Podcast: Play in new window
It’s this super-popular idea (which is good) which is often misunderstood (which is bad). It’s the reason an icecube melts in a glass of warm water. It’s the reason you can’t get an engine which is 100% efficient.
Our guest today is the author, blogger, and Wikipedia article subject Cory Doctorow. I got him onto the show by promising that I could explain death to his child. He did not realize two things: 1. you should not ever let me explain anything to your children, 2. I would spend most of the time explaining thermodynamics.
anyway, it’s a lovely show! If you listen past the end music, you’ll hear us trying to tell him how to explain death to a child.
Physicists: Jocelyn Read, Miles Steininger
Intro Music: Ted Leo and the Pharmacists
Exit Music: John Vanderslice
Cory asks an interesting question during the show, and only as I edited the show did I realize that it’s got a really pertinent answer. He asked us how evaporative cooling works. the answer is: Entropy and statistical mechanics. I don’t know why i forgot to tell him. anyway, here’s how it works (copied and pasted from an email I wrote. forgive the punctuation)
Imagine that you have a brick, and it’s 50 degrees C, and you pour some water on it, so the water temperature is 50 degrees C.
here’s where statistical mechanics and entropy come in.
the description of the water being 50C is NOT a description of the energy of each and every molecule in the body of liquid. it’s the average energy. So, among the molecules composing the puddle of water, some have an energy above 50C, some will have an energy lower than 50C. Indeed, some will even have an energy above 100C.
If those particular molecules are near the surface of the puddle, they can break away and turn into vapour.
so a body of water will always have SOME molecules which are evaporating away from it, even if the temperature is well below the boiling temperature.
So, because the temperature of the puddle is the average energy of the molecules, when the high energy molecules leave, the average energy decreases.
Like a rich person taking their money and moving it into the bahamas or something.
incidentally, a puddle of water in a closed room will usually reach an equilibrium. just as some water will evaporate, some of the water vapour will be statistically cold enough to condense.
so a puddle of water will cool until there’s enough vapour above it to make an equilibrium.
so what happens if you put a fan up to a puddle of water?
by blowing fresh air over the water (or the kid who just got out of the pool) you’re keeping the water from being in equilibrium with the air around it, and the puddle of water will just keep cooling due to statistical evaporation.