Physics inside a Microwave Oven.

By

Maarten Rutgers


DISCLAIMER:

Microwave ovens are designed to cook food and NOT to do scientific experiments. We do not recommend that you try anything described here yourself. If you do choose to do so, you are doing it at your own risk.
Normally, when food (water) is in the microwave, the radiation is continually absorbed, keeping the overall radiation levels low. Many of the following experiments required us to run the microwave nearly empty. Electromagnetic radiation (microwaves) can build up to high intensities. This can cause high levels of radiation to reflect back into the microwave generator causing it to overheat or to be damaged. Small leaks in the oven, otherwise harmless, may emit more radiation than usual and could potentially harm bystanders.

Now that you have been warned, lets start with the fun. This page and the photos and footage stem from the 1999 annual open house of the Ohio State University Physics Department. We performed the demos for a group of Ohio high school physics students.
I gratefully acknowledge Harold Whitt and Carl Acampado who helped develop these demos so they worked reliably in front of a large crowd. Brent Daniel is thanked for help in general, and with the video production work.

We will explore various bits of physics by sticking usual and unusual objects into a microwave oven. We bought a low power Samsung model for these demos.
We will be sticking the following things in the microwave:
For most of these we have photos and video, and will hopefully add some more later.


The light bulb:

We fill a drinking glass half full of water and place a light bulb in the glass, metal parts submerged in the water. When cooked for a few seconds we note that bulb lights up!

Why? Well, the electrons in the metal filament are jiggled around by the microwaves which causes it to heat up and glow. The water is only there to absorb excess microwaves, and it not totally necessary.

We have observed that the bulb can burn out after 10 seconds or more.

This demo can also be used to show what happens when a microwave oven is set to cook on low power. On 30%, the bulb lights for a short wile, and then goes off. Then on again, then off, and so on. The microwave generator is simply turned on and off to create an average power over time. The generator (magnetron) does not lower it's intensity, but pulses on and off.

Thanks to Mark Wexler. He showed me this demo back in 1992 or thereabouts.


The poptart:

The light bulb filament does not burn up since it is not surrounded by oxygen inside the bulb. Metals surrounded by oxygen and other combustibles can ignite and be destroyed. A poptart in its package is a good example. The shiny wrapper is made of mylar plastic coated with a very thin layer of aluminum. The aluminum is put there so that gases cannot easily pass through the membrane, keeping the food fresh longer. The aluminum layer is, however, a conductor of electricity and will heat up when subjected to microwaves, just like the light bulb filament. The result is a series of spectacular blue sparks as the metal is heated up so hot that it evaporates. The fireworks begin after about a second of cooking and end about as quickly. Once all the metal has been broken up in to small pieces, the show stops. The microwaves have a rather long wavelength (about 10cm) and cannot couple to things much smaller than that.

Click on this picture to see a Quicktime movie of the event.

If you have ever had the misfortune of putting a mug with gold letters, or a piece of fine china with gold leaf in the microwave you probably saw the same thing.


The Microwave CAM

Some of the pictures and movies in this web page show you a nice view of the interior of the microwave. To do this we drilled a 3/4 inch hole in the top of our microwave. When a microwave detector (you can buy these at your local hardware store) is held near this hole, no significant radiation levels were measured. This is not surprising, since the microwaves have a wavelength about 10 times larger than the hole and cannot get through. We placed a small "lipstick head" video camera over the hole and used it to collect video images. These video clips are silent, since this camera had no convenient microphone. The front view images and movies were simply made by pointing a camcorder at the front door of the microwave. (Again, the small holes don't let the microwaves through.)


The Compact Disc.

Now we replace the poptart with a compact disc. This is no different, since the disc is also a thin layer of plastic covered with a very thin layer of aluminum. A very beautiful display.

Click on this picture to see a Quicktime movie of the event.

Thanks to Prof. Louis Bloomfield of UVA for showing me this trick last year at the American Physical Society Centennial meeting. Check out his web site for more information than you could possibly imagine about your microwave oven. It includes most of the demos described here and much, much more. He also points out that now he has a use for all those CD's that come to his house as junk mail. He takes them to class and uses them for physics demonstrations.


Finding the hot spots in your microwave with fax paper.

If you have ever wondered why food can cook unevenly in the microwave, it is because of high and low points in the intensity of the microwave radiation field. As the waves bounce back and forth in the box, they interfere with each other. In some places they add up, and in other places they can subtract to zero intensity.

To visualize the hot and cold spots we do the following: Take a piece of blue foam insulation panel (1 inch thick Styrofoam) and cover it with a wet paper towel. Then we put a piece of thermal fax paper over this and place the whole combination inside the microwave oven. We cut the panel so it just covers the bottom.

When cooked for about 15 seconds, the water in the towel heats up where the microwave intensity is highest. The thermal paper changes color from white to black in these places. The following shows a top view of the panel after about 15 seconds of cooking: The image is about one foot wide.

Click on this picture to see a Quicktime movie of the event.

When cooked longer the whole paper eventually heats up, as steam from the wet paper seeps into the cold areas. If we repeat this carefully we get the same pattern every time. If we put a different amount of water on the towel the second time, we get a different pattern. The wetter the towel, the more it absorbs the microwaves, which affects the overall pattern.


Putting metal in the microwave.

Next we take the same fax paper setup which would give us reproducible patterns and place a large steel ball on top of the whole setup. The pattern now looks like this:

Click on this picture to see a Quicktime movie of the event.

The pattern is quite different! The ball reflected the microwaves around and changed the whole pattern. So, the pattern of hot and cold spots depends on what you put in the microwave. This is why microwave ovens have rotating tables in them. If one can never predict where the hot spots are, then one has to move the food around to make sure that on average all parts of the food see hot and cold spots alike.

Some microwaves have a little metal 'ceiling fan' in front of the microwave emitter. It is supposed to spin around and reflect the microwaves around so your food can sit still.

Note that it was not at all spectacular to put metal in the microwave in this case. Metal simply acts as a mirror for microwaves. The surface of the ball may heat up a little as it subjected to microwaves, but it has so much mass to absorb this energy. The surface of the ball might heat up a little from the microwaves, but there is so much metal inside to soak up the heat that the ball will not even feel warm to the touch. The thin metal layers on the CD and the food wrapper did not have this property. In that case the energy has no place to go and causes big temperature increases, leading to combustion.

Thanks go out to Don Alvarez, who first told me of this demo.


Grapes in the microwave oven.

This is a lesson that it is not necessarily metal which is most dangerous to put in the microwave.

Take a relatively think skinned grape and cut it in half, but not all the way. Leave a small bridge of skin connecting the two. Dab the excess juice from the grape and place the whole thing like an open book in the microwave oven. After 10 seconds of cooking, a large spark blows the two halves apart!

I first found this demo at the web site of Patrick R. Michaud.

Our suspicion is that the grape halves act as a small dipole antenna for the microwaves. Optimally such an antenna is 1/4 the size of the wavelength, which is pretty close for our grape. The skin flap is a conductor and currents run back and forth across it. As it heats up, it dries out, increasing it's resistance further, causing more heat, etc. Finally it ignites. The smell of burnt sugar fills the oven after it is turned off. The spark will not occur if the grape is too wet. Juice boils up and covers the skin bridge and there is no spark.

On rare occasion we noticed rising clouds of luminous plasma (often referred to as 'ball lightning' in other microwave oven web pages). While testing for repeatability (so we could demonstrate it in front of a large group of high school students) we ran out of grapes. Carl Acampado, one of our undergraduate assistants, started to cut used grape halves in half again, as shown in the next figure:

Click on this picture to see a Quicktime movie of the event.

When place in one of the hot spots on the bottom of the oven, we would be able to use the grape as a source for ball lightning very reproducibly. Anywhere from a few to 20 some plasma bursts are released from the grape.

Click on either picture to see a Quicktime movie of the event.

This next movie was taken through our microwave cam.

Click on this picture to see a Quicktime movie of the event.

The two front view movies have sound. The emission of the plasma sounds like a Jacobs ladder (must have for any self respecting mad scientist). The front view movies have this sound included in them. In the top view you'll note that we removed the rotating platter in the bottom of the microwave. This allows us to put the grape in a hot spot every time. The plasma leaves a lot of burnt sugar on the bottom of the microwave and started to eat away at the paint. After scraping the finish down to bare metal we found great improvement in the repeatable emission of plasma. There also seems to be more arcing between the grape and the bare metal oven casing than between the grapes halves themselves.

I don't understand in great detail what happens to start the plasma, but once it is created the microwaves will be absorbed by it readily (it is a conductor) and keep it alive. Since the plasma is hot it rises to the top of the oven, like a hot air balloon. On occasion such a cloud will sit at the top of the oven for many seconds. It likely consumes most of the energy in the oven so that no new plasma is emitted from the grape. Once it dies out, microwave intensity rises near the grape and the process starts over. After about 20 seconds on average, the grape starts boiling, and the liquid shorts out the conditions favorable for generating the plasma.


Candle in the microwave

A similar effect can be achieved by putting a burning candle into the microwave. The flame is the locus for a growing orb of plasma which rises to the top of the oven. We placed the candle in one of the hot spots which worked well for the grapes. At one point the flame itself became a large jet of plasma which quickly melted the candle. Interesting variations (see web sites below and links therein) will show you how to capture the plasma in an overturned fishbowl.  


Final note:

After all this you may ask yourself: what is really most dangerous about microwave ovens?

I personally feel that boiling water in a microwave is probably the most dangerous everyday thing you can do. Since microwaves can heat water so uniformly, and since the containers we put in microwaves are often very smooth, it is not unusual that the temperature goes above the boiling point without actually boiling the water.  This water is now called 'superheated.' A very slight disturbance can trigger the water into a roiling boil, making it practically jump out of the cup.  Typically, this happens when you take your mug out of the microwave, or when you are holding it and insert a tea bag or spoon.  Many people have been burnt like this. Chemists often encounter the same thing when they boil water and other liquids in the laboratory.  Commonly one adds little stone or Teflon flakes to the solution. These so called boiling chips act as a surface against which the water can easily boil, never allowing the liquid to become overheated. So, take care when you take liquids out of the microwave. Consider using hot mitts, and certainly place your mug on a surface before you add any coffee, tea, or sugar to it. Rapping your mug with a spoon before taking it out of the microwave may (or may not) trigger rapid boiling, in a sense defusing your beverage.


Other links:

As we now find out, there are many other web sites devoted to microwave pyrotechnics. Please visit them and see what else is being done.

Fun with Grapes - A Case Study

Unwise Microwave Oven Experiments

Hans Hochwald's microwave experiments.

Funny things to do with your microwave oven.

UVA's How Things Work on microwave ovens.

These pages contain many more links and I apologize if I left anyone out.


Copyright 1999, Maarten Rutgers.