How do Microwaves Work?

Most recent answer: 10/22/2007

Our county is conducting a science fair for all the local schools. My parter and I and doing research on "What happens when certain material is placed in a microwave?" I’ve searched everywhere but I can’t find a list on various materials. Could you please write back listing the results when these materials are placed in a microwave for about 10 minutes? - paper - plastic - metal ......why is this so forbiden? - glass - gold - silver - certain liquids .....and anything else that you can think of. Please write back before this Friday...if possible, because we will have to test your results for our grade. Thanks a whole lot!!! Thanks again! Sincerely, Neetu Mathew
- Neetu (age 13)
Huntington Catholic, Huntington, IN, USA

Neetu -

It sounds like you've got a really interesting science project. But let me say first that I would recommend that you should NOT do it! Or if you do do it, do it with a microwave that no one will ever want to use again, because you will probably destroy the microwave if you run it for very long without any water inside. Also, if you do do it, make sure there’s an adult around in case something catches on fire.

Microwaves work by shooting waves called microwaves through food. Microwaves are a lot like light waves, they are wavy patterns of electric and magnetic fields. The wavelength (distance from one field wiggle to the nect) is much longer for microwaves than for light. When the microwaves go through the food, they make the water molecules vibrate. This is because water molecules happen to have electrically positive and negative ends that are pulled back and forth by the electric fields of the waves. Some other molecules don't respond as much.

If you put a cup of water (or anything with water in it) into the microwave, nearly all of the microwave energy is absorbed by the water. If you run the microwave without anything in it, the microwaves aren’t absorbed by anything except for the microwave itself. There are certain parts in the microwave that can be damaged by absorbing microwaves because they get too hot. This is why you should not run a microwave empty or without anything with water inside in it. (Remember that food has water in it.)

Microwaves can go straight through most (but not all) kinds of paper, plastic or glass with no effect, so running the microwave with just paper, plastic, or glass inside should in most cases be about the same as if there were nothing inside at all. (This is why many microwaveable foods are in containers made of these things.)

But microwaves can not go through metal (including gold and silver). When they hit the metal, they are reflected, just like light off a mirror.  If the metal has sharp points or bends or odd things about its shape, you can get sparks or arcs of electricity between two points on the metal. This can cause the microwave to catch on fire. (This why you should never put metal twist-ties in the microwave. The sharp points of the metal create sparks that cause the paper of the twist-tie to burn.)

In theory, if you were to put the metal piece inside of some water, it should cause the water to heat up more quickly, because the metal will reflect stray microwaves back into the water. But you should not put two pieces of metal within 1/2 inch of each other or the edge of the microwave. The microwave has some metal in its walls to reflect microwaves back into the food, and you can get electric arcs between the sides of the microwave and metal pieces inside. But I would not recommend trying anything with metal in the microwave (even if it /should/ be safe), because you run the risk of something unexpected happening.

Different water-based liquids should heat up fine. Cooking oils also absorb microwaves. You could probably make a very good experiment just out of looking at different liquids (and/or foods with water in them). You could also consider how well the microwave heats liquids in different shapes/sizes of containers...and possibly find out what the best type of container is to use in the microwave. Another thing you could do is to get some relatively solid food and find out which parts of it heat up first by measuring the temperature at different places inside after heating it for different amounts of time.

But you really shouldn’t do any sort of experiment where you run the microwave without any water inside, especially not with metal inside.

-Tamara (w. mods by mw)

(published on 10/22/2007)

Follow-Up #1: microwave sterilization?

I am a nursing instructor. Our class last night was on infection control in meal preparation. I asked one of my students how she would sanitize her cutting board(s). She hesitated a long time as if she hadn't really studied the homework assignment and then said, "I'd nuke it the same as I do with my kitchen cleaning cloths and sponges." I wasn't prepared for this response, and I asked her how long and at what power, and she said, "One minute." I had heard from other students that kitchen sponges can be sanitized/sterilized in the microwave, but a minute seems too short a time. Also, how would one "nuke" a wooden cutting board safely or a glass board (and for what amount of time)? I feel I need to respond to her declaration, but none of the sites I can find address my questions specifically. What would happen if she were to put her cutting board into her microwave and run it for whatever length of time might be required to sanitize or sterilize it? Doesn't the absence of a water content predict destruction?
- Teddy (age 67)
Honolulu, HI USA
Your excellent question goes a little beyond what we can answer with confidence, but the physics aspects are worth mentioning.

Unlike ultraviolet light, microwaves don't do anything directly to kill viruses and bacteria. The only effect will be from plain old heat. Since microwaves don't heat things very evenly (even with a rotating platform), you have to be concerned about spots that are missed. Unlike for say a cup of liquid, where convection helps even out the heating, on these boards the temperature could be quite uneven.

Especially for the glass or plastic boards, it's not clear this process could even be done without damaging the microwave. Operating a microwave without some decent absorber of the energy can damage the magnetron tube. The wood may contain enough water to absorb help protect the microwave, but that still leaves the problem of uneven heating.

As you know, sterilization guidelines require that some temperature be maintained for some time. Taking a board out of a microwave after just a minute and letting it then start to cool just sounds like it would add to the other problems.

I suppose that with wet cloths and sponges, with heat carried around by the vapor, one has a better chance of getting things uniformly hot. You should check with someone who knows more to see if there is some study saying if that actually suffices for sterilization.

Mike W.

(published on 11/03/2011)