Every theorist (read that theoretical physicist) knows that without coffee there is no physics. Well, kind of. Every time I visit another physics department, after I am shown my office and how to get my computer connected to the web (also indispensable for physics), I am taken with reverence to the local coffee machine and I am explained with great detail on how it is supposed to be operated. Coffee seems to be the lubricant of good ideas.
So there it is. Now, no physics yet. Look carefully back and you will see that I have talked about physicist’s habits, but not really about physics.
So I thought it would be nice to describe how some really nifty coffee machines work. After all, there is a lot of physics in there that is worth expressing, and then it seems that they work by magic.
What I have in mind are some Italian Espresso machines. These are the simplest machines. They have no moving parts. You add water at the bottom, coffee grounds in the middle, you put them in a stove or some other source of heat, and a few minutes later you have coffee on the top.
So I drew a schematic of the machine, indicating the parts of it. There are no moving parts per se. The idea of how it works is simple, not magic. You heat the water at the bottom. Once you have injected enough heat in the bottom, you form a cloud of water vapor. Evaporating water costs heat. If you keep on applying heat, you get more water vapor, and the pressure of the water vapor rises. Once you have enough pressure, it pushes the water below it. The water is not very compressible, so it looks for a way out, and it finds it going upwards. There, the water is met with an obstacle. The coffee grounds. To get through it has to percolate. (Remind me to talk about percolation some other time). During this process it collects all the goodies from the coffee grounds. After it percolates, it keeps on rising. It still rises because the pressure from below has not ceased. So the coffee climbs and climbs, until it meets the exit and is collected on the upper compartment. The bonus: the upper compartment is cooler so it does not boil the coffee, which would make it taste horrible.
Voila! You have great coffee and a physics lesson. Some other time I will write a bit of the equations regarding this.