Discover the Science of a Microwave with Chocolate

by Robert Krampf

Combine science and chocolate to learn about how microwave ovens work.

This week’s experiment turned into a two parter.  It started out as one experiment, but it just kept getting longer and longer.  Over the years I have learned that people are much less likely to read a long experiment (much less try it), so I chopped it in half.

Part of the reason it got so long was that it is such a neat experiment.  How often do you get a chance to examine electromagnetic radiation and even measure its wavelength (next week) while melting and eating chocolate?

To try this, you will need:

  • a microwave oven
  • waxed paper
  • several chocolate bars
  • a large plastic, glass, or paper plate.  Do not use metal!

Start by looking at the inside of the oven.  If it has a turntable to rotate the food (most do), remove it.  We want the chocolate to stay in one place, not move around.

Cover the plate with waxed paper, and then place the chocolate bars (unwrapped) on the plate to form a solid layer.  You want the layer of chocolate to be as flat and even as possible.

Place the plate of chocolate in the oven and set the timer for 30 seconds.  Depending on your oven, you may have to cook it a bit longer, but I learned from experience (see this week’s video) that cooking too long gives you a LOT of smoke and a mess.

After 30 seconds of cooking, check the results.  You should find that there are spots where the chocolate is melted, and maybe burned, and other places where it is not melted at all.   Why?

Your microwave oven works by producing microwave radiation.  No, its not radioactive!  This is electromagnetic radiation, which also includes visible light, radio waves, ultraviolet light, radar, etc.  Microwaves can cause water molecules to vibrate, producing heat to cook your food.  OK, so why does your oven have hot spots, instead of cooking evenly?

Instead of just blasting microwaves around, your oven produces something called a standing wave.  The easiest way to imagine a standing wave is to look at one.  Get several feet of rope, and tie one end to a doorknob.  Hold the other end move back to take up most of the slack.  You don’t want the rope tight.  Start shaking the rope up and down, and notice the way the rope wiggles.  By adjusting how fast you shake the rope, you can find the point where it produces a stable pattern.  Some parts of the rope will always be moving up and down, while other points will not move much at all.  Its easier to see in the video than it is to describe, but you should recognize the pattern when you see it.  That is a standing wave.  The points where the wave is moving up and down a lot would be the part of the wave that produces a lot of heating in the oven, producing the burned spots.  The part of the wave that does not move much would not produce much heat, giving you the cooler spots in the oven.  That is why you need a turntable to move the food through the hot spots, to heat it evenly.

Reprinted with permission. © 2008. Robert Krampf’s Science Education

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