Kelvin

The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). Unlike the Celsius or Fahrenheit scales, the kelvin is an absolute temperature scale. This means it starts at the lowest possible temperature, known as absolute zero. At absolute zero, there is no thermal energy left to remove.

Scientists use this scale because it provides a direct way to measure the energy within physical systems. It is essential for studying thermodynamics, which is the science of heat and energy.

The kelvin scale is mathematically linked to the Celsius scale. A single change of one kelvin is exactly equal to a change of one degree Celsius.

To convert a temperature from Celsius to kelvin, you simply add 273.15 to the Celsius value. For example, if water freezes at 0 °C, it is 273.15 K on the kelvin scale. This relationship allows scientists to switch between everyday measurements and absolute measurements easily. The scale uses the same magnitude for its increments, meaning the "size" of a degree is identical in both systems.

Developing this scale required understanding how gases behave. Between 1787 and 1802, scientists like Jacques Charles and Joseph Louis Gay-Lussac observed something important. They found that at a constant pressure, ideal gases expand or contract their volume linearly with temperature. This is known as Charles's law. They noticed that for every degree Celsius change, the volume changed by about 1/273 parts. By looking at this pattern, they could extrapolate that a gas would reach zero volume at about −273 °C. This theoretical point became the foundation for absolute zero.

William Thomson, a British scientist later known as Lord Kelvin, proposed the first absolute scale in 1848.

His initial version was not perfect, but his underlying principles were correct. He worked closely with James Prescott Joule to understand the relationship between heat and mechanical work. Joule suggested that the formula for heat should include the specific heat capacity of water. By 1854, Thomson and Joule had formulated a much more practical absolute scale. This version was designed so that the difference between the freezing and boiling points of water remained 100 units, just like the Celsius scale.

In the 20th century, the way we define the kelvin became much more precise. For a long time, scientists used the triple point of water as a reference.

The triple point is the unique temperature and pressure where water exists as a solid, liquid, and gas at the same time. In 1954, the kelvin was formally added to the SI system. At that time, 273.16 K was defined as the exact temperature of water's triple point. This was more accurate than using the melting point of ice, which can fluctuate. Using the triple point allowed for much higher precision in scientific measurements.

In 2019, the definition of the kelvin changed once again to be even more fundamental. Instead of relying on the properties of water, it is now defined by a universal constant.

The scale is now fixed using the Boltzmann constant ($k_B$). This constant links the temperature of a system to its thermal energy. Every change of 1 K now corresponds to a specific, exact change in thermal energy measured in joules. This shift means the kelvin is no longer tied to a specific substance like water, but to the fundamental laws of physics.

The kelvin scale is vital for many different fields of science. It allows physicists to calculate the efficiency of engines and the behavior of particles in space. Because it starts at zero, it allows for direct mathematical ratios. For instance, a temperature of 200 K has exactly twice the thermal energy of 100 K. This is not possible with the Celsius scale, where zero is just an arbitrary point. By using the kelvin, scientists can describe the energy of the entire universe with total consistency.