The kelvin (K) is the unit of temperature in the International System of Units (SI) and one of its seven fundamental units.
It is widely used in science and technology due to its absolute nature and consistency with thermodynamic principles.
Origin and definition
The name of this unit honors the British physicist and mathematician William Thomson Kelvin, known as Lord Kelvin, who made important contributions to thermodynamics and proposed an absolute temperature scale.
The kelvin is defined from two fundamental points:
- Absolute zero (0 K): This corresponds to the lowest possible temperature, where all thermal motion at the molecular level ceases. This state represents the theoretical absence of thermal energy.
- Triple point of water: One kelvin is exactly 1/273.16 of the thermodynamic temperature of the triple point of water. This is the state in which water can simultaneously coexist as a solid, liquid and gas in equilibrium.
The equivalence between the Kelvin and Celsius scales is direct, since a difference of 1 K is identical to a difference of 1 °C. However, their reference values are different, since 0 °C is equivalent to 273.15 K.
Features of the Kelvin scale
The Kelvin scale is an absolute scale , meaning it starts at absolute zero, with no negative values. Unlike the Celsius or Fahrenheit scales , the Kelvin does not use the degree symbol (°). This is because it does not measure in relative "degrees", but in absolute terms of heat energy.
For example:
- 0 K (absolute zero) represents the theoretical absence of thermal motion.
- 273.15 K is the freezing point of water.
- 373.15 K is the boiling point of water (at standard pressure).
Key differences with other scales
- Celsius: Based on relative reference points (freezing and boiling of water). The Kelvin scale is shifted down 273.15 units, but the differences are equivalent (1 K = 1 °C).
- Fahrenheit: Uses different reference points and a non-linear relationship with the kelvin. To convert from kelvin to Fahrenheit, intermediate steps are required.
Uses of Kelvin
The Kelvin is essential in many scientific and technical disciplines due to its absolute nature and precision. Its applications include:
1. Thermodynamics and fundamental physics
The Kelvin is essential for describing thermal and energy phenomena, such as:
- Ideal gas behavior.
- Average kinetic energy of particles.
- Heat transfer.
2. Color temperature
In lighting and photography, the kelvin is used to measure the color temperature of light sources, based on the blackbody principle.
Color temperature is a measure that describes the chromatic characteristics of a light source, expressed in kelvin (K). It is based on the idea of how an ideal blackbody (an object that absorbs all incident electromagnetic radiation and re-emits energy based on its temperature) emits visible light at different temperatures.
As the blackbody heats up, the light it emits changes color, passing through different shades along the visible spectrum.
For example:
- Warm sources (reddish light): ~2000–4000 K.
- Cold sources (blue light): >7500 K.
3. Electronics and thermal noise
In electronic engineering, the Kelvin is used to analyse the thermal noise of circuits, known as Johnson-Nyquist noise. This noise is related to temperature and allows the efficiency and limitations of electronic systems to be assessed.
4. Astronomy and cosmology
In astrophysics, the kelvin measures extreme temperatures, such as the surface of stars or the cosmic background radiation (~2.7 K).
Curiosities and historical considerations
- Formerly: The term "degree Kelvin" (°K) was used, but since 1968 the use of the degree symbol has been eliminated.
- Erroneous analogy: The kelvin is sometimes confused with the degree Celsius due to the closeness of their magnitudes, although conceptually they are different.