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Name: andrew s phillips
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1999 


Question:
How did scientists come up with the exact figure -273.15 degrees celsius for absolute zero?



Replies:
Gee, I just gave a lecture on this topic today! OK, here's how the Kelvin temperature scale is defined; the triple point of water is a particular pressure and temperature at which ice, liquid water, and water vapor can all coexist simultaneously at equilibrium, in principle, staying that way forever (or until disturbed). The Kelvin scale assigns the temperature of the triple point to be exactly 273.16 K (NOT 273.15, note). Moreover, in the Kelvin scale temperature decrements are assigned such that the zero of temperature (called absolute zero) is at 0.000000...K, or in other words, if the temperature changes by 1/273.16, it has changed by 1K. This choice of the Kelvin scale makes it a "thermodynamic" temperature scale (any scale which goes to zero at the same temperature that an ideal gas would collapse to zero volume is a thermodynamic scale). On this scale, the melting point of ice is at 273.15K (at 1 atm) and the boiling point of water is at 373.12K (also at 1 atm). If we now DEFINE the Celsius scale T(C) = T(K) - 273.15, we end up with the melting point at 0.00 C and the boiling point at 99.97 C. Thus, we see that the reasons that the number 273.16K was chosen for the triple point's temperature were (1) it places the melting point exactly at 0.00 C and more importantly, (2) it forces the boiling point to be at roughly 100 C, which was the origin definition of the Celsoius degreee before the triple-point standard was chosen.

Whew! Hope this helped.

-prof topper



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