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# What is zero degrees Kelvin?

First, note that Kelvin is typically not expressed in degree(s), just Temperature Kelvin

(e.g., 50 K for 50 kelvin). ***(note2 at bottom, about kelvin vs degree kelvin)

Kelvin is a temperature scale based upon 0K being absolute zero*(note1 at bottom), which is currently the lowest possible temperature. This is not possible to actually achieve, but it can be determined as the point at which a 'perfect' (ideal gas) gas would have zero pressure and volume as it contracts on cooling. As most gases behave very much like a perfect gas and the deviation of real gases is well understood, this allows us to calculate the exact temperature at which this would happen. That temperature is zero Kelvin.

For the ideal gas this is the point at which the molecules (assumed to be perfect non-interacting mathematical points) stop all motion. So the pressure (which is caused by the gas molecules bouncing off the container walls) is zero. At this point also, the gas has zero kinetic energy.

(In reality quantum mechanics shows that particles must have a certain amount of energy even at zero Kelvin. This is termed the 'zero point' energy, and is manifest in a tiny amount of vibrational energy. So in reality there is residual motion in a gas at absolute zero, but this cannot exert any pressure as that would involve removing the residual energy which cannot happen. So in quantum terms it is the point at which no mechanical energy can be extracted from the system.)

The Kelvin scale is named for British mathematician and physicist William Thompson, 1st Baron Kelvin (1824-1907), who did much to unify the modern field of Physics.

Equivalent Temperatures in Other Temperature Scales

-273.16° Celsius

-459.67° Fahrenheit

0° Rankine**

-218° Réaumur

*Note1 - The Kelvin scale is indeed based upon the triple point of water, being assigned to 273.16 K; this point is beyond the scope of this discussion, however.

**Kelvin and Rankine are both based upon 0 as absolute zero, however Kelvin uses the interval of 1 K is equal to 1° Celsius, and Rankine uses the interval of 1° Rankine is equal to 1° Fahrenheit.

***Note2 - The accepted SI unit for temperature is K, not, degree(s) K. In scientific papers you will typically only find K, though it is still colloquially accepted to use degree Kelvin when you mean Kelvin.

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