The Skin Effect isn't what it sounds like, but it can help us understand what happens to a human being when they're exposed to different microwave frequencies. It also explains why your food comes out cold on the inside.
Image by Jim Cooke, photo via Winai Tepsuttinun/Shutterstock
Ever snipped a wire and seen that it's a hollow tube? Ever wonder why? The Skin Effect gives you your answer. Run a direct current - all the electrons rushing continuously from one place to the other - through a wire, and the current will move through the entirety of the wire. An alternating current - when the electrons get yanked back and forth very quickly - is different. An alternating current runs along the outside of an conductor. It rarely penetrates to the center. If an appliance, or a wire, is always going to use an alternating current for power, there's no reason to make a solid wire when only the outsides are going to be used.
The depth at which the electromagnetic field is reduced to 37% of its power on the surface is called the skin depth. The skin depth decreases as the frequency increases - so lower frequencies have more penetrating power than higher ones. The higher the permeability of the material, the shallower the skin depth. And the more conductive the material, the shallower the skin depth. So, if you wanted to shield someone from a very high frequency, you could cover them with a thin wall of material - and it could be any material. If you want to shield them from low frequencies, you would have to put up a thick wall of material, and it would probably have to conduct electricity very well.
The Skin Effect explains some of the baffling contradictions that come up in microwave cookery. Put one of those cheese-filled pastries in a microwave and when the timer goes off its center will be seconds away from nuclear meltdown. Put a lasagna or a piece of chicken in the microwave and half way through your meal you'll bite into something that's just been hacked out of a glacier. How can one heat so thoroughly and the other stay cold in the middle?
Part of this is due the way the microwave works. It creates an electromagnetic wave that makes liquid water molecules vibrate. The watery parts of a piece of food will heat up in a microwave, and heat spreads through water very well. The more watery the food, the more evenly it will heat. If the food is dry - or frozen - such as crust, it's pretty much indifferent to the electromagnetic waves, and it will hardly heat up at all. If the food is filled with a moist material (like cheese, or even better, cheeze) that material will heat up enough to cook the outside crust with a little heat leftover to scorch your tongue.
Take a food that's not particularly watery, but still a decent conductor, and the Skin Effect keeps anything from penetrating to the inside of the food. A large piece of meat will get hot on the outside, just the way it would in a regular oven. That heat will have to slowly penetrate inwards, also like a regular oven, so if your microwave is weak, or the food is particularly big or cold, it will still be cold on the inside.
So, what does this tell us about a human being in a microwave? Well, no matter what, it wouldn't be a pleasant experience. But it probably wouldn't cook your internal organs. Most of the damage would be superficial. (It would probably be a good idea to tightly cover your eyes, which are more watery than most things in your body.)
According to Microwave Interaction with Biological Tissues:
If a human being, for instance, is submitted to a microwave field, the internal organs are more protected at higher than at lower frequencies. As an example, the skin depth is three times smaller at 900 MHz, a mobile telephone frequency, than at 100 MHz, an FM radio frequency, which means that the fields are three times more concentrated near the surface of the body at 900 MHz than at 100 MHz.
Restaurants use microwave ovens that employ frequencies of about 900 MHz, while home microwaves have even higher frequencies, of 2.45 GHz. Assuming you get stuffed in one, it may concentrate on the very outside of your body, leaving your internal organs unscathed.
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