On 17 Apr, 10:20, demerzel <girol..._at_studenti.ph.unito.it> wrote:
> Salve, ho notato fra le proposte di tesi triennale di un mio prof di
> MQ una dal titolo "Sistemi a temperatura assoluta negativa". Non sono
> riusito a rintracciarlo per chiedere spiegazioni, qualcuno potrebbe
> spiegarmi in breve (o anche in lunga) la faccenda?
http://www.thenakedscientists.com/forum/index.php?topic=7285.0
"I know that negative temperatures do exists. It's explained even in
Zemansky's Thermodynamics:
temperature can be defined as:
T = ∂U/∂S
where U is the internal energy and S is the entropy.
In lasers, when the system is in "population inversion":
http://en.wikipedia.org/wiki/Population_inversion
the temperature so defined is negative, since U increases while S
decreases (because of the more ordered state given by the population
inversion).
Said simply: you increase the energy of the lasing system making
atom's electrons go up to higher energy levels. But in this way, when
a lot of them are in these higher states, the system is more ordered,
so the total entropy is lower.
So, during that process, ∂U is positive (U increases) while ∂S is
negative (S decreases).
So T is negative.
In this case, however, the system is not colder than a 0°K system,
but, actually, hotter!
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/negativeTemperature.html"
Received on Thu Apr 19 2007 - 14:18:58 CEST