|From: John||5/05/99 12:39:02|
|Subject: Absolute Zero||post id: 9674|
|I'm curious if anyone knows why Zero Kelvin is the lowest temperature. Does this mean we can't achieve a lower temperature in the lab or is it a scientific fact?|
Have we ever achieved absolute zero?
Also, is there any element/compound that will not freeze at this temperature?
|From: James Richmond (Avatar)||5/05/99 12:57:12|
|Subject: re: Absolute Zero||post id: 9678|
|The temperature of a substance is directly related to the energy of its molecules, which in turn depends on how fast the molecules are moving around, rotating and vibrating. Zero Kelvin is the temperature at which all molecular motion stops, so it is an absolute limit on temperature.|
The rules of quantum mechanics, in particular Heisenberg's uncertainty principle, actually prevent any molecule from ever being absolutely stationary. So it is not possible to cool anything down to absolute zero, although we can get very close. Laser cooling of atoms can achieve temperatures only a few billionths of a degree above absolute zero.
From memory, helium liquifies at normal pressure at about 4 degrees Kelvin. At normal pressure, it does not freeze (become solid) between 4 and zero degrees.
|From: Chris W (Plebeian)||5/05/99 13:01:21|
|Subject: re: Absolute Zero||post id: 9679|
|How was the temperature derived/fixed?|
|From: James Richmond (Avatar)||5/05/99 13:31:11|
|Subject: re: Absolute Zero||post id: 9688|
|The technical definition of the Kelvin temperature scale is based on pressures exerted by gases.|
A reference temperature is chosen and all other temperatures are compared to it. The particular reference used is the triple point of water, which is the particular pressure and temperature at which liquid water, solid ice and water vapour exist in equilibrium. This temperature we call 273.16 Kelvin.
This choice incidentally sets the size of one Kelvin as 1/273.16 of the difference between absolute zero temperature and the triple point temperature of water.
A degree Celcius has the same size as a Kelvin, but is shifted to make the freezing point of water 0 degrees. Therefore
OC = K + 273.15
The value of 273.16 was chosen for the triple point of water in order to make the boiling point close to 100 degrees Celcius (it's actually 99.975 degrees Celcius).
|From: Chris W (Plebeian)||5/05/99 17:24:55|
|Subject: re: Absolute Zero||post id: 9729|
|I think I missed the essence of the last post.|
We picked the triple point of water as a reference point... that fixes pressure, volume and temperature OK. I fail the next leap:
defining what we are dividing into 273.16 Kelvin?
Is it an extrapolation, using PV/T = constant, from the pressure at the triple point temperature to the point at which the pressure becomes zero?
(I guess that PV/T is undefined at absolute zero)
|From: ted||6/05/99 3:36:45|
|Subject: re: Absolute Zero||post id: 9782|
|T in that equation if memory serves is actually in K rather than C|
|From: James Richmond (Avatar)||6/05/99 23:10:18|
|Subject: re: Absolute Zero||post id: 10070|
|Is it an extrapolation, using PV/T = constant, from the pressure at the triple point temperature to the point at which the pressure becomes zero?|
Yes it is. Sorry I didn't make that clear. Other temperatures are defined by
T = 273.16 x (P/P3)
where P is the pressure measured in a gas bulb thermometer, at a given temperature T, and P3 is the pressure when the bulb is immersed in a triple point cell.
We can take a few measurements and extrapolate to zero pressure.
|From: Chris W (Plebeian)||7/05/99 9:14:27|
|Subject: re: Absolute Zero||post id: 10095|