Kelvin

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Baron Kelvin in 1906

The kelvin (symbol: K) is the base unit of temperature in the International System of Units (SI). It is named after the Scottish physicist and engineer William Thomson, 1st Baron Kelvin, who developed the concept of absolute temperature.

Definition

The kelvin is defined by taking the fixed numerical value of the Boltzmann constant, k, to be 1.380649×10⁻²³ when expressed in the unit J⋅K⁻¹, which is equal to kg⋅m²⋅s⁻²⋅K⁻¹, where the kilogram, meter, and second are defined in terms of the Planck constant, the speed of light, and the cesium frequency, respectively.

History

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Melting ice thermometer

The concept of an absolute temperature scale was first proposed by William Thomson, 1st Baron Kelvin in 1848. He suggested that there should be a scale where absolute zero is the null point, and the degree intervals are the same as those of the Celsius scale. This led to the development of the Kelvin scale, which is now widely used in scientific contexts.

Absolute Zero

Absolute zero, 0 K, is the lowest possible temperature where nothing could be colder and no heat energy remains in a substance. At this point, the entropy of a perfect crystal is exactly zero, according to the third law of thermodynamics.

Comparison with Other Scales

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Celsius and Kelvin thermometer

The Kelvin scale is related to the Celsius scale; the only difference is the starting point. While the Celsius scale is based on the freezing and boiling points of water, the Kelvin scale starts at absolute zero. The conversion between Celsius and Kelvin is straightforward:

K = °C + 273.15

Applications

The Kelvin scale is used in various scientific fields, including physics, chemistry, and astronomy. It is particularly useful in thermodynamics and statistical mechanics, where absolute temperature is a critical parameter.

Phase Diagrams

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Phase diagram

Phase diagrams often use the Kelvin scale to represent temperature. These diagrams show the conditions under which distinct phases occur and coexist at equilibrium.

SI Unit Relations

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Unit relations in the new SI

In the SI system, the kelvin is one of the seven base units. It is used to define the derived units of temperature-related quantities, such as heat capacity and thermal conductivity.

Temperature Scales Comparison

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Kelvin, Celsius, and Fahrenheit comparison

The Kelvin scale is often compared with other temperature scales, such as Fahrenheit and Celsius. While the Fahrenheit scale is commonly used in the United States, the Kelvin and Celsius scales are used worldwide in scientific contexts.

Kelvin Temperature Chart

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Kelvin temperature chart

The Kelvin temperature chart provides a visual representation of various temperatures in Kelvin, illustrating the scale's application in different scientific scenarios.

Related Pages

• Temperature
• Thermodynamics
• William Thomson, 1st Baron Kelvin
• International System of Units
• Absolute zero