Firstly, we need to know a bit about thermal equilibrium. When two objects are in thermal equilibrium there is no net flow of thermal energy between them. This means they are they same temperature. If one object was hotter than the other, there would be a net flow of thermal energy from the hotter object to the colder one - this means they are not in thermal equilibrium. However, the temperature of the colder one would increase and the temperature of the hotter one would decrease until they are in thermal equilibrium.
The zeroth law of thermodynamics states: if two objects are in thermal equilibrium with a third, then all three are in thermal equilibrium.
In order to measure temperature a scale is needed with two fixed defined points. We often use the Celsius scale with the freezing (0°) and boiling (100°) points of water as the fixed points. However, this cannot always be used as the boiling point of water changes depending on the surrounding atmospheric pressure (e.g up a mountain water boils at a lower temperature). This means that the boiling point of water is technically not really a fixed point.
Instead, we can use the absolute/thermodynamic temperature scale and use the triple point of water and absolute zero. The SI unit of this scale is kelvin (K). Thankfully, the increments on the absolute scale and Celcius scale are the same. There are 273.16 increments between absolute zero and the triple point of water meaning we can use the following relationship to switch between the two scales:
T(K) ≈ θ(°C) + 273
NOTE: The triple point of a substance is a specific temperature and pressure at which the substance can exist in all three states of matter, solid, liquid, and gas. For water is it 0.01° and 0.61kPa.
NOTE: The symbol 'K' for Kelvin is capital, this is because Kelvin was a person.
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