Wee Knight Science: How do you cool something?

I think we can all agree that refrigerators are made of magic. But, it may surprise you to know that there is also some science sprinkled in for funsies. Since I never got my letter from Hogwarts, I can't tell you about the first point, but I have journeyed far and wide (to the other side of the lab where the physicist--my co-blogger!--sits) to discover the secrets of

Let us begin with a quick refresher on thermodynamics. Hey, get back here! I swear it's not scary or hard. What it is is a great word to use to make your friends think you're smart. Look, we're just going to use this teensy equation, very harmless...

PV = nRT

where P is pressure, V is volume, T is temperature, n is number of molecules and R is the gas constant.

Super easy, right? Pressure, volume and temperature are proportional to each other! So, if we increase the pressure, either volume will decrease or temperature will increase--or, more likely, a little bit of both. Let's pop some numbers in there so you believe me. I'm going to ignore n and R for simplicity.

Let P = 5, V = 6, T = 30

P*V = T, therefore 5*6 = 30 (woo!)

Now, P = 6. Three options:

(1) V decreases to 5

(2) T increases to 36

(3) Mix of 1 and 2

What's that? You'd like a real world example? Alright, science fans, I whipped this one up just for you! Let us turn to our faithful friend, water.

Is that not just fabulous? Doesn't it make you feel good just looking at it?

As you know, if you take a room temperature pot of water and increase the temperature until it boils, it will turn to steam. Now, the temperature of the steam won't rise until all of the water evaporates (for reasons I will not get into because they aren't that interesting to most people). However, you will note that steam takes up a LOT more space than water does, which--oh my goodness here it comes--decreases the pressure.

Did you see what I did there?!

Why does it decrease the pressure? Because the molecules are farther apart! If you take a packed bus full of people and spread them out among 5 buses, those people feel less pressure on them from fellow bus-goers. Same deal with water and steam. Boom. We just proved that PV = nRT, the ideal gas equation, works in real life.

Now, armed with this thermodynamical knowledge, we are fully prepared to ~~battle~~ understand

If you have a refrigerator and you ever bothered to look at the back of it, you probably saw something like this: