Cooksbakesbooks asked via Twitter:
"Kitchen mystery: Popcorn kernels pop using cell phone energy; is this real, or a hoax? http://go.unl.edu/ocd"
And I answered briefly, but I saw nicoleradziwill wrote,
"can cell phones cook popcorn? so bizarre, i'm going to have to try this: http://twurl.nl/c4chr3 via @TS_Elliott"
The video in question is similar to the following (note that there's some strong language after the 2 minute mark, so stop before then if you'd rather not hear it. There's not much new after the first couple of iterations):
This particular mystery plays off of two basic ideas:
- Most people don't entirely know how microwave ovens work;
- There is lingering concern over the dangers of cell phones radiation.
Let's talk microwave ovens first. Microwave ovens work by sending out radio waves at one of a number of frequencies. For home microwave ovens, this is 2.45 GHz. The oven pulls in your AC power, converts it from a relatively low-voltage power source to a relatively high one, and then feeds that voltage into a magnetron, which turns the electricity into radio waves.
Some of the molecules in your food are dipoles, which means that they're basically like magnets. One side of them is positively charged electrically, and the other side is negatively charged. When the microwave sends its radio waves into your food, it alternately stimulates the positive and negative poles of these molecules. It's like having two electromagnets, one above your regular magnet, and the other below. Turn the top one on, and one end of your magnet will be attracted to that top magnet. Turn the top one off and the bottom one on, and the regular magnet will flip upside down. Do that over 2 billion times a second, and the magnet will vibrate quite a bit.
As we know from our various kitten analogies, such as the ones in Reducing Complexity, we know that heat is molecular motion. So when the microwave transfers energy into food, it increases the vibrations, and the food heats up. The waves travel into the food a bit differently than radiant heat (such as from your oven or grill) because the radio waves can go into your food up to a few inches before stopping, whereas radiant heat stops at the surface. This causes a band of heat to hit the top layers of your food much more rapidly than you would get from radiant heat, but after that the heat has to travel by the relatively slow con duction method in order to get to the bottom layers of your food.
What this means from a cooking standpoint is that, if you're heating up some soup or mashed potatoes, you want to stop the microwave and stir every few minutes to spread the heat more evenly throughout the food before continuing the cooking. If you're heating something solid, you want to ensure that it's either very thin or that you let the food rest for a while in between heating to keep from overcooking the first few inches before heating the lower levels of the food.
So, now let's talk cell phones. Based on my description above, can you guess the major difference between the microwave oven and four cell phones? You may think frequency of the waves at first, and it's true that microwave ovens in US homes don't match cell phone frequencies in the US, there are some ranges of cell phones that can match some ranges of microwave oven frequencies.
No, the big difference is power. You can spend all day under a lamp with a full spectrum lightbulb and not get worse than a little warm, but spend an hour or so outdoors and you are likely to come away with a sunburn. Concentrate that sunlight with a magnifying glass, and you can set things on fire. It's the same with a cell phone and a microwave oven.
The microwave oven has to draw a huge amount of power to cook something, but a cell phone doesn't need anywhere near that amount of power to communicate with a cell phone tower. If a cell phone were capable of cooking popcorn with its radio waves, you'd feel it warm you up whenever it would ring. Cell phones generate more waste heat just using the battery than they do from the radio waves they send out.
A quick update to mention that I told nicoleradziwill that the Mythbusters had busted this one, but they had declined to bust it on account of it's general kookiness. As Grant says, you have to heat the popcorn up to 220°F in order to pop it, and cell phones just don't do that. Sorry about misleading you there, nicoleradziwill, I misremembered the episode.