Microwave Fun!

Callie Russell & Marci Stucki

Table of Contents

Background    Statement of the Problem    Hypothesis    Method   

Data Analysis    Conclusion    Bibliography    Links   Return to Research

Background:

    Microwaves were invented in 1946 by Percy Spencer, an engineer who discovered them accidentally while testing a vacuum tube called a magnetron (Invention of the Microwave Oven). While he was testing the magnetron he discovered that the candy bar in his pocket had melted. He then tried placing other foods near the magnetron and discovered that it caused them to cook at a faster rate than conventional methods of cooking. Since then microwaves have been used for things such as telephone and television communication and treating soreness in muscles. But the most common use for microwave radiation is the microwave oven. Microwave ovens, which were introduced in the 1950s, have been found to be very useful in cooking many different types of foods in short amounts of time. And since 1971 they have been regulated by the FDA because of possible radiation leakage and danger to humans. Microwaves use a form of radiation known as electromagnetic radiation. They are waves of electrical and magnetic energy. They have certain characteristics that make them useful for cooking. These include being reflected by metal, passing through glass, paper, and plastic, and the ability to be absorbed by foods.
 

    Microwaves are able to produce heat within the food by causing the water molecules in the food to vibrate. The microwave is generated inside the oven in a tube called a magnetron. Because this causes the water molecules in the food to vibrate foods with higher water content are cooked more quickly than food with less water. Food cooked in a microwave is not radioactive even though radiation is used to cook it because as soon as the microwave is absorbed by the food it is changed to heat. Although, there is still a debate as to whether it is totally safe to eat food cooked in a microwave. There are some studies that say radiation is still present in the food and that all microwaves leak at least a small amount of radiation when being used.

    Because of these characteristics, microwaves are able to produce heat within the food by causing the water molecules to vibrate. The waves are generated within the Magnetron tube, and even though radiation is therefore used in cooking, there is no risk of contamination. In addition, all microwave ovens made after 1971 are covered by radiation safety standards enforced by the FDA (Microwave Oven Safety Standards). The acceptable limit of radiation is 5 milliwatts of radiation per square centimeter at approximately 2 inches from the ovens surface.

    Although microwaves do heat food fast, without rotation they are not able to do so evenly. This is because microwaves are a standing pattern of waves and it therefore creates hotspots (Beaty). This was nicely shown in an experiment by Alistair Steyn-Ross and Alister Riddell. They soaked paper in Cobalt Chloride and then heated it in a microwave oven. Because this paper is pink when wet and blue when dry a pattern of color remained after the paper was placed in the microwave. Usually the hotspots in a microwave are found and the halfway points of the waves which create a 3D pattern and therefore the spots from the experiment helped determine where the microwave got hottest (Beaty).
 

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Statement of the Problem:

    The purpose of this experiment is to find the hotspots in a microwave and prove that the microwave doesn't heat evenly.
 

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Hypothesis:

    If we soak a piece of paper in room temperature water and microwave it at 5 different heights then we predict that the hot spots of the microwave will be towards the center at any height because the suggested placement in the microwave is in the center. Hot spots will be determined by the dryness of the paper verses the damp spots.
 

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Method:

Materials:
1. Microwave Oven
2. Water
3. Ruler
4. Cups
5. Cardboard (11” x 12”)
6. 10 Sheets of Paper (8 ½” x 11”)
7. Camera
(8. FIRE)
 

Diagram

This is a picture of our diagram.  For each height we had a setup like this one.

Procedure:

    The first thing we did was remove the turn table in the microwave because the turn table would make the heat distribute evenly. We then cut a piece of cardboard the size of the microwave. We made a grid on the cardboard consisting of 9 squares to determine the area of hot spots in the microwave. We cut Styrofoam cups to make heights of 0, 5, 10, 15, and 20 cm to place the cardboard on. To begin, we soaked one sheet of paper in water for 30 seconds. Then placed the wet paper on the cardboard and proceeded to microwave for 35 seconds. We did two trials at each height of 0, 5, 10, 15, 20 cm.

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Data Analysis:

Data Table

    When the individual pieces of paper were micro waved we analyzed it to see where the hot spots were found. From our data table you can see that at a specific height our first and second trials were very similar. The graph shows that the hottest spots of the microwave occurred in the left back and the front right of the microwave. It also shows that the second hottest spot occurred in the center back of the microwave. As you can see from our graph, we found no results of hot spots in the center. A logical reasoning for this would be that heat isn’t distributed evenly because the heat flow occurs around the outside and never reaches the center. At the bottom of the microwave (0 cm) the hot spots were found in the left back, right center, and right front. At 5 cm from the bottom of the microwave the hot spots were found in the left back, left center, and left front. At 10 cm from the bottom of the microwave the hot spots were found in the center front and right front. At 15 cm from the bottom of the microwave the hot spots were found in the center back and right back. At 20 cm from the bottom of the microwave the hot spots were found in the center back and right front.

Data File

Below is a diagram of the different positions in the Microwave

 

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Conclusion:

    Compared to our hypothesis, that the hot spots would be found at the center of the microwave, our data showed differently. The data showed that the left back and right front were where the hot spots were found. This pattern continued throughout the distribution of heat, as it travels downwards from the top through the sides of the microwave until it gets to the bottom where it eventually begins to distribute evenly, making the middle the least hot of all. The experiment however was not full proof. First of all, there was no way to measure the amount of water which was absorbed into each sheet of paper. After a few trials of heating the cardboard with wet paper on top, the heat caused the board to slightly curve up on the ends. The height difference between the edge and center may have resulted in compromised data. Although we used a ruler for out measurements, there is always a possibility for human error. For additional results, we could have done trials with the paper vertical on each side and in the center of the microwave. This would have showed the progress of the wave at that point. Compared with the horizontal data, a vertical measurement could have helped in narrowing down the wave pattern. In conclusion, when putting food in the microwave next time, think about where it is placed.

 

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Bibliography:

1) Microwave Oven Radiation. http://www.fda.gov/cdrh.consumer/microwave.html

2) Beaty, Bill. Unwise Microwave Oven Experiments. http://www.amasci.com/weird/microexp.html

3) Invention of the Microwave Oven. http://www.ideafinder.com/history/inventions/story068.htm

4) Gallawa, Carlton J. Microwave Oven FAQ. http://www.gallawa.com/microtech/mwfaq.html

5) Bloomfield, Louis A. How Things Work. http://rabi.phys.virginis.edu/HTW//microwave_ovens.html

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Links:

1. Invention of the Microwave Oven. http://www.ideafinder.com/history/inventions/story068.htm
        This site was useful in gathering background information about the microwave.

2. Gallawa, Carlton J. Microwave Oven FAQ. http://www.gallawa.com/microtech/mwfaq.html
        The frequently asked questions about microwaves on this site were very helpful in understanding the technology behind it.

3. Fun Things to Do with Microwave Ovens. http://www.everist.org/special/mw_oven/
        This site included experiments that, although interesting, I wouldn't recommend performing with your own microwave oven. 

4. Beaty, Bill. Unwise Microwave Oven Experiments. http://www.amasci.com/weird/microexp.html
         This is a good reference for those wondering, "What if I did (insert crazy scheme here) with a microwave?"

5. Giancolli, Douglas C. Physics: Principles with Applications. New Jersey: Prentice Hall, 1998.
        Our class physics book...a fairly useful resource for those wishing to get a good grade.

6. Microwaves. http://imagers.gsfc.nasa.gov/ems/micro.html
        This site was helpful in understanding how actual micro-waves are used and depicted areas in our daily life where they are depended upon.

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