First of all, a microwave only heats water. And you probably know that all our food contains a certain amount of water, even the driest food still contains water. So by heating the water in our food, the food is heated.

Now for the explanation of why only water is heated, we first need a definition of heat. Actually, heat is no more or no less than the vibration of molecules and atoms. The heat you feel are only atoms vibrating in an object. The faster they vibrate, the hotter the object is.

Let me first explain how a normal cooking plate works. The atoms of the cooking plate start to vibrate as the plate heats up. They collide with the atoms of your cooking pot, and those atoms collide with your food, heating it up.

Now, why does a microwave heat water? To understand that, we must first answer the question “what is water?”. You probably all know the beautiful chemical formula of water, H2O. This means that our water molecule exists of two hydrogen atoms (H2) and one oxygen atom (O). See the image below.

Four electrons orbit around this molecule. The page linked below is an animation showing the movement of one of the four electrons. As you can see, the oxygen atom is larger, and therefore stronger. Because it is stronger, the atom pulls the electron more than the hydrogen atoms. This means that the electron will be around the oxygen atom more often than around the hydrogen atoms. When there are more electrons around the oxygen atom, the oxygen atom will be slightly negative, since electrons have a negative charge.

Click here for H2O electron movement animation

A microwave heats the food by producing, guess what, microwaves!

The next question we need to ask ourselves is “what are microwaves?”.

The answer is very easy; a microwave is a waving electric field. The animation linked below shows the movement of a microwave.

Click here for microwave animation

Since a microwave is an electric field, it has a positive and a negative side. Now, when the microwaves go through the water molecules, the water molecules will move along with the electric field from the microwaves since they both have an electric charge.

So actually, the water molecules start spinning around. Spinning around is movement, and what have we just learned? Movement = heat!

That way, a microwave heats food.