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# How come that absolute 0 is at -273.15C?

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Thinking about absolute 0 temperature (might have something to do with cold weather outside? ), it struck me as odd that Celsius scale defined as 100 divisions between water freezing temperature and water boiling temperature at mean atmospheric pressure on earth has such a "round" number on Kelvin scale, or that absolute zero is located precisely at -273.15C.

At first I thought that -273.15 is some kind of general round off (like speed of light being 3x10^5 km per second, but it is closer to 299 792 458 m/s), but then I found on Wiki article on absolute zero following quote:

"by international agreement, absolute zero is taken as −273.15° on the Celsius scale"

This is very confusing, to me at least. If there is such a thing as absolute zero and it represents lowest energy state of QM system, I find it very odd that we can have agreement that it's value should be precisely -273.15C (compared to 1/100 division of water related temperature ranges based on earth atmospheric pressure), or if it is not matter of agreement (and I'm failing to see how can physical quantity be matter of agreement if "measurement stick" is predetermined), it is very odd that it's value is such a round number. I would have expected something along the lines: "With our current technology best estimate for absolute zero on Celsius scale is -273.149792832 +/- 0.000000013".

Does anyone have idea of what is going on?

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Absolute zero is 0 Kelvin. The reason it is - 273.15 centigrade/Celsius is, as you say, that scale is in reference to the freezing and boiling points of water at normal atmospheric pressure.
The term absolute zero is used to identify the temperature where there is zero thermal energy - it isn't possible for a substance to be cooled below that poing because there is no more energy to remove.

I believe that Lord Kelvin decided to use the same divisions per degree as for celcious to make translation from one scale to another simple.

Michael

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I'm not an expert, but I might hazard that the Celsius scale has been redefined for simplicity at some point. It doesn't really perform any hard scientific logic to where the scale starts and ends (since the properties Celsius represents at either end of the scale vary depending on the environment!) and if we had determined some base for 0°k, which is far more important scientifically, the SI people may well have just decided to round Celsius off for simplicity.

That's just a theory, but it may have some truth to it since as you say such round numbers are not common in physics. The closest thing I can find to support this is that the "triple point" of water is 273.16°k (0.01°c). Perhaps they set the Celsius scale to that end?

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10 minutes ago, pipnina said:

I'm not an expert, but I might hazard that the Celsius scale has been redefined for simplicity at some point. It doesn't really perform any hard scientific logic to where the scale starts and ends (since the properties Celsius represents at either end of the scale vary depending on the environment!) and if we had determined some base for 0°k, which is far more important scientifically, the SI people may well have just decided to round Celsius off for simplicity.

That's just a theory, but it may have some truth to it since as you say such round numbers are not common in physics. The closest thing I can find to support this is that the "triple point" of water is 273.16°k (0.01°c). Perhaps they set the Celsius scale to that end?

Smack on!

From wiki on Celsius:

" By international agreement, since 1954 the unit degree Celsius and the Celsius scale are defined by absolute zero and the triple point of Vienna Standard Mean Ocean Water (VSMOW), a specially purified water. This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 °C. The temperature of the triple point of water is defined as exactly 273.16 K (0.01 °C)."

So there are "decimal digits" but they are now related to freezing and boiling points of water at 1 atm pressure, rather than other way around.

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.

Edited by Charic

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24 minutes ago, Charic said:

Whilst you're discussing the various temperature scales, you know what annoys me ?  its when the uk has  a heatwave one day, and the forecasters  revert too Fahrenheit  measurements, making us believe it's hotter than it really is.   Just stick with Celsius to avoid any confusion.

Sorry but I can only relate to "proper temperatures" the only thing I do to convert in my head is 16C = 61F to get a rough idea what they're on about

Dave

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On ‎20‎/‎01‎/‎2019 at 16:08, Davey-T said:

Sorry but I can only relate to "proper temperatures" the only thing I do to convert in my head is 16C = 61F to get a rough idea what they're on about ﻿

Dave

I think if my memory serves me correctly -40F is the same as -40C. But don't quote me.

K

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19 minutes ago, KevS said:

I think if my memory serves me correctly -40F is the same as -40C. But don't quote me.

K

Too late

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There's always one

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So, as above, it's just about definition of the scale otherwise you don't truly know what one Kelvin corresponds to.

0K as a thing is an extrapolation downwards from the ideal gas law, but obviously materials deviate from that as you approach. At 0K, delta_S = 0 for adiabatic processes, and perfect crystals can theoretically form where S=0.

Edited by coatesg

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