# 273 + degree Celsius = Kelvin. Why 273?

Temperature conversion:

273 + degree Celsius = Kelvin

Actually why is that 273? How does one come up with this?

My teacher mentioned Gann's law (not sure if this is the one) but I couldn't find anything related to this, which law should it be?

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–  Christoph Sep 19 '12 at 9:31
Historically, the Celsius scale was defined so that water froze at 0 and boiled at 100. Since this is an arbitrary choice, it has no fundamental physical meaning, thus the offset of 273. The Kelvin scale is defined so that zero is the lowest temperature possible, though for historical consistency, 1K is defined so that the triple point of pure water (very close to the freezing point) is 273K, so that 1K is about the same as 1 $^\circ$C. –  genneth Sep 19 '12 at 9:33
@genneth: Triple point of water is 273.16K, to be precise. –  Siyuan Ren Sep 19 '12 at 10:44

One Celsius (or Kelvin) degree as a temperature difference was defined as 1/100 of the temperature difference between the freezing point of water and boiling point of water.

We call these points 0 °C and 100 °C, respectively. The number 100 arose because we're used to numbers that are powers of ten because we use the base-ten system. The Celsius degree is "one percent" of the temperature difference.

There also exists the minimum temperature that one may achieve. In Kelvin degrees, it's sensible to call this minimum 0 K: that's a primary condition on the "natural" temperature scales such as the Kelvin scale. If one insists that one Kelvin degree is equally large as one Celsius degree as a unit of temperature difference, it just turns out that the melting point of ice has to be 273.15 K and the boiling point of water is 373.15 K. It's because the temperature difference between the boiling and freezing points is 2.7315 times smaller than the temperature difference between the minimum allowed temperature, the absolute zero, and the freezing point of water.

This number 2.7315 can't be explained in simple words. It is a fact about water, a fact of Nature that may be derived from the laws of quantum mechanics. One has to "simulate" what the water molecules are doing all over this large interval of temperatures and this is just what comes out. We know it is so from experiments.

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Sometimes I wonder why people don't go to a logarithmic scale (kind of like decibels), at least for low temperatures. Then there would be no lower limit. –  Mike Dunlavey Sep 19 '12 at 12:44
Dear Mike, exponential notation is enough to deal with low enough temperatures. What's important is that temperature is just energy per degree of freedom and energy is additive. So all nonlinear redefinitions of $T\to f(T)$ would be unnatural. Also note that for the ideal gas, we have $pV=nRT$, so at a fixed pressure (or volume), the volume (or pressure) is proportional to $T$. We don't express pressures and volumes in terms of their logarithms, either, so it would be equally unnatural for the temperature. –  Luboš Motl Sep 19 '12 at 13:32
The log scale is always a sort of "policymaking issue", making it easy for people to say simple numbers instead of powers of ten which are "complicated constucts" for most laymen. It's the only justification for decibels, pH, Richter scales, and other things: you just don't want the laymen to ever pronounce "ten to the power of [a number]", that's the only reason. But people achieved just a billionth of kelvin, that's the record, and those small numbers aren't extreme and one can express them in words like billionth without using "ten to minus [a big power]". –  Luboš Motl Sep 19 '12 at 13:35
@MikeDunlavey - There is, in fact, a lower limit, regardless of how the scale is calibrated. Temperature is a measure of the transmissible energy contained in the molecules of a substance. It is theoretically possible for said molecules to have no detectable energy (all energy they do have is inherent in their atomic structure, is not transmissible, and to remove that energy would be to destroy matter); that's zero degrees of temperature. We've gotten to within a fraction of a degree last I checked. However, you can't have less than no energy. –  KeithS Sep 19 '12 at 16:17
Dear Tobias, just to be sure, the Celsius scale was defined so that 0 deg C was the freezing point - it was until 1954. With this definition, the triple point was about 0.01 deg C. In 1954, the definition was changed but 0 deg C isn't the triple point. instead, 0.01 deg C accurately was fixed to be the triple point, so the freezing point is still approximately 0 deg C, with accuracy better than 0.01 deg C. See en.wikipedia.org/wiki/Celsius_degree –  Luboš Motl Sep 20 '12 at 9:40

Because it is the Celsius value at which all molecular moverment stops, aka absolute zero or 0 in kelvin scale

http://www.beyondmech.com/physics-mechanis/pm-topic-5.html

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I am not sure what your answer adds to the already accepted one. You also give a link to a webpage, but there should be at least some explanation why you are linking to this specific page. –  Bernhard Dec 6 '12 at 21:37