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Temperature conversion:

$$273 + \text{degree Celsius} = \text{Kelvin}$$

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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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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    $\begingroup$ 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. $\endgroup$
    – Mike Dunlavey
    Commented Sep 19, 2012 at 12:44
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    $\begingroup$ 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. $\endgroup$
    – Luboš Motl
    Commented Sep 19, 2012 at 13:32
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    $\begingroup$ 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]". $\endgroup$
    – Luboš Motl
    Commented Sep 19, 2012 at 13:35
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    $\begingroup$ @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. $\endgroup$
    – KeithS
    Commented Sep 19, 2012 at 16:17
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    $\begingroup$ 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 $\endgroup$
    – Luboš Motl
    Commented Sep 20, 2012 at 9:40
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The Kelvin temperature scale is an absolute temperature scale. That is 0 K is absolute zero. It also has the property that temperature intervals on the Kelvin scale are the same as on the Celsius scale. That is a decrease or increase of one Kelvin is the same as a decrease or increase of one degree Celsius. To meet these two requirements it is necessary to choose 273.16 K as the temperature of the triple point of water. According to Halliday and Resnick, "Fundamentals of Physics" second edition, 273.16 K was adopted as the temperature of the triple point of water at the Thirteenth General Conference of Weights and Measures in 1967 at Paris.

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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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    $\begingroup$ 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. $\endgroup$
    – Bernhard
    Commented Dec 6, 2012 at 21:37

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