero (−273.15 °C) and retaining the same dimension. At the age of 24, Kelvin introduced the thermodynamic temperature scale, whose unit was named in his honor in 1968.

It is one of the base units of the International System of Units. Since the 26th General Conference on Weights and Measures, held in November 2018, it has been defined using the Boltzmann constant, which has been given an exact numerical value, without uncertainty.

Defined by setting the numerical value of the Boltzmann constant, k, to 1.380 649 × 10^-23, when expressed in the unit J K^-1, equal to kg m2 s^-2 K^-1, where the kilogram, meter, and second are defined in terms of h, c, and Δν_Cs.

From the exact relationship k = 1,380 649 × 10^-23 kg m2 s ^-2 K^-1 obtains the expression for the kelvin as a function of the constants k, h and Δν_Cs:

${displaystyle 1K={frac {1,380649times 10^{-23}}{(6,62607015times 10^{-34})(9192631770)}}{frac {h Delta nu _{Cs}}{k}}approx 2,2666653{frac {h Delta nu _{Cs}}{k}}}$

The effect of this definition is that the kelvin is equal to the thermodynamic temperature change that gives rise to a thermal energy change kT of 1.380 649 × 10^-23 J.

Previously, the kelvin was defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It is represented by the letter K, never "°K", since currently its name is not "degrees". kelvin", but simply "kelvin".

Since the increase in one degree Celsius coincides with that of one kelvin, its importance lies in the 0 of the scale: the temperature of 0 K is called « absolute zero”, which corresponds to the point at which the molecules and atoms of a system have the minimum possible thermal energy. No macroscopic system can have a lower temperature. The temperature measured in kelvin is called "absolute temperature" and is the temperature scale used in science, especially in physics or chemistry work.

Kelvins are also used as a reference for color temperature in photography, video and film lighting. When a blackbody is heated, it emits light of a different color depending on its temperature. In this way, each color can be associated with the temperature at which a blackbody should be to emit that color. It is necessary to emphasize that the color temperature associated with a body is not related to its real temperature. For example, 1600K is the color temperature for sunrise or sunset. The color temperature of an ordinary tungsten filament lamp is 2800 K. The light temperature used in photography and graphic arts is 5500 K (to be considered "daylight", which does not prevent other parts of the image from being used). scale to refer to tungsten light or some led lamps) and that of the sun at noon with a clear sky is 5200 K. Light on cloudy days is bluer and is 6000 K or higher, even as low as 11,000 K.

Practical uses

Color temperature

Kelvin as a unit of noise temperature

Main article: Noise factor

For electronics, the kelvin is used as an indicator of how noisy a circuit is relative to a final noise floor, that is, the noise temperature. The so-called Johnson-Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from Boltzmann's constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise.

Multiples of the SI

The following is a table of the multiples and submultiples of the International System of Units.

International kelvin (K) Multiples
Value	Symbol	Name	Value	Symbol	Name
Submultiplos			Multiple
10⁻¹ K	dK	decikelvin	10¹ K	daK	decakelvin
10⁻² K	cK	centikelvin	10² K	hK	hectokelvin
10⁻³ K	mK	milikelvin	10³ K	kK	kilokelvin
10⁻⁶ K	μK	microkelvin	10⁶ K	MK	megakelvin
10⁻⁹ K	nK	nanokelvin	10⁹ K	GK	gigakelvin
10⁻¹² K	pK	picokelvin	10¹² K	TK	terakelvin
10⁻¹⁵ K	fK	femtokelvin	10¹⁵ K	PK	petakelvin
10⁻¹⁸ K	aK	Attokelvin	10¹⁸ K	EK	exakelvin
10⁻²¹ K	zK	zeptokelvin	10²¹ K	ZK	zettakelvin
10⁻²⁴ K	yK	Yoctokelvin	10²⁴ K	YK	Yottakelvin
10⁻²⁷ K	rK	rontokelvin	10²⁷ K	RK	ronnakelvin
10⁻³⁰ K	qK	Quectokelvin	10³⁰ K	QK	Quettakelvin
Common units are in bold.

This unity of the International System is thus named in honor of Lord Kelvin. In the units of the SI whose name comes from a person's own name, the first letter of the symbol is written with capital (K), while his name always starts with a tiny letter (kelvin), except in case you start a sentence or a title.
Based on The International System of UnitsSection 5.2.

Temperature and energy

Statistical physics says that in a thermodynamic system the energy contained by the particles is proportional to the absolute temperature, being the constant of proportionality the Boltzmann constant. That is why it is possible to determine the temperature of some particles with a certain energy, or to calculate the energy of some particles at a certain temperature. This is done from the so-called equipartition principle or theorem. The equipartition principle states that the energy of a thermodynamic system is

${displaystyle E_{c}={frac {n}{2}}k_{B}T}$

where:

${displaystyle k_{B},}$ is the Boltzmann constant
$T,$ is the temperature expressed in kelvin
$n,$ is the number of degrees of freedom of the system (for example, in monoatomic systems where the only possibility of movement is the translation of some particles compared to others in the three possible directions of space, n equals 3).

Unicode character

The symbol is encoded in Unicode at code point U+212A K kelvin sign . However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004B K Latin capital letter k instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126 Ω ohm sign, U+212A K kelvin sign, and U+212B Å angstrom sign. In all three cases, the normal font should be used."

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