Don’t try to find the answer to that question in food books. With only one exception, every book in my food library, including those devoted exclusively to microwave cooking, either evades the question entirely or gives the same misleading answer.
Evading the issue only reinforces the less-than-helpful notion of a magic box. But promulgating a wrong answer is even worse.
The ubiquitous non-explanation is that “microwaves make water molecules rub up against one another, and the resulting friction causes heat.” This misinformation rubs me the wrong way, because friction isn’t involved at all. The idea of water molecules rubbing up against one another to make heat is just plain silly. Just try to start a fire by rubbing two pieces of water together.
Nevertheless, you’ll find the friction fiction even in some of the instruction manuals that come with the ovens.
Here’s what really happens.
Some of the molecules in food, particularly water molecules, behave like tiny electric magnets. (Techspeak: The molecules are electric dipoles or, in other words, they are polar.) They tend to line up with the direction of an electric field, just as the magnet in a compass tends to line up with Earth’s magnetic field.
The microwaves in your oven, which have a frequency of 2.45 gigaHertz or 2.45 billion cycles per second, are producing an electric field that reverses its direction 4.9 billion times a second. The poor little water molecules go absolutely nuts trying to keep up by flipping their orientations back and forth 4.9 billion times a second.
In their agitation, the frenetically flipping, microwave-energized molecules bang up against neighboring molecules and knock them around, sort of like the way in which an exploding kernel of popcorn scatters its neighbors. Once knocked, a formerly stationary molecule becomes a fast-moving molecule, and a fast molecule is by definition a hot molecule. Thus does the microwave-induced molecular flipping get transformed into widespread heat.
Please note that nowhere have I said anything about friction between molecules. Friction, if I may remind you, is the resistance that keeps two solid surfaces from sliding freely over one another.
This resistance saps away some of the energy of movement, and that sapped energy has to show up somewhere else, because energy can’t just disappear into nowhere. So it shows up as heat. That’s fine for high-friction rubber tires and even low-friction hockey pucks, but a water molecule doesn’t need to be rubbed by some kind of molecular masseuse to get hot in a microwave oven. All it has to do is get knocked around by a fast-flipping neighbor that has swallowed a microwave.
Oddly, microwave ovens aren’t very good at melting ice. That’s because the water molecules in ice are tied pretty tightly together into a rigid framework (Techspeak: a crystal lattice), so they can’t flip back and forth under the influence of the microwaves’ oscillation, much as they may feel the urge. When you defrost frozen food in your microwave oven, you’re heating mostly the other, non-ice parts of the food, and the resulting heat then flows into the ice crystals and melts them.
If you use a synthetic sponge to wipe up the sink and counter, you may want to sterilize it now and then, especially after you’ve handled raw meat or poultry on the counter (which you shouldn’t do anyway; do it on disposable waxed paper). You could boil it in water, but a quicker way is to put it, dripping wet, on a dish and zap it in the microwave oven for one minute on high.
Be careful in removing it; it’ll be too hot to touch. Some people put their sponges in the dishwasher, but many dishwashers will not reach sterilizing temperatures.