Question

Why 0k is equal to -273 degree celcius ?

Answer 1

The basic reason for the different values of temperature scales is the different calibration method used.
The absolute zero is taken as zero on the Kelvin scale. As absolute zero is the point below which cooling is not possible, there is no negative on the Kelvin scale.

Now the celcius scale is based on the triple point of water. (triple point. The temperature and pressure at which a substance can exist in equilibrium in the liquid, solid, and gaseous states. The triple point of pure water is at 0.01°C (273.16K, 32.01°F))

On calibrating, the base of celcius scale is taken as 0 deg.C
And according to celcius scale, the lowest achievable temperature is -273 deg.
So naturally 0k is equal to -273 degree celcius