Like if I spill water and it’s dry when I come back in a couple hours how did it do that without getting up to its boiling temp?
Is it the same type of thing like when my hair is wet and it’s in the process of drying? Because it isn’t at the 212° F or I’d feel it, right?
In: 3
Something you have to understand is that “evaporate” and “boil” are not the same thing.
Fluids can be thought of as like a bunch of tiny marbles sloshing around in a huge bowl, like a mosh pit. They can flow past one another and bounce around all they like, but they’re contained by the walls of the bowl. But the top of the bowl isn’t closed–it’s open. All the marbles need is enough speed to fly out of the bowl and not come back. With just the right series of lucky ricochets off of other marbles, one marble can occasionally get knocked with enough speed to fly out of the bowl. That’s essentially what evaporation is.
When a marble does get knocked out of the bowl like this, it necessarily has to do it at the expense of the marbles it knocked into just before it peaced out. Those marbles are now moving slower than they were before. The average energy of all the marbles in the bowl combined has thus gone down with the stray marble flying off.
Since temperature is just the average speed of all the marbles in the bowl (and also, the vibrations of the bowl itself, which will help keep the marbles jostling), and losing a marble makes the total energy go down, the temperature has also decreased. This is why a damp towel feels cool to the touch, or why you feel so cold when you step out of the shower. Water evaporating off of you is stealing your heat away. This is also how sweating keeps you cool.
Water, and any other fluid for that matter, can evaporate an essentially any temperature. Particles just need that lucky kick to get away. The colder the fluid is, the lower the chances this will happen, of course, since all the marbles that are there to ricochet off of will be moving slower overall. So warmer fluids will evaporate faster than cooler ones.
“Boiling” is the point where *all* the particles in the fluid are going fast enough that *all* of them can escape without needing a boost from a lucky ricochet. It’s not the point where evaporation starts, it’s the point where evaporation *must* happen. So, a fluid cannot get hotter than its boiling point. Any extra energy you dump into the fluid to make it warmer only makes it evaporate faster.
Latest Answers