The three temperature scales

A temperature scale is built by choosing two reproducible fixed points and dividing the interval between them into a conventional number of degrees. Historically, three main scales have become established.

Celsius ($°$C). Calibrated on the melting point of ice ($0\,°$C) and the boiling point of water at atmospheric pressure ($100\,°$C). It is the everyday scale in Europe.

Kelvin (K). The absolute scale: $0$ K corresponds to absolute zero, where molecular agitation ceases (at least classically). There are no negative temperatures in kelvin. The conversion from the Celsius scale is:

$\ev{T_K = T_C + 273{,}15}$

An important fact: temperature differences are identical on the two scales, because they differ only by a shift of the origin:

$\Delta T_K = \Delta T_C$

Fahrenheit ($°$F). A historical scale, still used in the USA. Ice melts at $32\,°$F and water boils at $212\,°$F, so the interval between the two fixed points is $180$ degrees rather than $100$. The conversion is:

$T_F = \tfrac{9}{5}\,T_C + 32$

Which scale to use when

In physics, kelvin is used almost always, in particular whenever ratios of temperatures appear ($T_1/T_2$), as in the gas law or in radiation: a ratio only makes sense on an absolute scale. For differences $\Delta T$ alone, however, it makes no difference whether you work in $°$C or in K.

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Topics: Thermology Concepts: Temperature

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