Testing Patterns of Microwave Radiation With the Heating of Water

by Stephanie Culnane

 

Table of Contents

Introduction

Background Information

Research Question

Hypothesis

Procedure and Materials

Results

Conclusion

Bibliography

 Links

Return to research 2001

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Testing the Patterns of Microwave Radiation With the Heating of Water

 

Introduction: 

            This experiment was done to show the way that microwaves heat unevenly and to test the patterns of radiation in the microwave.  Water heated in different spots is used to show how if you were heating something in the  microwave such as food, how it would be hotter and colder in different spots because of the patterns the waves make. 

 

 

 

Background information:

           

            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 the 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 1950's, 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.

            All microwaves made after 1971 have been regulated by the FDA to limit the amount of radiation leakage possible from the microwave.  The limit of radiation is 5 milliwatts of radiation per square centimeter at approximately two inches from the oven surface.

            Microwaves are good at heating food faster, but they don't always seem to heat evenly.  The reason for this is the microwaves are in a standing pattern of waves and the pattern creates hotspots inside where certain spots receive more heat than others.  Usually the hotspots in a microwave are found at the halfway points of the waves which create a 3D pattern (Beaty).  An experiment done by Alistair Steyn-Ross and Alister Riddell tested the heat pattern in a microwave.  They wanted to test the pattern of melted cheese on a cooked pizza.  They used paper soaked in cobalt chloride.  This type of paper is pink when wet and blue when dry.  And so it left a pattern of color when put in the microwave.  The spots helped determine where the microwave got hottest (Beaty).

            Another theory on why microwaves don't heat evenly is that the waves follow different paths when they are in the microwave, and when the crests and troughs of the wave interfere with each other it can affect the heat.  When a crest of a wave rides on the trough of another wave it's destructive interference and they cancel each other out and cause that area of the microwave to not heat as much as the other areas.  This is why turntables are used in microwaves.  To distribute the heat since there are certain spots that will heat hotter than others.

 

Research Question: Find the hotspots in a microwave and prove that the microwave doesn't heat evenly.  Also attempt to find the 3D pattern that the waves in the microwave are supposed to create.  To do this water will be heated in different spots in the microwave.

 

Hypothesis: When the water is heated some spots will be hotter than others because of the uneven heat distribution of the microwave.  My best guess on the pattern of the heat is that it will be hotter towards the middle of the microwave then in spots closer to the outside.  But in order to test if there is an actual 3D pattern I will have to elevate the water and so a trial that way also.  And then in order to prove that most microwaves have the same type of radiation pattern I will use another microwave and test the heat distribution in that one also to show that the heat will be mostly in the center even though it's a different microwave.

 

Procedure and materials:

            To test the heat distribution of the microwave, I used the following Materials:             

                        a microwave

                        small plastic cup

                        water

                        thermometer

                        medium plastic cup

 

                       

                                               

Procedure:

            The first thing I did was fill a large cup full of water and let it sit for about an hour so that it would be close to room temperature.  Then I removed my turn table from the microwave and used a pencil to make a grid in my microwave with sixteen squares of equal size.  I took the temperature of the water, which turned out to be approximately 17º C.  Then I took the glass turntable out of my microwave so that the water would heat according the waves in the microwave.  If I were to leave the turntable in and let the water rotate the heat would be distributed and the experiment would not work correctly.  I then filled up my cup of water with approximately 100 milliliters of water and placed it in one of the sixteen spots on the gird I had lightly drawn in my microwave.  I microwaved the water for twenty seconds with the microwave set on high power to get the temperature to raise as much as possible.  I took the temperature of the water after it had been microwaved and recorded that.  I then waited three minutes in order for the cup that the water was in to return to room temperature and so that the microwave would not contain more heat when I did the next trial.  This was so that my results would be as accurate as possible.  Since the heat doesn't leave the microwave right away I didn't want to do the next trial right away.  I repeated my procedure with heating the water and taking the temperature for all sixteen of the squares, waiting three minutes in between each of them. 

 

Other Microwave Experiments at Unwise Microwave Experiments

Results:

The data for the experiment is as follows:

 

Trial #1

Back of Microwave (temperature in ºCelcius) text version

36º C

33º C

37º C

32º C

34º C

48º C

38º C

48º C

45º C

42º C

43º C

50º C

26º C

37º C

36º C

29º C

Front of Microwave

 

            This is a model of the different spots that the water was placed in the microwave.  There doesn't seem to be a definite pattern, but the results do agree with my hypothesis that there will be more heat in the middle of the microwave.  There are lower temperatures towards the outsides of the microwave, with significantly lower temperatures on the front outside corners of the microwave.  These may be because it is closer to the door of the microwave, where there may be small cracks where the opening of the microwave door is.  And radiation leakage from microwaves has been studied, and although most microwaves don't leak enough to harm a person, there still may be leakage of the radiation which could've contributed to the lower temperatures in the spots closer to the door of the microwave.  But to be sure that these temperatures were correct, I did another trial using the same procedure.  I left the water in the microwave for twenty seconds on high, and I waited three minutes in between each time so that the cup and microwave would cool down after being used.

 

 

Trial #2

Back of Microwave (temperature in ºCelcius) text version

37º C

35º C

37º C

34º C

34º C

48º C

39º C

47º C

43º C

42º C

44º C

48º C

27º C

37º C

35º C

28º C

Front of Microwave

 

            The second trial had similar results to the first trial with a majority of the heat being in the center of the microwave.  This helps prove that there is more heat in the center of the microwave.  Then to determine whether there is a 3D pattern in the microwave to prove what I found in my research, I elevated the cup of water in the sixteen different spots. To do this I used a medium sized plastic cup and turned it upside down.  Then I set the small plastic cup with the water in it on top of the medium plastic cup.  This elevated the water approximately five inches.  I then repeated my procedure that I used for the first two trials of microwaving the water on high for twenty seconds and waiting three minutes in between each time to allow the microwave and cups to return to room temperature.  I also did two trials with the cups elevated like I did for the first part of the experiment to see if the results were similar both times.

 

Trial #1 (elevated)

Back of Microwave (temperature in ºCelcius) text version

37º C

31º C

35º C

32º C

35º C

42º C

33º C

47º C

43º C

42º C

39º C

48º C

26º C

33º C

37º C

27º C

Front of Microwave

 

            The results for the first trial where I elevated the water were similar to the other two trials were the water wasn't elevated.  The heat was closer to the middle of the microwave, with exceptions in a couple spots.  However, there still didn't seem to be a 3D pattern of shown in the microwaves.  I then did a seconds trial to make sure that the results from the first trial were accurate and had no mistakes. 

 

Trial #2 (elevated)

Back of Microwave (temperature in ºCelcius) text version

39º C

34º C

37º C

33º C

33º C

46º C

38º C

44º C

42º C

44º C

43º C

46º C

28º C

35º C

32º C

28º C

Front of Microwave

 

            The second trial where I elevated the water was similar to the first trial so that shows that it is less likely I made a mistake in taking the temperature in the first trial.  This trial is also similar to the temperatures in the trials taken before the water was elevated in the microwave.  It seems that there isn't really a significant difference between when the water was elevated and when it was not elevated.  Therefore there really isn't a specific 3D pattern that can be found with experiment.  However, the heat is still closer to the center of the microwave as was predicted.  After I elevated the water in the microwave, I repeated the experiment done in the very first two trials where the water wasn't elevated in a different microwave to prove that in general microwaves will have more heat towards the center of the microwave than on the outsides, especially in places near the door.  I microwaved the water in the same cup used in the first experiment, and used the same amount of water, 100 milliliters.  I also made sure the water was all at the same room temperature when I started the experiment.  And I also waited three minutes for the cup and the microwave to return to room temperature after the microwave was used for the twenty seconds on high.

Trial #1

Back of Microwave (temperature in ºCelcius) text version

32º C

33º C

31º C

38º C

39º C

38º C

38º C

35º C

40º C

38º C

37º C

41º C

24º C

31º C

32º C

26º C

Front of Microwave

            According to this trial the second microwave that I used for this experiment is similar to the first because the middle of the microwave is hotter than the outsides of it.  Even though the overall temperatures of the microwave aren't as hot there is still the same general pattern of the hot spots of the microwave being towards the middle.  With the two spots in the front corners of the microwave being significantly cooler just like with the other microwave.  But just to make sure I did another trial in the second microwave. 

 

Trial #2

Back of Microwave (temperature in ºCelcius) text version

35º C

35º C

30º C

33º C

38º C

38º C

38º C

36º C

40º C

36º C

36º C

40º C

26º C

32º C

31º C

25º C

Front of Microwave

 

            The second trail for this microwave was similar to the first and proves that the hot spots in the microwave are mainly in the center.  This agrees with the hypothesis and also the results from the experiments using the first microwave.  The majority of the heat is in the center of the microwave.  And there is still no definite pattern for the waves. 

 

Conclusion:

            The uneven heat distribution was proven by the two trials of heating water in different spots in the microwave.  However, there was really no evidence of a set pattern made by the waves.  But the heating of water may not be enough to distinguish the pattern because according to Bill Beaty, the patterns of microwave oven radiation are 3D and probably cannot be tested using the heating of water.  But the experiment did prove my hypothesis that the heat would be hotter towards the center of the microwave.  But the experiment may not be completely accurate because of many factors.  Although I let the cup of water sit for an hour in order to obtain room temperature, the temperature could have also changed throughout the duration of the experiment because I used water from the same container for all sixteen of the areas tested in the microwave.  And even though I waited approximately three minutes before testing each area of the microwave, after using the microwave a few times, it took longer to cool down each time and there still may have been heat left in the microwave.  This may have made the microwave heat up more than it was supposed to each time it was used after that, and this could have affected the water temperature.  And also as I explained before, there still could be radiation leakage from the microwave, even though if there is the amount would be quite small and might not have an affect on the temperature of the water.  The experiment might also vary if I were to put the water in the microwave for a longer period of time instead of just twenty seconds.  The results might also be different if I were to put the microwave on a different power level such as medium or low.  The heat different might not be as different in certain spots if the heat is not as high in the microwave.  When the water was elevated in the microwave there didn't seem to be a significant difference in the water temperatures, but the temperatures still agreed with hypothesis of the microwave being hotter in the middle of it than on the outside edges.  Because the microwaves didn't seem to make a specific 3D pattern, testing the heat distribution of a microwave in this way may not be the best way to do it.  Although the information agreed with my hypothesis, if I were to guess that the microwaves went in a specific pattern, such as all of the hot spots being in diagonal lines, this experiment would not prove that.  However, the experiment also proves that in general turntables do help at least a small amount in distributing the heat to the food being cooked in the microwave.  If a turn table were to be used in the microwaves used in the experiment, we would see that in general they would work because most of the heat is in the center and when it is turned the food is being turned to go through the hot and cooler spots in the microwave.  I also took out the turntable in the experiment using the second microwave so I could use the same conditions that I used to test the heat in the first microwave.  Other things that could affect the heat in the microwave are the number of watts and the amount of power the microwave puts out.  However, I don't think that this will affect the patterns of the waves themselves.  It would affect how hot the water will get in the twenty seconds that it is heated, but it shouldn't affect the patterns of the waves.  Since the temperatures in the experiment using the first microwave were overall higher, that microwave probably has a higher wattage than the seconds microwave used.  But if the water in the second microwave was to be hated longer it would probably reach the same temperatures as the first microwave.  Meaning that the wattage has an effect on temperature, but not the overall distribution of the temperature, which tells us the patterns of the waves.  However, with the background research done, it's hard to tell whether there is a pattern of waves because the waves or are moving, or if the waves are standing and there is a pattern because they overlap each other.  The opinions found on this topic seem to contradict each other.  If there was a moving pattern of waves, it seems that a turntable would not be necessary because the hotspots would always be moving along with the waves.  However, because two trials were done in the microwaves and the spots seem to be in the same places both times, it seems logical to conclude that the same spots will be hotter every time a person is to use their microwave.  It can be determined from that that the waves in the microwave are standing, and that the uneven are created when the waves collide with each other.  When the crests and troughs of the waves collide it causes the spot to be cooler than normal.  So at places where the waves don't collide, the hot spots are created.  Although the experiment wasn't successful in determining a specific pattern for the waves, it did determine that there are hotter spots in the middle of the microwave.  It also gives more evidence that the waves are probably standing and colliding with each other rather than moving around.

 

Links to Related Sites : http://ighawaii.com/natuarally/newsletter/micro.html: site about microwave radiation and health effects of radiation leakage

                                           http://www.doh.wa.gov/ehp/rp/rp-oven.htm: talks about microwave radiation and the way microwaves work

                                           http://www.fda.gov/cdrh/consumer/microwave.html:  mostly about health effects of microwave radiation

                                           http://www.minerals.csiro.au/winc/microwav/htm: microwave radiation

 

 

                                                                                 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