How does evaporation produce its cooling effect? (For example: licking the hand, then blowing across it.)
Preliminary theory:
Pretend liquid water, say, is made up of submicroscopic particles. Pretend these mass points are in constant, random motion. Temperature (T) measures the violence of this motion - faster motion equals higher T. The molecules must attract each other; otherwise the liquid would almost instantly disappear - turn, suddenly into a gas.
The three mechanisms of cooling:
1. "Increasing the number of particles":
A lump of liquid water is, in a sense, a single particle. The gas it becomes when it evaporates is, in a sense, billions of particles. There has been an increase in the number of particles.
Defining temperature more precisely: let T be proportional to the kinetic energy (Ek) (Ek is a measure of the "smashing power" of a flying brick, say.) of the average particle.
If a box has two particles flying about inside it randomly, like ricocheting rifle bullets, and each particle has an Ek of 2, then T = 2. If, now, the two molecules collide and stick together - forming a single, heavier particle - then T = 4. (The two stuck together have the sum of the Ek's of the two on their own.) The temperature has risen from 2 to 4. Equally the reverse process - an increase in the number of particles - would lower T; would produce cooling.
Since evaporation involves an increase in the number of particles there is cooling. This presumably has something to do with entropy. (Sporus admits not knowing how to count particles, even "in principle".)
2. Useless Work:
Turning into a vapor involves a large increase in volume. The atmosphere has to be pushed back to make room. It resists with its own pressure. So creating this hole in the atmosphere to accommodate the new gas requires work energy, a la "force x distance."
When each particle escapes the liquid and enters the atmosphere it has to push atmosphere particles out of the way and further apart. This slightly increases the volume of the Earth's atmosphere - the top of the atmosphere is pushed slightly further out into space. Lifting the atmosphere requires work to be done against gravity. This work comes from the evaporating particle pushing the atmosphere particles upwards, using its own Ek and therefore becoming slowed - cooled.
3. Escaping the liquid may raise a particle's potential energy (Ep):
If a slow moving particle shoots away from the surface of the liquid into the atmosphere in a bid to escape, it will only succeed if it has sufficient speed. Too little Ek and it will not be able to overcome the attraction of the particles, behind it, still in the liquid. It will fall back into the liquid like a ball thrown up into the air falls back to Earth. Thus it is the particles with the greater Ek that escape first.
This leaves behind a liquid consisting in the slower moving - colder - mass points. The liquid cools.
Prima facie the vapor given off as the liquid evaporates is also cool, since the escaping particles decelerate as they fly free: like a ball thrown into the air. But this overlooks the possibility that at longer ranges the attractive forces between the particles change to repulsive - accelerating the particles again up to higher speeds. (Such an arrangement of forces is common.) Thus the vapor might be cooler or warmer or neither.
Whether the net effect: (Cooling of liquid + Temperature change of vapor) is cooling or warming or neither depends on Ep(gas) - Ep(liquid). If this is positive there is cooling. (See: Latent Heat.)
The sum of effects 1, 2 & 3 produces the net cooling.