Science In Real Life

Usually the titles for these essays refer to a substance or idea that we’ve all at least heard of. It’s hard to wonder about the science behind a concept one has never been exposed to. That’s why if you’ve read at least this far, I’m rewarding you with a subtitle: Or, the Science of Your Refrigerator. There now, isn’t that better? The principles I’m about to elucidate will also explain the science behind your car, anything else that does work, and why we can never have a perpetual motion machine, no matter how good you were this year.

So, what does a refrigerator do? The hunky delivery guys from Whirlpool have just installed my brand new shiny refrigerator. I fire it up and I put in a hunk of cheese. The cheese was at room temp, and after an hour in the fridge it’s at a chilly temperature of whatever-I-set-the-fridge-to. Where did the heat go? Remember from the essay about heat that there is no such thing as cold, and heat is the averaged-out random motions of atoms and molecules. Setting aside the utterly silly notion that the refrigerator dumps cold on the cheese, it must therefore remove heat.

It does this by allowing the heat from the cheese to flow to something else. Fortunately for the continued preservation of our food, different substances switch from liquid to gas and back again at different conditions of temperature and pressure. There are tubes in your refrigerator containing a coolant. As the coolant passes through the section of tubing that is in contact with the main volume of the refrigerator, it absorbs energy and becomes a gas. The molecules of cheese bumped into the molecules of the fridge shelf, which in turn bumped molecules of the casing, and so on until the molecules of coolant were bumped out of their liquid state and into a gaseous one. Everybody do the bump!

So now we have cold cheese. But the process isn’t over, or else we’d only be able to cool the cheese once. The gaseous coolant now flows through a compressor. Powered by electricity, the compressor, well, compresses. It applies pressure to the coolant until it becomes a liquid again, and the process repeats. But unlike the cheese, this is no free lunch. You can’t just remove heat and make it go away. The process of compressing the coolant A) costs energy, and B) generates heat. It all has to do with a concept called entropy. Entropy is a measure, loosely speaking, of disorder. It measures the number of possible ways a system can be rearranged without changing its fundamental state.

Quick, switch two air molecules. Is the air in the room any different? No, you can still breathe just fine. Since you can switch any two (or three, or eleven, or six hundred thousand) air molecules without really changing how the air interacts with other objects, the air has a lot of entropy. Imagine an ice crystal. Swap two frozen water molecules and the crystal structure has to break. Swap enough of them and the whole ice cube falls apart. So the ice cube has low entropy, or at least it did before you went and put it in your soda.

Back to our coolant. Liquids have lower entropy than gases do. So I have my gaseous coolant and I want to make it a liquid. I know, let’s use a magic gremlin! I hire him (they work for lint and firstborn children, so I don’t have to pay him much) to take all the coolant molecules one by one out of their gas and into a liquid state. This is hard work. Magic gremlins have lots of moving parts inside them, and as he does this those parts rub up against one another, creating friction. Friction leads to heat, heat leads to entropy and entropy leads to the dark side. Replace “magic gremlin” with “compressor” and “firstborn children” with “electricity” and you can see why your refrigerator actually raises the temperature of any room it sits in. Ever reach behind a fridge? It’s hot back there.

Let’s generalize this. Your refrigerator is what we physicists call an engine running in reverse. It’s taking a mechanical work process (the compression of the coolant) and using it to move heat. If we were to run the engine forward, it would take heat and use it to do mechanical work. This is where your car comes in. Igniting the gasoline generates heat, which moves the piston, which turns the axles, which rotates the wheels, all in the house that Jack built.

All energy sources work this way: find a process that can be used to generate heat, then use the heat to move matter. If we want electricity, we use the moving matter to power an electric generator (see hydroelectric power). But there are limits. You can abstract the processes of any engine into a cycle between two heat sinks, one hotter than the other. By moving heat from one reservoir to another, we can trick it into doing work along the way. But as I’ve illustrated, the very nature of mechanical work is such that additional heat is generated. There is no escaping it. Insulation is futile. Any would-be perpetual motion machine is eventually worn down by its own waste heat.

There are various types of thermodynamic cycles that generate work from heat. But one is special; it is called the Carnot cycle. It is special because it it possesses the maximum possible efficiency a cycle can have. There is a somewhat mathematical proof that boils down to: try to construct a more efficient cycle. G’won, try. Surprise, it turns out to be the same cycle. See here (PDF) for details, but be warned — that way lies mathematics.

The rule about a Carnot cycle being of maximum efficiency leads directly to the second law of thermodynamics, which is a formalized statement of what I’ve been saying all along: Entropy always increases. The process of trying to decrease entropy increases entropy. To wrap this essay up, I give you the three laws of thermodynamics in both their formal physics wording, and in a form more applicable to daily life.

1) Energy can neither be created nor destroyed. <—> 1) You can’t win.
2) The entropy of a system never decreases.    <—> 2) You can’t break even.
3) As temperature approaches absolute zero, entropy approaches a constant minimum. <—> 3) You can’t quit the game.