Don't Use the Popcorn Button

February 5, 2021

So today we're talking about microwaves.

Two events led us here:

Tacos

A few weeks ago I was microwaving some tacos. I happened to be wearing my Sony noise cancelling headphones—probably jamming out to some Caamp.

But then everything went fuzzy. Then back to normal. Then fuzzy...

So I took my tacos out and held my head at varying distances from the microwave. Sure enough, the sound alternated between fuzzy and the excellence I've gotten used to. The closer I was to the microwave, the fuzzier the sound.

Cookies

Last week I was baking some sourdough starter cookies (I'm about 10 months late on this pandemic trend). I had to soften the butter and I'm not patient enough to let it sit and get to room temperature. So microwave it was.

About 30 seconds later, the stick of butter was soft. But curiously, it almost looked like someone held a heat gun in the dead center of the stick. There was a little lagoon of butter.

It's not called a microwave

Technically, the box that heats your food is called a microwave oven. And it heats the food using microwaves. Which are electromagnetic radiation.

Radiation is a scary word because bombs. But radiation is just transmission of energy through waves.

A microwave sits on the electromagnetic spectrum between radio and infrared waves.

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Microwaves are kinda awesome. When you shoot them through food, dielectric heating happens. Water is positively charged on one end and negative on the other.

The electric fields of the wave constantly change direction. This causes the water molecules to vibrate. This vibration releases heat. Which makes your Bagel Bites scald the roof of your mouth.

Most microwaves operate at 2.45 GHz. In other words, the waves change direction 2.45 billion times per second.

So why doesn't your plastic melt in the microwave? Because it doesn't have water in it. Check out the molecular structure of high density polyethylene—the most common kind of plastic.

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If it's been a while since middle school, water is H2O. No oxygen in HDPE means no vibrating molecules. Which means no heat.

Now you might be thinking, "but there's probably not any water in metal. So why can't I put that in the microwave?"

Great question. Remember dielectric heating? It only works on dielectric stuff (i.e. things that don't conduct electric force). All metals conduct electricity. Some do it really well.

So if you nuke a fork, it will do this. And if there's other stuff in the microwave (like Bagel Bites), you're gonna need a fire extinguisher.

Why isn't the microwave oven hot when I stick my hand in?

The microwaves are vibrating the molecules in the food, not the air. Unless there's a ton of water in the air, it won't get hot. This is why you can't have nice things crust.

Some microwaveable packages—like the ones Hot Pockets come in—do have specially designed metal or like-metal reflectors though. These get hot just like a metal fork, except super smart nerds planned it all out so instead of catching your house on fire, it burns a delicious crispy crust on your Hot Pocket.

Ok so what about my butter lagoon?

Remember that the waves are changing direction at 2.45 GHz (AKA 2.45 billion times per second). Now look at this.

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Imagine a bunch of those shooting around your microwave. The peaks and troughs are the hottest points. And they're trading places 2.45 billion times every second.

But where the wave intersects the line is functionally always going to be in the same place. Since there's no change there, it's not going to affect the molecules in your food.

If your microwave didn't have a spinning turntable, you might have seemingly random bits of cheese on your nachos that didn't melt. Ideally, the spinning counteracts that.

But my butter lagoon? It was perfectly placed. I put the bowl almost dead center in the microwave. In my microwave, that's a hot spot. And because that's also the point around which the turntable rotates, the butter never escaped the hot spot—resulting in a perfectly shaped dairy lake.

Ok well what about your headphones?

Bluetooth and microwaves use the same frequency—2.45 GHz. So the microwaves were screwing up the connection between my phone and headphones.

Why was it on and off? Microwaves—at least all the ones that I've ever used—aren't always producing microwaves. Turn your microwave on for a minute and listen. You'll likely notice intervals where it's "working" and when it's not.

If you still want to go deeper and find out how your microwave actually makes the microwaves—it's called a magnetron, watch this video (6.5 minutes).