How Do Things Cool With Evaporation?
You know water evaporates – that’s when it turns from a liquid to a gas. You probably also know that a hot pot of water will cool off in part because of evaporation. However, did you know that a cup of water at room temperature will also cool off? Yes, even if the water starts at room temperature it will cool off to below room temperature. I think this is awesome.
But how does this work?
Insert an NMR tube holding a cm^3 of water into a 300 MHz nuclear magnetic resonance spectrometer, a uniform 7.046 tesla magnetic field. Water’s thermodynamic nuclear temperature promptly goes beyond infinity (K, C, F, or R – your choice) to a very large negative temp kelvin. Cf: adiabatic demagnetization of paramagnetic salts.
The population inversion at room temp, 300 K, is about 48/million protons excess, curiously (10^6)[(e^(1/k)-1] where k = Boltzmann’s constant in appropriate units. Cool!