Why do we use Kelvin as an si unit to calculate temperature?

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Kelvin is defined such that 0 K is the complete absence of any thermal energy. As such, it’s useful to have a scale where 0 actually corresponds to, well, absolute zero (compared to, say, Celsius, where 0 is just the freezing point of water).

Because unlike Celsius or Fahrenheit it doesn’t have 0 at arbitrary point.

If you have something at 10 K and 100 K it has 10x the thermal energy.. unlike in other systems

This is the basis for SI system… Simplicity and interconnectivity

Celsius is based off water, under regular conditions. At 0 it freezes and 100 it boils. Simple stuff, easy, useful in everyday life.

But scientists deal with all sorts of scenarios that are not boiling a tea kettle to make afternoon tea, or putting an ice cube into a cup of whiskey.

They think about things small and huge, unimaginably small or huge, at different altitudes (where the boling point of water is lower than 100). they think about weird scenarios like tons of air pressure, or no air pressure, or what happens if we’re on Mars instead of Earth!

0 and 100 in Celsius make sense for us, but they stop making sense when you start thinking about everything that is not riding your bike down the street on a breezy summer afternoon with no cares in the world.

So we have Kelvin. Instead of starting at some arbitrary point, of water freezing at 0 in your home under day to day conditions of you riding your bike, they start at what the universe declares is actual 0, we call that absolute zero. And Kelvin starts there. But celsius isn’t all bad… so we kept it mostly in tact, we just moved 0 down a whole lot.

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.**

It starts at 0, and has the same unit size as a commonly used temperature scale (one Kelvin is equal to one degree Celsius). Being a scale starting at zero can be important for some calculations, and is just better practice to begin with.