PFE007: Microwaves

Everybody's got one and they are (essentially) necessary for eating nowadays. I'm talking about a pair of thumbs.

Okay, not really, but microwaves are everywhere. But I can assure you that my microwave is not micro, nor wave like (try big and boxy).

Let's first talk about regular ovens. A regular oven is either gas or electric, but either way there's a heat source at the top/bottom that heats the air next to it. As the air moves around the oven, it begins to heat the food - from the surface inwards. This isn't a particularly efficient way of transferring heat to food (it is good at killing stuff that'll make you puke your dinner) as you have to heat up all the air, then slowly heat your way through your chicken casserole.

Microwave ovens were, of course, discovered by accident (1945). Some guy was doing some crazy physics experiment and noted that the chocolate bar in his pocket melted. Whoa. Something managed to heat up chocolate but not his pants? Let's all take a moment to appreciate this.

He then modified his experiment, made some popcorn, and didn't tell anyone about it.

This machine turns electricity from the wall into microwaves into hot leftovers. Each step is tricky and special. To get from electricity to microwaves, you're going to need some fancy stuff to jack up the voltage, but then mainly just a magnetron (that's what originally liquefied the chocolate). Basically, a magnetron spins the electrons round and round. This creates microwaves of just the right length (you have to build your magnetron just the right size to get just the right length). And tada! Microwaves!

The topic of what is, "just the right length" seems to consistently be a question of confusion. In fact, my graduate professor (who's name I won't mention) said in his quantum lecture today, that microwaves are designed to be the right frequency to resonate water molecules. The wavelengths for absortion of water molecules run at about $1\mu m$ or $10^{-3}$ millimeters. That's really, really small.

As it is, microwaves run on a scale of about 5 inches (it's actually about 12.2 cm or 4.8 inches). Why this value? Well, it turns out, that the government regulates this stuff, and did so before they started putting microwaves in everyone's kitchen. This wavelength had been set aside for sciency stuff (I guess why they were using it back then) and if you run at a different length then you can mess up all the cell phones and wifi's and radios and TVs nearby.

You can actually see this for yourself on your own microwave. I looked at my (rather dirty) microwave and found that it lists 2450MHz as the frequency. This tells you how many times the microwaves go back and forth per second. More than 2 billion times. How can you get a length out of this? Microwave radiation is, essentially, the same thing as light, radio waves, and x-rays. They all move at the same speed, the speed of light. So you can use a simple formula to turn that frequency around into: 4.8 inches. Exactly as expected.

Back to the heating of food (let's be real, that's all we really care about). While microwaves aren't designed to resonant with water, they do heat water better than a lot of other things. Because of how water molecules are shaped, they try to line up with the field produced by the magnetron. But the magnetron will flip it's direction very quickly, causing the water to also flip direction.

All this flipping back and forth heats up the water molecules which in turn heats up the rest of the food. As for how the food actually heats up, it basically depends on where the water is in it. (Some other things will heat up too from this effect, but usually it's less). In theory then, your plate/bowl shouldn't heat up too much. In practice: I keep burning myself.

Are they scary? Can they give me cancer? The short answer is no and no. The only way you can hurt yourself from a microwave (outside of violating any of the safety concerns listed below) would be to actually cook yourself. That is, get the microwave to run with the door open. But even then, all that would happen is you would get hot (and probably burned) - essentially no different from sticking your arm in a regular oven. And plus, getting your microwave to microwave your hand sounds like a lot of work to burn yourself. The point here, is that when the door is closed so little radiation leaks out that the OCD FDA doesn't bother to test regularly (see the third and fourth paragraphs for more information).

Safety concerns (seriously):

1) Stuff in microwaves gets hot. I mean come on, but I still burn myself all the time because I'm an idiot.
2) Liquids can superheat. Because the temperatures in a microwave can increase so fast, if the conditions are right, a liquid may fail to boil until it's been picked up causing it to boil suddenly and explosively. I don't know how to try this and it sounds painful anyways.
3) Similarly, if your plastic container is still tightly sealed, it too can explode (this is much more likely). The pressure from the water inside heating up can blow the top off a container while it is inside the microwave or when you go to open it.
Duh.
4) Metal. Okay, so sometimes it's okay to microwave metal. I mean, the microwave is made out of metal, right? It turns out the shape of the metal is really important. Pointy things are more problematic. Since microwaves can push electrons around inside an object, and metals conduct that electricity, a charge separation can grow (think feet+carpet+winter). But if there is a smooth edge along the entire object, that charge won't be concentrated. In the event of a fork, however, the charge builds up on the point ends (your finger on your body). This can cause a massive spark or even a sustained spark. Either way, you're going to get toxic gas and a possible nasty microwave fire. And you'll look like an idiot.
5) A CD is partly metal, partly plastic, and is pretty smooth, so you should be good right? Well, having done this (I can't find mine, but here's another)

it didn't blow anything up, but there was an awful smell, probably from burning plastic. Likely killed a few brain cells. While pretty, there are a lot of prettier things out there.

So yeah, basically if you remember to open you leftover container and don't put any metal in your microwave you should be good to go. Also bad idea (apparently): firecrackers. Sounds like fun though.

That's microwaves.