How a Microwave Heats Food

Open the door of a microwave and there is nothing inside but a glass tray and a metal box. No flame, no hot coil, no obvious source of heat. And yet a cup of cold water comes out steaming in ninety seconds. Where does the heat come from?

It comes from the food itself. Specifically, from the water inside the food. To see why, you have to know one strange fact about a water molecule: it is lopsided. The oxygen end of the molecule has a slight negative charge, and the two hydrogen ends have a slight positive charge. A molecule with two charged ends like this is called a polar molecule. You can picture it as a tiny magnet, except the poles are electric instead of magnetic.

Now, inside the metal box of the oven, a part called the magnetron is producing radio waves at a frequency of about 2.45 billion cycles per second. These are a kind of electromagnetic radiation — the same family as light and radio, just at a particular wavelength. The wave is essentially an electric field that flips direction back and forth, very fast. When it points one way, the positive end of each water molecule gets tugged forward and the negative end gets tugged backward. A tiny fraction of a second later, the field flips, and the molecule has to spin around to follow it.

So the water molecules in your food are being yanked back and forth, billions of times a second, trying to keep up with the flipping field. As they twist, they bump into the molecules around them. Those collisions are what we call heat. The official name for this whole process is dielectric heating: an oscillating field causing polar molecules to rotate, and the friction between molecules turning that motion into thermal energy.

This explains a few things you have probably noticed. A dry ceramic plate does not get hot directly, because it has almost no polar molecules to spin. It only warms up because hot food sitting on it conducts heat into it. A piece of pizza, on the other hand, has a soggy center and a drier crust, so the center heats much faster than the edges. And ice melts surprisingly slowly in a microwave: in solid ice, the water molecules are locked into a crystal and cannot rotate freely, so they barely respond to the field until a little liquid water forms and starts the process.

There is one more piece. The waves bouncing around inside the oven do not fill the space evenly. They overlap and cancel in some spots and add together in others, making a pattern of hot zones and cold zones. That is why food heats in patches, and it is why the turntable exists — to drag the food through the pattern so no single cold spot stays cold.

A microwave, then, is not really an oven in the old sense. It does not heat the air around your food and let the warmth seep in. It reaches through the food and shakes the water directly.