Microwave
Microwave Mischief
and Madness
Heather Hosack, Nathan Marler,
and Dan MacIsaac, Department of Physics
& Astronomy, Northern Arizona University, Flagstaff AZ 86011-6010; danmac@nau.edu
M
icrowave ovens use electromagnetic
radiation to heat and cook food. A
typical microwave oven uses a mag1
netron to produce about 1 kW of radiated power at a frequency of 2450 MHz, wavelength of
12.2 cm, electric field strength of 2 kV/m, and
power density of approximately 2.8 MW/m3.2
Microwave radiation penetrates foodstuffs
and is absorbed by the flexure of polarized water
and food molecules, which rub against one another synchronously with and at harmonics of
the oscillating electric field component of the radiation. Molecular vibration in the right frequencies is also known as thermal energy or heat;
hence, microwave electromagnetic radiation becomes thermal energy for cooking foodstuffs.
This effect is sometimes called dielectric heating.
There is NO notable molecular bond resonance
effect in microwave ovens, and some materials
with more tightly locked molecules (such as
frozen foods) don’t flex very well and don’t microwave well.
The walls of a microwave are made of metal,
which allows electrons to freely move in closed
paths and cancel out the electric fields in the
oven walls. Thus, microwave radiation cannot
penetrate the closed box walls of the oven (or the
metal mesh covering the window in the front
door) but are reflected back into the oven and
through the food until they are absorbed. If the
microwave oven is operated without a load (material to absorb the microwave energy), microwaves will reflect about until they re-enter the
magnetron and eventually damage it; therefore,
you should always operate the oven with a load.
A small glass of water is sufficient.
Electrically isolated pieces of metal placed in a
14
Fig. 1. Lighted bulb.
Fig. 2. Broken bulb.
microwave oven cannot allow electrons to move
in closed paths; electrons will “pile up” on the
edges of the metal object and may arc over
through the air to another part of the metal. For
example, small pieces of metal placed in an electric field of 2 kV/m and separated from one another by only 1 cm can experience an electric
field difference of:
V
m
⌬V = (2 ⫻ 103 ᎏᎏ)(1 ⫻ 10–2m) = 20,000 V,
which will produce a spark discharge in the microwave between the pieces of metal.3 Electrons
THE PHYSICS TEACHER ◆ Vol. 40, May 2002
➤ Grape races. Prick one end of several seedless
grapes with a toothpick and place on a glass
plate. Also demonstrates water’s dielectric
properties. Try a grape cut in half with a
knife, leaving a thin flap of connective tissue
(electrons try to move through highly resistive grape-skin, and plasmoids may form).
Microwave Demonstration
Safety Issues & Guidelines
Fig. 3. Lighted CD in microwave.
accelerating though the air will slam into nitrogen molecules in the air, which will emit ultraviolet and blue light via electronic excitation and
emission. Hence, microwaving metal objects
will produce extremely hot, bluish sparks.
Some Microwave Oven
Experiments
Prepare the oven by taping a paper file card
over the light in order to darken the interior.
Darken the room for your audience. Consider
using an old glass plate that you won’t miss underneath objects in the oven. Times vary with
each experiment; always use high power, but
note the duration in parentheses.
Small spark discharges (relatively safe)
➤ CDs (6–8 s). Electrons arc across thin foil
tracks and destroy the CD by vaporizing the
thin metal, constantly changing the discharge paths. Do not breathe the smoke or
melt the plastic (see Fig. 3).
➤ Small Christmas tree ornaments. See CDs.
➤ Frozen or fresh-diced carrots. Arcs between
the corners. Electrons collect on corners
(points of most curvature) and edges, causing
arcing.
➤ Bar of soap. Ivory or Irish Spring (with
whipped-in air bubbles for flotation) works
well; wet the topside. Microwave until it
bubbles with steam and re-sculpts itself (because of water’s dielectric properties).
THE PHYSICS TEACHER ◆ Vol. 40, May 2002
When performing these microwave demonstrations please
follow and provide these safety guidelines to the audience:4
Protect Yourself and Others — Don’t microwave yourself. Never try to defeat the safety interlock and run
the microwave oven with the door open; severe tissue
damage can occur. Perform experiments in a fume
hood or well-ventilated area. Don’t burn yourself —
most hot items (especially glass trays and bulbs) look
just like cold items, except they will burn you. Sealed
items (like eggs) and containers can explode when
heated. Use gloves or oven mitts. Have bystanders
maintain a distance of three feet, or use a shield.
When demonstrating for a class or a crowd, consider
using a video camera on manual focus.
Beware of being scalded by hot fluids — In a ceramic
container it is possible to superheat water; some pockets can be hotter than 100⬚C. Superheated water will
flash boil or geyser out of the container if boiling is
suddenly triggered by vibration, for example, from an
object (like a spoon) or a powder or your upper lip. If
you are superheating fluids, protect yourself with
appropriate eye protection or a face shield and long
sleeves.
Protect the oven — Especially if it belongs to Mom or
the department secretary! Use a small container of
water in a back corner to absorb excess microwave
energy and avoid magnetron damage. Run these
experiments for a maximum of 30–60 seconds and
allow cool-down time.
Fluorescent lightbulbs – Dramatic, but breaking or melting these will cause release of toxic mercury vapor.
Don’t microwave these until they melt. Incandescents
are fair game (see Fig. 1).
Plasma and vaporized metal are extraordinarily hot
— Some scientists refer to plasma as the fourth state
of matter. If making plasma in the microwave oven,
watch for molten glass and plastic, and turn off the
oven if plasmoids lodge against the walls of the oven.
(Have a fire extinguisher on hand just in case.)
Vaporized metal and plasma can melt through the
oven walls, set fire to plastic liners and paint, and
crack and melt glass (see Fig. 2). Careless experiments
with plasma in a microwave oven will result in you
buying a new one (and maybe unwanted attention
from the fire department).
15
Hot objects and larger discharges (less safe)
➤
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➤
➤
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16
Remember to put a glass of water in the
oven for a load.
Lightbulbs. Put incandescent lightbulbs
(burned-out or working) into a microwave,
either in a glass of water, or just standing in
an empty glass. Additional microwaving of
vaporized tungsten will melt through the
bulb (Fig. 2); standing the bulb in water
eliminates this. Rapid electron motion excites the atoms of the very thin and resistive
filament.
Small fluorescent tubes or neon bulbs (3–4 s).
Twist the neon bulb leads together and place
them in a small ball of tinfoil. Disposable
camera flash units (xenon strobe bulbs) and
fluorescent tubes work well. Watch the heat
— additional microwaving can melt the
bulbs.
Kitchen scour pads (Cu, steel wool). Ignites if
enough time is given. Electrons will collect
on irregularly shaped edges of tightest curvature and arc.
Aluminum-foil rings (10-cm diameter, twist a
constriction at one point) or copper coils.
These arc, create plasma, and generate ultraviolet light (this heavily loads the magnetron,
so keep it under 10 seconds and allow cooldown time). The aluminum will melt glass,
so use old glass beneath it. Be sure to leave a
gap in the ring; electrons will collect on irregularly shaped edges and arc.
Copper wiring. These create arcs between the
wiring. Most thin metal wiring will work
well for this. Electrons will collect on irregularly shaped edges and arc.
Superheated water. Wear goggles and oven
mitts. Boil and cool to remove air bubbles;
heat in a smooth ceramic container. Water
will superheat (exceed its boiling point) and
flash-boil after a nucleation seed, such as instant coffee, is introduced. An infamous accident for people microwaving coffee.
Ball lightning (plasma). Place wooden
kitchen matches in putty, then in the oven.
Ignite the matches, then blow them out after
a short time. Microwave the smoke, which
ignites and moves about as plasmoids.5
Wooden toothpicks placed alongside candlewicks also work. One current theory for
the little-understood phenomenon of ball
lightning involves excited carbon microparticles. Be careful not to set fire to the interior
of the oven.
Acknowledgments
This student project was supported by the NAU
Department of Physics and Astronomy Physical
Science Teacher Preparation program, and the
Arizona Teacher’s Excellence Coalition
(AZTEC). Photography by Rebecca Davis,
Sarah Bickford, Nathan Marler, and Heather
Hosack.
References
1. Magnetron and microwave ovens are explained at
http://www.gallawa.com/microtech/how_
work.html and http://rabi.phys.virginia.edu/
HTW/microwave_ovens.html. Other explanations of how microwave ovens work can be found
in Craig Bohren’s letter “How does the microwave oven really work?” Am. J. Phys. 65, 12
(1997), as well as in Eugene Hecht, Physics: Algebra/Trig. (Brooks-Cole, Pacific Grove, 1998).
2. A description of basic calculations for the microwave is available at http://www.pueschner.
com/engl/basics/calculations_en.html.
3. Metal arcing in microwave ovens and a discussion
of why it happens can be found at http://www.
gallawa.com/microtech/metal_arc.html.
4. For good safety reference in regard to microwave
oven experiments followed by similar activities,
see http://www.everist.org/special/mw_oven/
index.htm.
5. There is a wealth of scientific literature on microwave-driven plasma, ball lightning, and speculative links between the two phenomena. Bibliographies can be found at http://www.sciam.
com/askexpert/physics/physics30.html and
http://www.eskimo.com/~billb/tesla/ballgtn.html.
THE PHYSICS TEACHER ◆ Vol. 40, May 2002
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