Heat Distribution in Microwave Ovens

Background Information

In today’s society, the microwave has become an essential part of our industry.   Its’ usage varies from drying ceramics to detecting speeding cars and treating sore muscles (Michale Oudlaug).  However, the most common use of microwave energy is in microwave ovens.

A microwave is a high frequency electromagnetic wave, one millimeter to one meter in wavelength.  This energy was first transferred to create a microwave oven by Dr. Percy Spencer.  He was experimenting with new vacuum tubes called Magnetron, which are derived from the nucleus of a high voltage system.  Then, he noticed a candy bar melting in his pocket next to the vacuum tubes.  As a result, the melting candy bar triggered him to conduct more experiments using the Magnetron tube with different types of food. He discovered that electromagnetic through magnetron tube causes food to cook at a fast rate (Gallawa).  He soon directed the microwaves into a metal box, not allowing any energy to escape; the first microwave oven was created.

The electromagnetic produces heat within the food and causes the water molecules to vibrate at a faster speed.  As a result, the food is heated at a faster rate.  However, although microwaves heat food faster, without a regulating device, such as a rotating plate, they are unable to distribute the heat evenly.  The cause of this is the standing pattern of waves that create hot spots within the appliance.  This problem was carefully tested by Alistair Steyne-Ross and Alister Riddle.  They heated a Cobalt Chloride soaked paper in a microwave oven and examined the change in color on the paper.  This was possible because the paper has a characteristic of turning pink when wet and blue when dry (Beaty).  So where are the hot spots in the microwave oven?  Our hypothesis states that the middle of the microwave oven will be the hottest while the top will be warmer then the bottom and the edges.

Method: TOP

In order the measure the distribution of heat inside a microwave oven we used a General Electric Co. Spacemaker 500 watt microwave oven.  We then made a cardboard stand which would hold up our fax paper (Perfect print, high sensitivity fax paper).    We measured the dimensions of the microwave to be 8x12X19 inches.  Then, we cut the cardboard to 16x10 ˝ inches, adding supports to the ends of a board in order to adjust the height using scotch tape and a ruler to measure.

Figure 1

Next, we cut 26 sheets of 11 x 8 ˝ inch fax paper.  We connected two sheets, total of 13 sets, of papers to be 11 x 17 inches per connected sheet in order to cover the whole surface of the oven.   Using a paint brush, we brushed room temperature water onto the spread of fax paper, placing it glossy side up in the microwave oven, and heating for 1 minute and 30 seconds.  Repeating this step 5 times we tested the paper at the bottom of the microwave oven, 2 inches, 4 inches, 6 inches and at the top.  After recording and analyzing the data on the brushed sheets of water, we repeated our steps, this time soaking the sheets for 1 minute, instead of brushing them with water.  Again, placing and heating the paper at the bottom, 2 inches, 4 inches, 6 inches, and at the top of the microwave oven.  After recording this data, to prove our results accurate, we did a second trial of each step.

Data analysis: TOP

The results show that at the bottom of the microwave the hottest spot is the innermost section of the microwave, near the back, about 6 inches from each side. They also show that the second hottest spot, at the bottom of the microwave, is the front and about 6 inches from each side.  The soaked and not soaked papers show similar results at the “bottom” paper.  Next, we raised the height to 2 inches from the bottom.  The results show that, when dividing the paper in half, both the center of the right and left sides is the hottest.  In other words, the spots about 3 inches from each side of the paper at the middle part are the hottest spots.  The soaked paper show this more clearly then the non-soaked paper, although the resulting spots are in the same areas.  Then, we analyzed the middle of the microwave, 4 inches from the bottom.  We found that the only hot spot was the center of the right side of the paper, when dividing the paper in half.  The spots are about 5 ˝ inches from the side of the paper.   At the 6 inch level from the bottom, the hot spots are at the front of the microwave, about 1 inch from the sides.  This time, the non-soaked sheet’s results were clearer, although the non-soaked and soaked papers show similar resulting spots.  Finally, at the top of the microwave, our data shows a distinct pattern from which resembles the circular generator at the top of the microwave where the heat is released.  There is a circular outlining starting about 4 inches from the sides; especially strong on the sides of the circle.  Then, the next hottest spots show up in the far left and the front right corners, where it mimicked the semi circle outlining.

Bottom:                                               2”

4”                                                          6”

Top:

Conclusion: TOP

Compared to our hypothesis, that the middle at the top would be the hottest sections, the result showed differently.  The result show that the top and around the edges are the hottest spots and the least hot was in the middle.  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.  It seemed that remaining heat at the bottom travel back up through the middle, taking on a “W” shape.  Another possible conclusion is that instead of the electromagnetic waves making circular patterns from the top to the bottom and back up, it went in an “S” shaped pattern.

W wave distribution                            S wave distribution

The experiment however was not fool 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 all our measurements, there is always a possibility for human error.

For additional support, we could have done a trial with the fax 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.

Graph & Relevant Data: TOP

The following graph is drawn to clarify the most common hot spots that are shown in the results.  The domain indicates the fax paper divided in sections in order to point out the specific spots.  Also, the range indicates the number of hot spots that appeared on the certain spots.

Frequency of Hot Spots

Data File: Text | Excel

*In order to clearly understand the divided sections of the fax paper, indicate the diagram depicted on the next page.

Diagram of fax paper samples

http://www.usmicrowaves.com  The us Microwave company made this site to give consumers all the information they could want, starting with the background and technology of what makes them run, to ways you can repair any problems you might be having with it.

http://www.gallawa.com/microtech/history.html This site was great in describing how microwaves were first discovered and how the experiments were accidental in leading to this modern day necessity.

http://imagers.gsfc.nasa.gov/ems/micro.html  This link gives many examples of what microwaves are used for aside from their use in microwave ovens.

www.mercola.com/article/microwave/ This article goes into detail on how the microwave ovens work, and some of the hazards that they provide when cooking food with them and that we should be careful when heating certain items.

http://home.howstuffworks.com/microwaves.htm This describes the technology behind how the microwave energy is used to cook the food, and how they are able to cook the food more evenly then conventional ovens.

http://www.ziprealty.com/buy/appliances.jsp - Very cool web page that pretty much has everything about Microwave ovens, including some history (Thanks Abby!!!!!!)