# What is Temperature?

Consider a closed box containing a gas. The **temperature** \(\class{blue}{T}\) of this gas describes the *average kinetic energy* \( \overline{W}_{\text{kin}}\):

Here \(k_{\text B}\) is the Boltzmann constant with the value \( k_{\text B} = 1.38 \cdot 10^{-23}\, \frac{\mathrm{J}}{\mathrm{K}} \). And \( f \) is the number of **degrees of freedom** of a gas particle. For example, if a gas particle can move in all three spatial directions but does not rotate, then \(f=3\).

Temperature is therefore a measure of how fast the particles in a body are moving. The faster they move, the larger the temperature. The slower they move, the smaller the temperature. In the extreme case, when the particles are not moving at all, the temperature is \( \class{blue}{T} = 0 \, \mathrm{K} \) (Kelvin).

Absolute temperature is measured in the unit "Kelvin" and the lowest temperature is just \( \class{blue}{T} = 0 \, \mathrm{K} \). You can calculate the absolute temperature in degrees Celsius with the following formula:

Here, \( \class{red}{\vartheta} \) (pronounced: »theta«) is the temperature in »Degrees Celsius« and \(\class{blue}{T}\) is the temperature in the unit »Kelvin«. You can see from the formula that the physically lowest temperature is \( \class{red}{\vartheta} = -273.15\, ^\circ \mathrm{C} \).