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What is Kelvin?

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In the international system unit, Kevin is the base unit of temperature and its symbol is K. Kelvin definition is prepared by fixing the numerical value of Boltzmann constant k to 1.380649×10−23 J/K. It was named after physicist and engineer of Glasgow University, 1st Baron Kelvin. Absolute zero is used as the null point in this scale. This is the principle on which the absolute thermodynamic temperature scale is based. Pure water freezes at 273.15K and boils at 373.15 on a Kelvin scale.


Who Invented Kelvin Scale?

William Thomson, later ennobled as Lord Kelvin, wrote in his paper “On an absolute thermometric scale” in the year 1848, the sheer need for a scale where the null point of the scale should be absolute zero or infinite cold. The scale should be able to use degree Celsius for its unit increment. While researching his paper he found that -273o Celsius was equal to zero based on the then air thermometers. This scale later came to be known as Kelvin thermodynamic temperature scale. Also, 273 is considered as the negative reciprocal of 0.00366. This value is the acknowledged expansion coefficient of gas per oC relative to the ice point. This was significantly consistent with the currently accepted value. 


Kelvin Scale Definition

The modern definition of the Kelvin scale was given by Resolution 3 of the 10th General in the year 1954 where the triple point of the water was identified and entitled as the second defining point, assigning the temperature to exactly 273.16 Kelvin. Then in 1967 and 1968, the symbol of a degree Kelvin was replaced by the symbol K. This was done to make the thermodynamic temperature Kelvin more useful while defining the magnitude of unit increment. It was later held in Resolution 4 that the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water is equivalent to the unit of thermodynamic temperature. So, when asked “what is K in temperature measurements” can be answered with this. 


Recent Changes in Kelvin

Effective from May 20, 2019, the General Conference on Weights and Measure in 2018 decided that the unit would be defined in a manner where the Boltzmann constant would be equivalent to 1.380649 X 10-23 Joule/Kelvin. Originally this unit was defined as 100/27,316 of the triple point. With the Kelvin scale, many formulas and physical laws can be articulated in a simplified manner. This is one of the significant reasons the Kelvin scale is recognized as the international standard for the measurement of scientific temperature. The earlier scientist had an assumption that humans cannot recreate the temperature of absolute zero which occurred between -273.15oC to -459.67oF. 

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Measurements of Kelvin Unit

In physical science, it is the primary unit of temperature measurement and is not the same as degree Celsius which has a similar magnitude. Kelvin scale helped in identifying the degree of temperature from ice point to boiling point which is 100 degrees. This is the reason why there are two fixed temperatures on the Kelvin scale. In terms of centigrade, these are 0o C or ice point and 100o C which is the boiling point. 


Kelvin Scale and ITS-90 

ITS-90 or International Temperature Scale 1990 published by the Consultative Committee for Thermometry consists of 17 reference points that are based on highly reproducible states, naturally occurring triple points or the phase transitions taking place in naturally occurring substances. The following are among the reference points:

  • The triple point of Hydrogen = 13.8033 K

  • The triple point of Oxygen = 54.3584 K

  • The triple point of Mercury = 234.3156 K

  • The freezing point of Tin = 505.078 K

  • The freezing point of Aluminium = 933.473 K

  • The freezing point of Gold = 1337.33 K


Did You Know?

  • There is no negative Kelvin scale because Kelvin itself is an absolute zero temperature scale. So, 0K in Kelvin refers to absolute zero. This is the point where gas molecules have no or zero thermal energy. 

  • The scientists earlier thought that without adding energy to the system it is not possible to reach absolute zero as the system was always warmer than that. This assumption came to a conclusion when the German physicist managed to push particles in the paradoxical temperature in the year 2013. It is essential to understand that with arbitrary precision a number can be measured on any scale. However, as the Kelvin scale is a physics-based scale it is considered to be the most accurate one.

FAQ (Frequently Asked Questions)

1. Why is Kelvin used instead of Celsius?

Ans: When there is an alteration in Celsius or Fahrenheit it is said to be not directly related to kinetic volume or energy because the scales do not start at zero. However, scientists suggest that the application of the Kelvin scale to measure temperature is directly related to kinetic volume and energy and thus it can be considered as an absolute temperature scale. Zero kinetic temperature or energy is represented by 0K. So, when there is a significant change in the degree of temperature, for instance from 100o-200o, Kelvin represents the increase of the kinetic energy system. An increase in the volume can be considered as the increase in temperature as these two are directly related. The scale is based on the physical properties of gas and therefore can be calibrated anywhere in the world precisely with a universal constant and proper equipment, unlike Celsius. 

2. What are the Advantages of using Kelvin in Temperature Measurement?

Ans: The scales of Fahrenheit and Celsius were built either on some combination of chemical and water or were built on freezing point or boiling water. Scientists required the use of an energy scale where zero would reflect the complete absence of thermal energy which was showcased by the Kelvin temperature scale and hence they used the Kelvin temperature scale. There is zero presence of any thermal energy as the temperature lowers along with less vibration in the material to trigger thermal excitations. Also, another important reason why the Kelvin scale is used for measuring temperature is that this scale is more convenient than the scales of Reaumer or Fahrenheit.