Why do we use Kelvin as an si unit to calculate temperature?
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For the math to work, you need to use a scale where 0 is actually nothing. 0 Kelvin is 0 thermal energy, and it goes up from there.
Eg. something at 100K has twice as much thermal energy as something at 50K. Something at 300K has 3x the thermal energy as something at 100K etc. That property is critical for thermodynamics, which is the sort of math that SI units are catering to.
0C is about 273K, which we defined as 0 both because it marks water’s freezing point and because it makes the temperatures we encounter in daily life have nice small numbers when we use Celsius. It would be cumbersome to say it’s 308K on a scorching hot day and 270K when it’s snowing.
And more importantly, something that’s 40C does NOT have double the thermal energy of 20C!! That’s 313 and 293K respectively. The hotter thing is only has like 7% more thermal energy not double! That’s why chemistry formulas have to use temp in K. If some reaction rate scales with temperature, warming it from 20 to 40C will not double it, it’ll go 7% faster because there’s 7% more thermal energy.
**TLDR For double the temp to have double the thermal energy (and so on), you need 0 temp to be at 0 energy. Kelvin is the scale that does that.**
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