Celsius Scale

Celsius, or centigrade, is a scale and unit of measurement for temperature. It is one of the most commonly used temperature units.

Learning Objective

Explain how the Celsius scale is defined

Key Points

The degree Celsius (°C) can refer to a specific temperature on the Celsius scale as well as a unit to indicate a temperature interval, a difference between two temperatures or an uncertainty.
The Celsius scale is currently defined by two different temperatures: absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW; specially purified water).
Based on this, the relationship between Celsius and Kelvin is as follows: $T_{Celsius} = T_{Kelvin} -273.15$ .

Terms

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.
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
standard atmosphere
an international reference pressure defined as 101.325 kPa and formerly used as a unit of pressure

Full Text

Celsius, also known as centigrade, is a scale to measure temperature . The unit of measurement is the degree Celsius (°C). It is one of the most commonly used temperature units in the world. The unit system is named after the Swedish astronomer Anders Celsius (1701-1744), who developed a similar temperature scale .

Temperature Scales

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

Thermometer

A thermometer calibrated in degrees Celsius

From 1743 until 1954, 0°C was defined as the freezing point of water, and 100°C was defined as the boiling point of water, both at a pressure of one standard atmosphere, with mercury as the working material. Although these defining correlations are commonly taught in schools today, by international agreement the unit "degree Celsius" and the Celsius scale are currently defined by two different temperatures: absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW; specially purified water). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature and which uses the symbol K. Absolute zero, the lowest temperature possible (the temperature at which matter reaches minimum entropy), is defined as being precisely 0K and -273.15°C. The temperature of the triple point of water is defined as precisely 273.16K and 0.01°C. Based on this, the relationship between degree Celsius and Kelvin is as follows:

Phase Diagram of Water

In this typical phase diagram of water, the green lines mark the freezing point, and the blue line marks the boiling point, showing how they vary with pressure. The dotted line illustrates the anomalous behavior of water. Note that water changes states based on the pressure and temperature.

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

Besides expressing specific temperatures along its scale (e.g., "Gallium melts at 29.7646°C" and "The temperature outside is 23 degrees Celsius"), the degree Celsius is also suitable for expressing temperature intervals -- differences between temperatures, or their uncertainties (e.g. "The output of the heat exchanger is hotter by 40 degrees Celsius" and "Our standard uncertainty is ±3°C"). Because of this dual usage, one must not rely upon the unit name or its symbol to denote that a quantity is a temperature interval; it must be clear through context or explicit statement that the quantity is an interval.

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