FHSST Physics Units:Temperature

Temperature

In everyday life, many people measure temperatures in Celsius. But in physics, we prefer to use kelvin.

As we all know, Celsius temperatures can be negative. This might suggest that any number is a valid temperature. In fact, the temperature of a gas is a measure of the average kinetic energy of the particles that make up the gas. As we lower the temperature so the motion of the particles is reduced until a point is reached where all motion ceases. The temperature at which this occurs is called absolute zero. There is no physically possible temperature colder than this. In Celsius, absolute zero is at -273 °C.

Physicists have defined a new temperature scale called the Kelvin scale. According to this scale absolute zero is at 0 K and negative temperatures are not allowed. The size of one unit kelvin is exactly the same as that of one unit degree Celsius. This means that a change in temperature of 1 kelvin is equal to a change in temperature of 1 degree Celsius—the scales just start in different places. Think of two ladders with steps that are the same size but the bottom most step on the Celsius ladder is labelled -273, while the first step on the Kelvin ladder is labelled 0. There are still 100 steps between the points where water freezes and boils (at 1.0 atmosphere of pressure).

                         |----|   102 degrees Celsius    |----|  375 kelvin
                         |----|   101 degrees Celsius    |----|  374 kelvin
 water boils  --->       |----|   100 degrees Celsius    |----|  373 kelvin
                         |----|   99  degrees Celsius    |----|  372 kelvin
                         |----|   98  degrees Celsius    |----|  371 kelvin
                                             .
                                             .
                                             .
                         |----|   2   degrees Celsius    |----|  275 kelvin
                         |----|   1   degree Celsius     |----|  274 kelvin
 ice melts    --->       |----|   0   degrees Celsius    |----|  273 kelvin
                         |----|   -1  degree Celsius     |----|  272 kelvin
                         |----|   -2  degrees Celsius    |----|  271 kelvin
                                             .
                                             .
                                             .
                         |----|  -269 degrees Celsius    |----|  4 kelvin
                         |----|  -270 degrees Celsius    |----|  3 kelvin
                         |----|  -271 degrees Celsius    |----|  2 kelvin
                         |----|  -272 degrees Celsius    |----|  1 kelvin
 absolute zero --->      |----|  -273 degrees Celsius    |----|  0 kelvin
 

(NOTE TO SELF: Come up with a decent picture of two ladders with the labels --water boiling and freezing--in the same place but with different labelling on the steps!)

This makes the conversion from kelvin to degree Celsius and back very easy. To convert from degrees Celsius to kelvins add 273. To convert from kelvins to degrees Celsius subtract 273. Representing the Kelvin temperature by T_K and the Celsius temperature by T_°C,

$\begin{array}{ccc}{T}_K& =& T_{oC}+273\end{array}$ or $\begin{array}{ccc}{T}_{oC}& =& T_K-273\end{array}$

Converting between kelvin and Celsius is additive -- so a difference in temperature of 1 degree Celsius is equal to a difference of 1 kelvin. The majority of conversions between units are multiplicative. For example, to convert from metres to millimetres we multiply by 1000. Therefore a change of 1 m is equal to a change of 1000 mm.

Although it seems as though there is not much reason for the scientific community to use the Kelvin scale over the Celsius scale, there is actually a very remarkable difference in using the Kelvin scale other than the reminding effect stated above. It happens that the idealised form of some materials varies proportionately according to the Kelvin scale, such that some required values can be simply found by multiplication and division. In such calculations, it is just a chore to use the Celsius scale.