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Using Everyday Examples in Engineering (E3)
Cooking with Microwaves
Peter Y. Wong
Tufts University
Introduction
Students are very familiar with using microwave ovens to make popcorn or to reheat
leftovers. In this class, they will learn more about cooking with microwaves and how it
relates to heat transfer and thermodynamics.
Engage the students with either physical props (e.g., microwave and marshmallow
experiments) or some multimedia videos (i.e., there are many experiments with
microwave ovens online).
Follow up with a classroom discussion about some of their experiences with using a
microwave.
•
What things should not be put into a microwave? Many students will answer with
“metal objects.” Show them the packaging in a commercial food product that crisps
food. There is metal in there.
•
Why is it necessary to rotate the food every once in a while? Students will respond
with “even heating.” Ask why is the heating uneven. Where are the microwaves
coming from?
•
Is it safe to look at the food in the microwave? Students might say “yes, but my
mother told me not to look too long.” Why does light pass through but not the
microwaves?
•
Do foods heat faster in microwaves than toaster ovens? Students may say “It
depends” and then add examples such as toast and pizza for toaster ovens and cups of
water and leftover chicken for microwaves. What is the difference between the two
kitchen appliances?
These questions can help engage the classroom to re-examine their thinking about how
the food gets heated and how microwave ovens work.
This material is based upon work supported by the National Science Foundation (NSF) under Grant No. 083306. Any
opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not
necessarily reflect the views of NSF.
Background on Microwaves
Microwaves are part of the electromagnetic spectrum and are used for many technologies
including communication, radar, and food processing. The electromagnetic spectrum
consists of various types of radiation, characterized by wavelength (λ) and frequency (f).
The frequency of the wave can be visualized as the number of wave crests that move by
an observer in one second (Figure 1). Microwave radiation refers to the region of the
spectrum with frequencies between ~109 Hz to ~1011 Hz. This type of radiation lies
between infrared radiation and radio waves.
Figure 1. How to measure the frequency of a wave. In the case shown here, the
frequency of the wave is 2 Hz, or 2 cycles per second.
Waves with shorter wavelength (higher frequency) have higher energy. The frequency of
visible (orange) light at λ = 600 nm is approximately 5x1014 Hz. Toaster ovens with
either quartz bulbs or resistively heated elements shine with a bright orange and heat food
surfaces by thermal radiation. The frequency of radiation used in microwave ovens is
approximately 2.45x109 Hz (2.45 GHz), with a wavelength of 12.25 cm. This means that
the microwave radiation used to heat food is actually less energetic than visible light.
Why, then, are microwave ovens considered so efficient at heating food? The answer is
related to the interaction of that wavelength with food- in particular water molecules. In
fact, the interaction was discovered by accident. If you have time, you may talk about the
invention of the microwave cooking. An engineer discovered that his chocolate bar
melted in his pocket while working with RADAR technologies.
There are other examples of technologies developed by accidental discoveries. Some
examples are hook-and-loop fasteners, penicillin, saccharin, Teflon coated pans, and
vulcanized rubber. This is also an opportunity to discuss the relation between science and
engineering.
Water Molecules and Microwaves
Water molecules (H2O) are polar; that is, the electric charges on the molecules are not
symmetric (Figure 2). The alignment and the charges on the atoms are such that the
hydrogen side of the molecule has a positive (+) charge, and the oxygen side has a
negative (-) charge.
Figure 2. Polar nature of water molecule
Electromagnetic radiation have electric charge as well; the "wave" representation shown
in Figure 1 is actually the electric charge on the wave as it flips between positive and
negative. For a microwave oscillating at 2.45x109 Hz, the charge changes signs nearly 5
billion times a second (2 times 2.45x109 Hz).
Electric charges react similarly to how magnetic charges do - opposites attract. When
oscillating electric charge from radiation interacts with a water molecule, it causes the
molecule to flip (Figure 3). Microwave radiation used in ovens take advantage of the
water molecules flipping characteristic along with the flipping dissipation due to other
neighboring water molecules. The dissipated energy is in the form of heat which can be
inferred from a temperature measurement rise when the water is in liquid state.
Figure 3. Interaction between microwave and water molecule
So microwaves work well with foods that have water in them. Microwaves also work
with fats and sugars, but not as efficiently. Interestingly, when water is solid (i.e., ice)
then the microwaves do not flip the water molecules and there is no dissipation or heat
addition to the ice. So the defrost cycle on microwaves are designed to heat some small
amount of water on the frozen food in a short period (on the order seconds), wait for that
heat is be conducted through the food (a few seconds again), and then repeat until enough
of the water has melted. Steam also has a weak interaction with microwaves.
Inside a Microwave Oven
Microwaves are produced by a device
called the magnetron which consists of
four major components: (1) anode block
(hollow cylinder with vanes pointed into
the center), (2) cathode filament, (3)
pair of permanent magnets, and (4)
antenna. The top view of the anode
block is shown in Figure 4. The spaces
between the vanes are resonant cavities.
The cathode filament is a cylindrical rod
located at the center of the anode block.
It serves as the source for electrons
during emission of microwave radiation.
Figure 4. Top view of anode block
Two permanent magnets are
located at the top and bottom of
the anode block, as shown in the
side view in Figure 5. These
magnets create a magnetic field
inside the anode block that is
parallel to the cathode filament.
The antenna is positioned so that
it starts in a resonant cavity and
terminates in the waveguide.
The waveguide transfers the
microwaves to the cooking
chamber, much like how fiber
optics are used to transfer light.
Figure 5. Side view of the magnetron
The events that lead to the production of
microwaves begin when the cathode emits
electrons. Since the filament and the electron
both have negative charge, electrons accelerate
outward, toward the vanes. The changing electric
field from the electrons induces a magnetic field
around itself. This magnetic field interacts with
the magnetic field from the permanent magnets.
The interaction of a moving electric charge in a
magnetic field results in a net force. In this case,
the magnetron has been designed so that
electrons move clockwise around the cathode
filament, forming a cloud (Figure 6).
Figure 6. Electron motion in anode block
As an electron approaches a vane, a
positive charge is induced within the
vane as electrons are repelled.
Meanwhile, a negative charge is
induced in a neighboring vane due to
an accumulation of repelled electrons.
Negatively charged vanes then repel
electrons rotating within the cloud
forming a pinwheel shape (Figure 7).
Vanes closest to the "spokes" have a
positive charge, and neighboring ones
are negative. As the pinwheel rotates,
vane charge oscillates from positive to
negative creating an alternating
current within resonant cavities. The
antenna carries current to waveguide.
Figure 7. Electron cloud and induced currents
The final step in a microwave
oven is to release the radiation
within the cooking chamber.
When radiation exits the
waveguide, it is often reflected
by a stirrer blade to evenly
distribute radiation throughout
the cooking chamber (Figure
8). Once entering the chamber,
the radiation is reflected by the
chamber walls until it is
absorbed by the food. However,
the spreading is not even
throughout the chamber.
Figure 8. Microwave chamber
A drawback in microwave ovens
are hot and cold spots. Waves
reflecting around the chamber
interact with each other through a
phenomenon called interference
(Figure 9). Waves overlapping so
that crests match one another
interfere constructively, forming a
hot spot. Waves interfering
destructively result in a cold spot.
Figure 9. Interference of waves
Cooking with a Microwave Oven.
Microwave heats food by agitating the water molecules in liquid state. Most foods we
consume contain over 70% water by weight, making this an effective way for heating
foods. However, the negative side is that food with low water contents take longer to heat
in a microwave. There are also hot spots and cold spots to consider in cooking large
pieces of food.
As mentioned previously, frozen foods take longer to heat because the water molecules in
solid state are not moved as much as in liquid water. Ask students to graph the power
duty cycle for a microwave in the defrost cycle. If you have an actual microwave in the
classroom or lab, the students can make observation (especially by listening to the
microwave) to determine the duty cycle.
At the other extreme, foods heated in a microwave cannot become hotter than the boiling
point of water, or 100°C. After water turns to steam, there is a large drop off in
interaction between the 2.45 GHz frequency and steam. In fact, the preferred frequency
for interaction jumps to 22 GHz. Thus, foods typically cannot be browned in a
microwave since browning occurs at temperatures well over 100°C. This can also be
observed with reheated pies that do not have a crisp crust like a conventional-oven baked
pie would have. A specially designed metal susceptor can be used in food packaging to
allow for browning.
Do microwaves cook foods from inside to outside? No, but food in a microwave oven is
heated much faster than in a conventional oven. Microwave radiation has the ability to
penetrate several centimeters into the food similar to how radio waves can go through
concrete walls or deep into tunnels (up to a point). 2.45 GHz microwaves penetrate into
food several centimeters down and can heat quicker than through conduction alone. If the
food were larger than several centimeters, the middle of the food still needs to be heated
by conduction. So some microwaves will also go through a duty cycle to allow the heat to
spread through the food via thermal conduction.
If marshmallows are placed throughout a microwave oven, there will be some a visible
indicator of the uneven heating (hot spots and cold spots) in microwaves. The hot spots
will have marshmallows enlarging more rapidly due to the heating of the water (and then
the air) in the marshmallows. This uneven heating can be reduced by using a rotary
platform; as well as stirring the food every few minutes.
You can watch the cooking of the foods in a microwave; although it is through a dim
window. Looking closer at the window will review a grid like panel that prevents the
microwaves from leaving the cooking chamber. This works because the spacing in the
grid (which is metal) is much less than the wavelength of the microwave radiation (12.2
cm). The grid does allow visible light (400-800 nm) to pass through since the wavelength
is much smaller than the grid openings. Students can investigate this shielding by
searching for Faraday cages also.
Evaluation
Here are some end-of-class questions to pose to students to help gauge their
understanding of the class content:
1) What are three advantages of microwave ovens over conventional ones?
2) What are three disadvantages to microwave ovens?
3) What technology developments have helped to overcome some of the
disadvantages?
Possible homework problems:
1) Explain in detail the interaction of microwaves and an ice cube that eventually
becomes saturated vapor.
2) Compare a conventional oven recipe with a similar microwave-suitable one (e.g.,
cooking a cake).
3) Research whether or not food cooked in a microwave is germ-free.
Useful Links
How Microwave Ovens Work by HowStuffWorks.com
http://www.howstuffworks.com/microwave.htm
An interactive tutorial on Microwave Ovens from University of Colorado, Physics 2000
project.
http://www.colorado.edu/physics/2000/microwaves/index.html
Invention of the Microwave Oven from American Heritage
http://www.americanheritage.com/articles/magazine/it/2005/4/2005_4_48.shtml
References
Incropera, F.P. and De Witt, D.P., Introduction to Heat Transfer, Second Edition, John
Wiley & Sons, New York, NY, 1990.
McGee, H., On Food and Cooking: The Science and Lore of the Kitchen, Simon and
Schuster, New York, NY, 1984.
Horn, J., Cooking A to Z: The Complete Culinary Reference Source, New and Revised
Edition, Cole Group, Santa Rosa, CA, 1997.
Wolke, R.L., What Einstein Told His Cook, W.W. Norton & Company, New York, 2002.
© 2011 Peter Wong. All rights reserved. Copies may be downloaded from www.EngageEngineering.org. This material
may be reproduced for educational purposes.