# Why does Kelvin (temperature unit) not have “degrees” like Celsius or Fahrenheit?

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Or probably the other way around, why do Celsius and Fahrenheit have “degrees” in its unit?

In: 8

A degree is a change in temperature that is measured against a scale. The size of a degree is arbitrary. For example, originally the Celsius scale was set such that 0°C was the freezing point of water, while 100°C is its boiling point.

Kelvin is different because it’s an absolute scale. 0K is absolute zero — the point at which gas molecules have no thermal energy.

I can’t remember exactly why, but I’m pretty sure has something to do with the fact that Kelvin is an exact measurement of temperature while Celsius and Fahrenheit are an approximate

Kelvin is it’s “own unit” of measurement. Like how a foot in the US is something entirely different than a meter on the rest of the world. It’s used for measuring extreme temperatures because, as stated above – it’s 0 point is at no molecular movement so the scale stops at 0.

Because when they defined the unit, they defined it that way: https://www.bipm.org/en/committees/cg/cgpm/13-1967/resolution-3

They could have defined it differently, but they decide not to, there is nothing absolute about the size of 1Kelvin, it’s the same as the size of 1 degree Celsius, the only difference is it’s starting point.

When you measure something in “degrees”, that term implies that there isn’t an absolute “starting point”. You’re measuring away from some arbitrary other point.

Neither the Fahrenheit nor Celsius scales, at their time of creation, had absolute starting points. It was assumed that the temperature could be extended up or down infinitely in either direction. So the inventors of these scales simply picked a temperature to call the zero-mark and created an arbitrary temperature difference to call “1 degree”.

This is also apparent in the other common use case for degrees, measuring rotations around circles. Circles don’t have a “start” or “end” point. Every point around a circle is functionally identical to all the others. So you have to pick an arbitrary spot and call it zero, and every angle away from that spot gets measured in degrees. You can wrap around and around that circle infinitely far, either forwards or backwards. The circle just doesn’t end no matter which way you go, so the concept of an absolute starting point is meaningless.

The Kelvin scale is different. It was created after (one could even say in direct response to) the discovery of absolute zero temperature. Now, we don’t have to pick an arbitrary spot and call it zero, because we now actually do have a true, absolute baseline that very definitely, literally means “zero temperature”. So it is now meaningful to talk about having 1 or 10 or 100 “units of temperature” instead of “degrees of temperature”. And the “Kelvin” is that unit.

The degree units are there. It’s just a different way of referring to them.

Fahrenheit – Scale was formed by setting zero as the lowest temperature he could achieve from a mixture of ice water and salt. 100 was normal human body temperature (which, as we know, he didn’t measure properly so we now end up with something weird like 98.6 as normal body temperature.)

Centigrade (Celsius) – Scale was formed by setting zero as the freezing point of fresh water at sea level, and 100 as the boiling point. The endpoints are phase change points that are fairly stable.

Rankine and Kelvin are temperature scales that use absolute zero as their zero point, and use Fahrenheit and Celsius temperature units, respectively. Kelvin is better known since it is related to the Celsius scale, which is now universally accepted as a standard in scientific circles.