Saturday, July 30, 2011

Microwave Ovens

Have you ever wondered how your microwave warms up your food, or worried that this mysterious process might be mutating what you ingest or even that those microwaves are escaping the box and penetrating your insides? Well, hopefully I can shed at least a little light on this magic hotbox and allay, well, some of your fears anyway.
Speaking of light, let me start with explaining what a microwave is. As in the actual, electromagnetic wave. You may have heard that light travels in waves, and that each color has a different wavelength. The rainbow shows the spectrum, and these colors are always in the same order...based on their wavelengths and frequencies. These visible waves are only a fraction of the spectrum. Other electromagnetic waves include radio waves, infra-red, x-rays, etc. (see spectrum below definitions).
Wavelength: Physics . the distance, measured in the direction of propagation of a wave, between two successive points in the wave that are characterized by the same phase of oscillation. Or:
Frequency: the number of cycles or completed alternations per unit time of a wave or oscillation. Symbol:  F; Abbreviation:  freq. Or:
1 Hz means that an event repeats once per second.
To demonstrate where your everyday waves fall on a spectrum, including the microwaves we are preparing to discuss, here is a helpful little picture:

Fantastic. Now we realize that the force we are working with in a Microwave Oven is electromagnetic, and have a basic understanding of the spectrum.
      The most important part of your microwave oven is the Magnetron. This nifty device is what is actually creating the microwaves and sending them into the box. When power is supplied electronically (i.e., plug it in and turn it on), the magnetron produces simultaneous electric and magnetic fields that oscillate at the right frequency to create microwaves. These microwaves emit from the magnetron, and are reflected off metal surfaces: A metal fan sends waves into the oven segment, the metal lined walls of the microwave oven reflect the waves throughout and back and forth.
      So now you have microwaves being reflected around the oven. What happens when you add food? First, let’s look at the make-up of the food you eat. Do you know what is common in almost everything you ingest? It’s water molecules. It is these molecules of H2O that are the key to microwaves increasing the temperature of the object inside the oven. This is due to the fact that these molecules are polarized.
      When the Oxygen (O) atom combines with the two Hydrogen (H2) atoms, it pulls the electrons partly from the Hydrogen…creating a negative charge on the Oxygen end of the molecule. This in turn creates a positive charge on the H2 end of the molecule, and there you have your polarization.
      As the fluctuating electric field of a microwave passes by these water molecules, it causes the polarized water to in turn fluctuate…and at the speeds that they begin to fluctuate, heat is in turn created! The excited water is turning the work energy into heat and that is a thermodynamics lesson for another post.
This leaves us with a couple of questions still from the introduction.
Are these microwaves escaping and cooking your insides? The answer is no. Take a look at your microwave oven. All the walls, save the door, are all-metal surfaces. The door is a mesh design of metal and amazingly, those holes in the mesh are TOO SMALL to allow the escape of the microwaves. Fabulous, right?
Are these microwaves mutating your food? Pure water molecules are not going to be changed by the effects of microwaves. That being said, the complex carbon chains that make up the rest of most foods may be affected by the microwaves, but to what extent, I don’t know. If you know, please feel free to post a response, I would love to hear what others have to say!
Experiment:
Try (with adult supervision, if you are a minor) microwaving an ice-cube or ice-cubes. What happens? Can you explain why?
Formula (for the math-happy science geeks):
C = λ(ν) or speed of light = wavelength times frequency
Electromagnetic waves always travel at the speed of light, and therefore, the higher the frequency, the shorter the wavelength.


References:

Bloomfield, Louis A. How Things Work. John Wiley & Sons, Inc. 2006. p 432.

 Fischetti, Mark. How the Microwave Works. Scientific American. Oct 30, 2008.
http://www.scientificamerican.com/article.cfm?id=how-the-microwave-works
viewed at link on Jul 30 2011.

Definitions from: http://www.dictionary.com/

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