# From Celsius to Kelvin - why +273?

Hi,

I am puzzled by the use of the number 273 when converting from Celsius to Kelvin and vice versa.

I know that Celsius scale is based on ice point and steam point, whereas Kelvin scale is based on triple point of water, which is very very close to ice point.

I am a teacher and i was asked the above question by a student. My first gut impulse is that we assign numbers as a form of starting point for comparison. It is like asking why pi is 3.14.

But just to set my mind at ease, are there further deeper reasons how this number 273 is chosen?

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D H
Staff Emeritus
The zero on the Celsius scale is rather arbitrary. What makes the freezing point of water at standard pressure so special?

The zero on the Kelvin scale is anything but arbitrary. Absolute zero is a very special temperature. The reason for the 273.15 offset that is what is needed to make a temperature whose zero point is absolute zero have the same scale as the Celsius scale.

Hello

Centigrade scale has ice point (melting ice at 1 atm) and steam point (steam at 1 atm) taken for the scale, 0 degree celsius and 100 degree celsius respectively.

For Kelvin scale, its just that the scale starts from absolute zero ( the coldest temperature ). If absolute zero is in centigrade scale then its -273 degree celsius.

As for the pi question, I think that it is a constant for something to be true... lets say you know the area of a circle which is 78.6cm^2 with radius 5cm. To get 50cm^2 you need to multiply radius with radius and an unknown, which is pi.

Please correct me if im wrong thanks!

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You've got to be careful as the Pi analogy is confusing.

Pi is not just a number assigned as a starting point, the value is something special (the name is arbtrary, but the value isnt) very much like absolute zero which the Kelvin scale is based off. So Pi is very special number that is infinitely long, and the only reason why we comonly use the approximation of 3.14 is becuase it strikes a balance betweeen ease of calculation and accuracy.

Scales that compare one thing to another such as farenhheit and celsius are correct to be compared to something that is a starting point to make things easier.

Just nit picking mate, but its area not vol. :P

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oops i will correct it

rcgldr
Homework Helper
To clarify this a bit. Celsius scale was define to be 0 degrees at the freezing point of water (at 1 atm pressure), and 100 degrees at the boiling point of water (at 1 atm pressure). I doubt that at the time of this definition anyone knew what the limit for how cold things can get was. This was detemined later on the Celsius scale, it turned out to be -273.15.

Pi is a transcendental number.

http://en.wikipedia.org/wiki/Transcendental_number

However it's definition as the ratio circumference / diameter of a circle is exact. If pi is used as a unit of magnitude, then the issue is eliminated.

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Pengwuino
Gold Member
Maybe this would help but I doubt it - make the analogy to star brightness. A 0 magnitude star is standardized and anything dimmer goes up in value. However, brighter objects like the sun have a magnitude in the negatives! Then you can even go on to make the comparison to absolute magnitudes being like the kelvin scale. I guess the problem is, if you get deeper into the argument, there is no "absolute 0" for brightness (although you kinda have the inverse where you could have.... "absolute infinity" being a point in the sky that has 0 light intensity.... thus is just space, but that's kinda silly and non-technical).

I'm trying to think of a good analogy but I'll be damned if I can...

Also you can't use Celsius for the gas laws that divide by a temperature, because you can't divide by zero. So you must use kelvin.