How do microwave ovens work?

posted Nov 30, 2014, 2:51 PM by Patrick Poole   [ updated Dec 1, 2014, 7:21 AM ]
Face in the microwave
I hope you had a wonderful Thanksgiving full of friends, family, and food. Now that you’ve awoken from post-feast slumber, you might be lucky enough to still have heaps of leftovers to eat for the next few days. I bet you’ll warm those things up in your microwave oven—have you ever wondered how that works so much faster than your regular oven? Or, why it’s not a good idea to put metal inside?

Water molecules are polar, in that one side is more positively charged and one more negatively charged

You may know that a water molecule is comprised of three different atoms: two hydrogens and one water, hence H2O. Water has widely different properties than hydrogen or oxygen would alone—both of these are present in their gaseous phases in air, and won’t condense down into their liquid forms unless you get them incredibly cold. But as soon as they bond together in just the right way they form water, which is liquid at room temperature, and can be solid if it’s only made a little bit colder.

Mickey mouse ears

The chemistry and physics of why water is different than its constituent parts stems from the bonds that hold the molecule together. In the case of water the two hydrogen bonds tend to make the water molecule hold a certain shape, sometimes described as “Mickey Mouse ears”. As a result of this shape, the two hydrogens are bunched on one side, making that more negatively charged, while the positive oxygen dominates the opposite side. As a result water is called a polar molecule, in that it has these positive and negative poles (sort of like North and South poles on a magnet).

Molecules can vibrate in different ways, any of which can be described as heat

We’ve talked before about atoms and molecules vibrating. Hotter materials will tend to move more, but there are a few ways in which that motion can occur. Molecules may be able to translate (move side to side, in and out, or up and down) or rotate (spin about different axes) within the bulk material, but some of these motions are inhibited by the neighboring atoms or molecules, especially in a solid material. Even if they can’t move around in space, the atoms in a molecule can bend and flex around its bonds in a number of different ways: stretching, rocking, wagging, twisting, etc. These are all called degrees of freedom of the system, because they are different modes of possible motion that are allowed. You can see them below, and read more here.

Symmetric streching (raise the roof)
Asymmetric stretching (wild party)
Scissoring (YMCA party)

Wagging (to the beat)
Wagging (wave party)
Rocking (rock on)

All of these motions describe some sort of energy inherent to the building blocks of the material—in this way they are all heat. When you heat up a material by convection, conduction, or radiation, you’re causing the atoms and molecules to move in many of these ways, probably. Certain materials are more likely to move in some ways than others, though, like in a solid—if you’ve got a harness on in a roller coaster you are constrained from translating yourself side to side, but you can still wave your arms around.

Light is one way to cause atoms or molecules to move within a material

Light is a traveling, oscillating electric and magnetic field. That electric field can push and pull on atoms and molecules, especially if they are polar like water. The electric field will push on the positive nucleus a little bit and pull on the negative electrons a bit. This forcing isn’t enough to pull apart the electrons and nucleus (at least, not until you have a very big electric field, but that’s another post), but it’s enough to cause to water molecule to rotate until it is lined up with the direction of the electric field.

The light field is varying over time according to the frequency of the light wave, though. So just as soon as the water molecule has oriented itself properly, the electric field has changed and is pointing the other way, which spins the water molecule again.

The wavelength of the microwave radiation in a microwave oven excellent at vibrating water molecules, which bump around and transfer heat

Most commercial microwave ovens operate at a frequency of 2.5 gigahertz—that’s 2.5 billion times per second. That turns out to be a pretty good frequency for causing the water molecules to rotate back and forth. As they are spinning about they will bump into other molecules surrounding them, causing those molecules to move as well—in this way energy is transferred from the microwave radiation to the material, causing it to heat up.

Microwave rotation dance

Microwave radiation can penetrate evenly a few centimeters (about an inch) into most foods, and it will still reach even deeper than that. All of the nearby water molecules will be spinning because of these electric fields, and so microwave ovens can heat something much more quickly because the entire volume of food is being heated at once. So you can think of microwave ovens as boiling the water inside your food to cook it.

Interestingly, you typically won’t get browning of your food inside a microwave. That’s because the temperature only gets as high as boiling water, which isn’t hot enough to cause the chemical reaction that’s responsible for caramelization, or food browning. You can read more about the yummy Maillard reaction here.

You can still make hotter than boiling water temperatures with microwaves, though. The oscillations from microwaves affect other molecules than water, including some fats and oils. When you pop a bag of popcorn, the oils inside the bag are getting heated by the microwaves, which then cook the surrounding popcorn kernels.

Bonus physics—Metals in the microwave

If you’ve ever been forgetful enough to put something metal in the microwave, you know it can have catastrophic results. Here the electric field of the microwave light is pulling on electrons just like before, but with a big difference: in a metal the electrons are not bound to a single nucleus, and are instead free to move around. Moving electrons are a current, and so the microwave oven actually turns the metal into a small antenna. That isn’t so bad, except that electrons moving around like this can cause a lot of heat (this is how light bulbs work, but that’s another post), and if enough electrons build up near a sharp edge in the metal you can get a spark to form in the air. While that can look cool, it’s dangerous and can very easily damage the microwave beyond repair and even start a fire! So, always remember not to put metals in your microwave.


Bonus physics—Microwave shenanigans

Even without going to metals, there are a multitude of neat physics experiments you can do with microwaves: everything from blowing up marshmallows to making plasma plumes! I’m going to talk about these in future posts, so stay tuned.

Thanks to Tiago Becerra Paolini (wikipedia) for the molecular vibration gifs.