Kelvin: Unit of Temperature for Thermodynamics

Kelvin scale was built up with no reference to a cold or hot object. It centers only on the energy in a molecule and thus it became the thermodynamic scale of temperature. The idea for the Kelvin scale came from a discovery in the 1800s of a relationship between the volume and the temperature of a gas. Scientists theorized and proved that the volume of gas should become zero at a temperature of minus -273.16 degc, the triple point of water at 611.73 Pa. Scientists further theorized that at -273.16 degc the molecular motion ceases and molecules have no energy. Kelvin defined "absolute" as the temperature at which molecules would stop moving, or "infinite cold.

This is how the triple point of water became the lower end of the Kelvin scale. At temperatures near 0 K (?273.15 °C; ?459.67 °F), nearly all molecular motion ceases, and ΔS = 0 for any adiabatic process, where S is the entropy. In such a circumstance, pure substances can (ideally) form perfect crystals.

The kelvin is the base unit of temperature in the International System of Units (SI), abbreviated as "K". Kelvin unit directly links itself with energy through Boltzmann constant k = 1.380649×10?23?J/K [Joule / Kelvin]. The main advantage is that this allows measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant which is a fixed number and does not depend on how low or how high the temperature is. The Kelvin scale (absolute temperature) is used in the definitions of many fundamental thermodynamic relationships.

How Kelvin is different from Celsius and Fahrenheit: A change of one kelvin is the same amount of temperature change as one degree Celsius, but the Kelvin scale is “absolute” in the sense that it starts at absolute zero, or what Kelvin and other scientists called “infinite cold.” (0 K = -273.15 degc. The triple point of water is - 273.15 K at 611.73 Pa, it is equal to 0.01 degc which is the freezing point of water. Like other temperature scales, the Fahrenheit and Celsius scales, the freezing and boiling points of water are factors in establishing the scale’s range, there are 100 degrees between the temperate at which water freezes at (273.16 K) and boils (373.16 K).

How Kelvin scale [K] developed? How does Kevin scale address various issues?

How triple point of water became the lower end of the Kelvin scale?

Kelvin scale was built up with no reference to a cold or hot object. It centers only on the energy in a molecule and thus it became the thermodynamic scale of temperature. The idea for the Kelvin scale came from a discovery in the 1800s of a relationship between the volume and the temperature of a gas. Scientists theorized that the volume of gas should become zero at a temperature of minus - 273.15 C.

Detail

Charles’s law plots of temperature and volume and temperature and pressure are shown above. A remarkable feature of these plots is that if the straight line is extrapolated down to a value of zero volume, the corresponding temperature (the intercept on the temperature axis) is -273.15 degrees Celsius, which corresponds to 0 kelvin, the absolute zero temperature. This relationship shows that absolute zero is the lowest possible temperature because it corresponds to a gas volume of zero, and it is physically impossible to have a volume less than zero.? On the right-side plot when pressure vs. temperature data is extrapolated to a pressure of zero, it intercepts the temperature axis at exactly -273.15 degrees Celsius (absolute zero temperature). This observation makes practical sense because, at absolute zero, the thermal molecular motion of the gas molecules nearly ceases. This molecular motion causes gases to exert pressure; therefore, minimizing the kinetic energy of the gas molecules minimizes the gas pressure. This is how the triple point of water became the lower end of the Kelvin scale.

Kelvin scale and Carnot cycle:? The Kelvin scale was also influenced by the Carnot engine, a theory that examines the relationship between pressure, work, and temperature. It is one of the fundamentals of physics and thermodynamics and is a measure of the efficiency of an ideal engine. The fact the efficiency of the Carnot engine depends on two temperatures and the lower the sink temperature the bigger the efficiency, obviously a Celsius scale does not fit in this concept because even at 0 degc gas water lots of kinetic energy to extract work out of it. The enthalpy and entropy of a cooled ideal gas reach their minimum value, at zero kelvin. The fundamental particles of nature have minimum vibrational motion at absolute zero.

Boltzmann’s contribution to the Kelvin scale: Boltzmann made a significant contribution to the application of Kelvin in thermodynamics. For the first time, Boltzmann used and introduced kelvin into a thermodynamic equation and he established the value of k can be used to relate kinetic energy and temperature of molecules. Boltzmann proposed that the average kinetic energy of molecules is proportional to their kinetic energy, ? mv2 = 3/2 KbT, known as the Boltzmann equation, m is mass, v is velocity, The Boltzmann constant (Kb) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas kelvin. Thus, kelvin became an integral part of almost every thermodynamic equation that exists today. Monatomic ideal gases (the six noble gases) possess three degrees of freedom per atom, corresponding to the three spatial directions, which means thermal energy of 3/2 KbT per atom. Boltzmann constant has undergone revisions from time to time. As per the latest revision in 2019, Kb stands at 1.380649×10?23 joule per kelvin (J?K?1)