MadSci Network: Physics

Re: Why do some onions spark when heated by microwave?

Date: Wed Sep 6 10:26:16 2000
Posted By: Adrian Popa, Director Emeritus, Hughes Research Laboratories
Area of science: Physics
ID: 968028200.Ph

Your question is full of interesting physics and my answers are a
bit complicated. Iíll try to make them understandable. I
answered a similar question about exploding and arcing
grapes and cranberries which can be found in the Mad Science
archives at the following URL:

Iíll repeat the grape discussion and then discuss where diced
onions are a bit different from the water filled grapes.

First let me define 3 terms:

Dielectric - so called because it permits the passage of the
lines of force of an electrostatic field but does not conduct
current. Some times called an insulator
(e.g. plastic, rubber, glass, Teflon, distilled water).

Dipole antenna - two linear conductors (commonly equal in
length) separated at the center by a transmission line feed
(e.g. TV rabbit ears antenna).

Plasma - a high temperature ionized gas (e.g. neon sign,
welding torch, gas laser, anelectric arc)

Microwave ovens come in many different designs and sizes and
your experience with grapes (onions) may not be able to be
repeated in another design of oven. I found that I had to
place the grapes that I tested in the center of my oven,
raise them up on top of an inverted water glass. Also, my
experiments did not work with a cup of water inside the oven
along side of the grapes. The cup of water inside the oven is
a safeguard load for the microwave magnetron power tube. A
microwave oven should NEVER BE OPERATED with out some material
to cook inside the oven and all manufactures tell you to put a
cup of water in the oven if you want to test the oven.
Heating just a few grapes (onions) with nothing else in the
oven could possibly damage the microwave tube if the water in
the grapes evaporates to quickly.

Microwave ovens generate between 500 watts and one thousand
watts of microwave power at a frequency of 2400 megahertz
(MHz or 2400 million cycles per second), corresponding to a
wavelength of 12.5 cm (4.9 inches)in air. However, the oven
wavelength in the dielectric distilled water is reduced to
1.4 cm (0.55 inches). As you shall see this is an important
part of the answer to your question. Also, for an arc to occur
in the air inside a microwave oven, a microwave field of at
least 30,000 volts per centimeter must be generated! With
the grapes (onions) separated by one millimeter (0.04 inches)
we need to generate a 3000 volt potential between the grapes
(onions)to generate arcing! This is a considerable amount of

First, I found that the experiment would not work with a cup
of water in the oven to help protect the microwave tube. The
grape experiment required all of the power my oven
could produce (800 watts) and it could not share this power
with a cup of water.

Not being able to have a cup of water in the oven causes me
to caution anyone about trying to repeat this experiment
because they might damage their microwave oven.

For my experiments I purchased large 2.54 cm (1 inch) diameter
red grapes. To protect the oven's microwave tube, I limited
the oven timer to ten seconds for each test so that the
grapes could not dry out during the experiment. I also used
fresh grapes for each experiment so that I knew they were
water filled and not dried out.

I found that single grapes would eject steam out of the stem
hole forming little rocket engines which often propelled the
grapes about the oven. If the stem was left in the grape,
so that the steam could not escape, the grape skin would
quickly rupture in a small explosion as it was heated. This
reminded me that most microwave oven instruction books
tell you not to cook eggs in the shell in an oven for the
steam build up inside the hard shell will eventually cause
the egg to explode causing a big mess in the oven.
When I put two grapes close together with the stem holes close
to each other the arching and sparking you described occurs.
The arc made a 120 Hertz buzz following the pulsing
power of the microwave magnetron tube. When I placed the stem
holes tightly together the arcing would not occur.
When I heated single grapes and touching grape pairs together
in the oven for 10 seconds, the touching pairs were at a
significantly higher temperature after heating than the single
grapes were! This suggests some form of enhancement of the
microwave field in the grape pairs (perhaps a dipole antenna).
I also repeated these experiments with large cranberries.
They have less water in them than grapes and are smaller in
diameter. I was able to produce smaller arcs between cranberry
pairs; however, I had to cut a small hole in the berry ends
to let the steam escape to form an arc.

There are two general classes of antennas, metallic conducting
antennas and dielectric antennas that concentrate
electromagnetic fields. The common antennas most people are
familiar with are antennas made from conducting wires and rods
such as the rabbit ears on indoor TV antennas or the multirod
TV antennas on millions of roof tops. Dielectric
antennas include various geometric solids including cylinders,
rectangular boxes, spheres and plastic focusing lenses.
Non conducting dielectric materials are used for microwave
cooking ware because they are relatively transparent to
microwave energy. Also dielectric heating of food, particularly
the water molecules in food, is the key principle used in
microwave cooking. Dielectric spheres one or more wavelengths
in diameter form a special class of microwave antenna structure.
When a dielectric sphere is immersed in a microwave field the
spheres concentrate the electric field lines along an axis as
shown in Figure A (see reference). If the sphere is
slightly elongated, the field will usually align with the longest
axis. This is exactly what water filled grapes (which are one or
two wavelengths in diameter) will do in a microwave
oven. The concentrated microwave field inside the grapes
quickly heats the grapes to a high temperature after only 10
seconds of heating. (see diagram in Mad Science archive reference)

Next I put two grapes with their stem holes tightly together
and the pair of grapes form a larger more efficient dipole
like antenna as the microwave energy field flows between the
two coupled grapes. I believe this is why the coupled grapes
are much hotter after 10 seconds of heating than single grapes

Finally, I slightly separated the stem holes of the grape pairs
by about one millimeter. As the grapes are heated each grape
emits a jet of steam toward the other grape and the
concentrated microwave fields from the spheres reach more than
3000 volts exciting the steam into a plasma state. The plasma
forms a short circuited conductor between the dipoles and we
get the arching in the region of steam between the grapes.
When the grapes have expelled their steam pressure I found that
the plasma extinguishes and the arc goes out.
This explanation is based on spheres filled with pure water and
we know that the grape juice is acidic not just pure water.
A more complex reaction is occurring within the grapes.
However, the external resultant arcing would probably be about
the same for both cases.

I said in the beginning that this is a complicated experiment
with a complicated answer. It would take some expensive microwave
equipment and time to study the arcing grapes in
more detail in the laboratory. I hope this helps answer some of
your questions.

Diced onions do not contain as much water as grapes; however,
the steam pressure in them will be ejected from between the onion
layers forming a connecting conducting plasma similar to that in the
grapes. In the case of diced onions, depending on their size,
several chunks would need to be electrically aligned in a random manner
to form an antenna for arcing and burning.
These antennas will not be generate as great an internal electric
field within the onion hunks as in the sparking grapes, but the effect
is similar. The onion hunks form a dielectric transmission line and
the interconnected hunks form a sort of resonant antenna to build up
the large fields required for arcing and burning>

Best regards, your Mad Scientist
Adrian Popa

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