## Absolute zero and the Kelvin scale of temperature

Temperature is the measure of the hotness or coldness of an object. It is a property that allows us to quantify heat and determine which way the heat will flow from one object to another. When an object is cold, we say it has a low temperature and when it is hot, we say it has a high temperature. If we place our hands in water and the heat energy flows from the water to our hands then the water is at a higher temperature than our hands. If the heat energy was to flow from our hands to the water then the water would be at a lower temperature than our hands.

The temperature of a substance is a measure of the motion of all the atoms and molecules in the substance. When a substance is heated the speed of the particles increase as do their kinetic energies and the temperature rises. When a substance is cooled the speed of the particles decrease and so do their kinetic energies and the temperature drops. If the substance is cooled further the motion of the particles continues to slow down and their vibrations become less and less. Eventually a temperature can be reached at which point the atoms and molecules in the substance are at their lowest energy state and their movement virtually ceases. This is reached at a temperature of -273°C and is called absolute zero. This is the lowest possible temperature because the atoms and molecules are at their lowest energy state and therefore there is no energy for transfer.

## Kelvin scale

The Kelvin temperature scale takes its name after Lord Kelvin who developed it in the mid 1800s. It takes absolute zero as the starting point and temperature measurements are given the symbol K (which stands for "Kelvin"). Temperature differences on the Kelvin scale are no different to those on the Celsius (°C) scale. The two scales differ in their starting points. Thus, 0°C is 273K.

Converting from Celsius to Kelvin
Temperature in °C + 273 = Temperature in K

Converting from Kelvin to Celsius
Temperature in K – 273 = Temperature in °C