# Kelvin Scale

## The kelvinis a unit of measurement for temperature; the null point of the Kelvin scale is absolute zero, the lowest possible temperature.

#### Key Points

• 0K (absolute zero) is universal because all thermal motions of all matter are maximally suppressed at this temperature. Absolute zero is therefore the natural choice as the null point of the Kelvin scale.

• The Kelvin scale is used extensively in scientific work because a number of physical quantities, such as the volume of an ideal gas, are directly related to absolute temperature.

• To convert kelvin to degree Celsius, we use the following formula:$T_{Celsius} = T_{Kelvin} -273.15$.

#### Terms

• The coldest possible temperature: zero on the Kelvin scale and approximately -273.15°C and -459.67°F. The total absence of heat; the temperature at which motion of all molecules would cease.

• The unique temperature and pressure at which the solid, liquid and gas phases of a substance are all in equilibrium.

• A hypothetical gas whose molecules exhibit no interaction and undergo elastic collision with each other and with the walls of the container.

#### Figures

1. ##### Relationships Between the Temperature Scales

Relationships between the Fahrenheit, Celsius, and Kelvin temperature scales, rounded to the nearest degree. The relative sizes of the scales are also shown

2. ##### Temperature Scales

A brief introduction to temperature and temperature scales for students studying thermal physics or thermodynamics.

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units (SI) and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using absolute zero as its null point. In the classical description of thermodynamics, absolute zero is the temperature at which all thermal motion ceases Figure 2.

The choice of absolute zero as null point for the Kelvin scale is logical. Different types of matter boil or freeze at different temperatures, but at 0K (absolute zero), all thermal motions of any matter are maximally suppressed. The Kelvin scale is used extensively in scientific work because a number of physical quantities, such as the volume of an ideal gas, are directly related to absolute temperature.

The Kelvin scale is named after Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin (1824-1907), who wrote of the need for an "absolute thermometric scale." Unlike the degree Fahrenheit and the degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the primary unit of measurement in the physical sciences, but it is often used in conjunction with the degree Celsius, which has the same magnitude. The kelvin is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01°C, or 32.018°F). To convert kelvin to degrees Celsius, we use the following formula:

$T_{Celsius} = T_{Kelvin} -273.15$

Subtracting 273.16K from the temperature of the triple point of water, 0.01°C, makes absolute zero (0K) equivalent to -273.15°C and -460°F (Figure 1).

#### Key Term Glossary

absolute zero
The coldest possible temperature: zero on the Kelvin scale and approximately -273.15°C and -459.67°F. The total absence of heat; the temperature at which motion of all molecules would cease.
##### Appears in these related concepts:
base
A nucleotide's nucleobase in the context of a DNA or RNA biopolymer.
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gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
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ideal gas
A hypothetical gas whose molecules exhibit no interaction and undergo elastic collision with each other and with the walls of the container.
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Ideal Gas
Theoretical gas characterized by random motion whose individual molecules do not interact with one another and are chemically inert.
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kelvin
in the International System of Units, the base unit of thermodynamic temperature; 1/273.16 of the thermodynamic temperature of the triple point of water; symbolized as K
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magnitude
A number assigned to a vector indicating its length.
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matter
The basic structural component of the universe. Matter usually has mass and volume.
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motion
A change of position with respect to time.
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quantity
A fundamental, generic term used when referring to the measurement (count, amount) of a scalar, vector, number of items or to some other way of denominating the value of a collection or group of items.
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thermodynamic
Relating to the conversion of heat into other forms of energy.
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thermodynamics
a branch of natural science concerned with heat and its relation to energy and work
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thermodynamic temperature
Temperature defined in terms of the laws of thermodynamics rather than the properties of a real material: expressed in kelvins.
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triple point
The unique temperature and pressure at which the solid, liquid and gas phases of a substance are all in equilibrium.
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work
A measure of energy expended in moving an object; most commonly, force times displacement. No work is done if the object does not move.