Temperature is a fundamental measurement in science and everyday life. Over centuries, we’ve refined how we quantify and express it. The kelvin (K), the SI unit of temperature, has undergone a significant redefinition. This article delves into the concept of Kelvin, its applications, and the importance of its modern definition.
While the Fahrenheit and Celsius scales dominate daily temperature measurements (weather forecasts, cooking, etc.), the Kelvin scale, named after Lord Kelvin, plays a vital role in scientific contexts. Unlike Celsius and Fahrenheit, the kelvin is not expressed in degrees. Instead, it stands alone as the unit of measurement. For instance, “a material becomes superconductive at a temperature of 93 kelvins.”
A one kelvin change is equal to a one degree Celsius change. However, the Kelvin scale is “absolute,” starting at absolute zero, also known as “infinite cold” (0 K = -273.15 °C = -459.67 °F. Room temperature is about 294 K). This absolute scale contrasts with relative temperature measurements where objects are simply hotter or colder than something else. The thermodynamic temperature of an object reflects the average kinetic energy of its atoms and molecules.
triple point cell
According to classical physics, motion stops at absolute zero. However, quantum theory introduces “zero-point motion,” a random motion even at absolute zero, due to the Heisenberg uncertainty principle. This zero-point motion is not thermal and isn’t part of thermodynamic temperature. Therefore, at absolute zero, only quantum-mechanical zero-point motion exists.
Applications of the Kelvin Scale
The Kelvin scale is prevalent in the physical sciences. One common application is the “color temperature” of light sources. Incandescent bulbs emit yellowish light with a color temperature around 3,000 K. This means their spectrum resembles the radiation of a hot object at 3,000 K. “Daylight” or “full spectrum” lamps, with more blue light, have color temperatures between 5,000 K and 5,600 K, similar to the sun’s surface temperature (approximately 5,800 K). Many LEDs fall within or exceed this range.
Redefining the Kelvin: From Water’s Triple Point to the Boltzmann Constant
Historically, in 1954, the kelvin was defined as 1/273.16 of the thermodynamic temperature of water’s triple point—the equilibrium of water, ice, and water vapor. This triple point occurs at a consistent temperature (273.16 K) for a specific water formulation at a defined pressure. This provided a valuable and reproducible reference point.
Extrapolating from the water triple point to very high or low temperatures proved challenging. Consequently, 21 other defining points were specified internationally, from helium’s freezing point to copper’s freezing point. However, the kelvin is now redefined based on the Boltzmann constant.
The New Definition of Kelvin
The revised SI, approved in November 2018, defines the kelvin as follows:
“The kelvin, symbol K, is the SI unit of thermodynamic temperature; its magnitude is set by fixing the numerical value of the Boltzmann constant to be equal to exactly 1.380649 × 10-23 J K-1 [joules per kelvin].”
In essence, the kelvin is now defined by fixing the Boltzmann constant, which relates thermodynamic energy to temperature.
Why Redefine the Kelvin?
This redefinition provides a more stable and universally accessible standard for temperature measurement, independent of specific materials or phenomena like the triple point of water. It allows for more accurate measurements at extreme temperatures and ensures consistency across various scientific disciplines. Understanding the history and rationale behind this redefinition provides crucial context for appreciating the significance of the kelvin as the fundamental unit of thermodynamic temperature.
