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Temperature is a physical property of matter that quantifies how hot or cold a body is.

It is a scalar quantity which can be mapped directly to the real number line.

Absolute Temperature

Absolute temperature is a measure of the amount of heat energy in a body.

It is defined as:

$T = \dfrac 1 k \paren {\dfrac {\partial U} {\partial \ln g} }$


$k$ is a constant that relates the mean kinetic energy and absolute temperature of the body $B$
$U$ is the total energy of $B$
$g$ is the number of possible states in which $B$ can be.


Temperature is frequently, at elementary levels at least, considered as one of the fundamental dimensions of physics.

In dimensional analysis it is usually assigned the symbol $\Theta$.


There are several scales against which temperature is measured.

Each one has two reference points.

Temperature Scales
Name Unit symbol Absolute Zero Melting point of water Boiling point of water
Celsius ${}^\circ \mathrm C$ $-273.15 \ {}^\circ \mathrm C$ $0 \ {}^\circ \mathrm C$ $99.9839 \ {}^\circ \mathrm C$
Fahrenheit ${}^\circ \mathrm F$ $-459.67 \ {}^\circ \mathrm F$ $32 \ {}^\circ \mathrm F$ $211.9710 \ {}^\circ \mathrm F$
Kelvin $\mathrm K$ $0 \ \mathrm K$ $273.15 \ \mathrm K$ $373.1339 \ \mathrm K$
Rankine ${}^\circ \mathrm R$ $0 \ {}^\circ \mathrm R$ $491.67 \ {}^\circ \mathrm R$ $671.641 \ {}^\circ \mathrm R$

There are others.

As a general rule, only Kelvin is used in physics nowadays.

Celsius is usually used in the domestic context, for weather reporting and so on, in most nations, and sometimes seen in the teaching of physics, but usually at the most elementary levels in schools.

Fahrenheit is still used as the official temperature scale only in the US and Belize, although can still be seen on occasion in the contexts of weather reporting and health monitoring in the UK.

The Rankine scale is used in a few specialist engineering applications in the US and Canada.