Measuring temperature and temperature scales

​​In a nutshell

Temperature is a measure of hotness. If an object is hotter than another there is a net flow of thermal energy from hot to cold until thermal equilibrium is reached. Temperature can be measured with degrees Celsius on the Celsius scale or with kelvin on the absolute temperature scale.

Definitions

Equations

Variable definitions

Temperature

Temperature is a measure of hotness on a chosen scale. If two objects are in contact with each other and one has a higher temperature, there will be a net flow of thermal energy from the hotter object to the colder one. 

The thermal energy will flow until the objects reach thermal equilibrium. This is the point at which there is no net energy flow between them as they have the same temperature.

Temperature scales

There are different scales that measure temperature, the most popular being the Celsius scale with units $$\degree C$$​. A temperature scale needs two fixed points at defined temperatures, for the Celsius scale these are the freezing point of water at $$0 \degree C$$​ and the boiling point of water at $$100\degree C$$ with 100 increments in between them.

Another scale is the absolute temperature scale with the SI base unit kelvin $$K$$. The increments on this scale are the same as on the Celsius scale but they go down to absolute zero ($$0\ K$$), the lowest possible temperature. The freezing point of water is at $$273.15\ K$$ which means that to convert between the two scales you can use the approximation:

​$$T(K)\sim\theta(\degree C)+273$$​​

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Example

Convert $$20\degree C$$ into kelvin.

Firstly, write down the temperature:

$$\theta=20\degree C$$​​

Next, write down the conversion between Celsius and kelvin:

$$T(K)\sim\theta(\degree C)+273$$​​

Now substitute the number and calculate the temperature in kelvin:

$$T(K)=20+273=293\ K$$​​

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​$$20\degree C$$ is equivalent to $$\underline{293\ K}$$.