Microwaves and Microwave Oven
Prof. Tai Kai NG and Mr. Chi Wai CHAN
Department of Physics, Hong Kong University of Science and Technology
History
The idea of microwave oven was first conceived by Percy L.
Spencer in 1946 and finally patented in 1950. The mechanism
is based on the principle of frictional heat production using
microwaves. Because of the huge size of the early microwave
ovens – weighted over 300 kg and measured over 1.5 metre in
height – they were mainly used in large restaurants and food
outlets. The first domestic microwave oven was produced by
Raytheon Corporation in 1965.
(The first home
version microwave
oven)
What are Microwaves?
Microwaves form parts of the electromagnetic spectrum with typical wavelengths
from 1 millimetre to 10 centimetres – something in between light waves and radio
waves (Figure 1). Like any other electromagnetic waves, the microwave is composed
of both electric and magnetic fields, which are perpendicular to each other and
propagates at the speed of light throughout space(speed of light “c” in vacuum is
roughly 300,000,000 metres per second, or 300,000 kilometres per second) . Apart
from ovens, microwaves are also used in telecommunications, e.g., radars, wireless
computer networks and mobile phones. In fact, the entire universe is filled with
microwave radiation left by the Big Bang explosion of the early Universe.
(Figure1 The Electromagnetic Spectrum)
The Microwave Oven
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The core part of a microwave oven is the microwave generator. In the early designs,
the generator is a magnetron, which is a vacuum tube that converts electrical energy
into microwave energy. The microwaves are transported to the cooking chamber by a
wave-guide, analogous to electrical wires transporting electricity. The cooking
chamber is designed to keep the microwaves, so that microwaves bounce off the walls
like beams of light in a mirrored room around and absorbed by food. Metal wiring on
the glass window of the door keeps the microwaves from leaving the cooking
chamber.
(Microwave Oven)
Cooking with Microwaves
Microwave cooking is a process of exciting the water molecules in food. Food
substance that does not contain water, such as oil, usually does not get cooked by
microwaves. Water molecules are polar molecules with one end dominated by a
negative charge and the other by a positive charge. The typical frequency of the
microwave produced by a microwave oven is of the order of 2.5 GHz (1 GHz =
1,000,000,000 Hz). That is to say, the electric fields produced by the magnetron
oscillate back and forth at the rate of 2.5 billion times per second (defining 1 billion =
1,000,000,000). Since the water molecules are polarized, the water molecules in the
food will also get pulled back and forth at the rate of about 2.5 billion times per
second. This rapid back-and-forth motion between water molecules creates friction,
and hence heat. Typically, microwave can only penetrate about 3.5 to 5 cm into the
food. The centre of the food is cooked mainly by heat conduction. The hot and cold
spots on the food are caused by the interference effect of microwaves, similar to
bright and dark spots on a screen caused by interference of light after passing through
a grating. In order for microwaves to distribute evenly around the food, a turntable or
a rotating source is often used.
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The exact amount of microwave energy need to cook a piece of food depends on its
properties. In practice, the amount of microwave energy get absorbed and reflected
can be measured for each food substance. Food substance contains a substantial
amount of water is a good absorber of microwave energy. When water is changed to a
different state, such as ice, it becomes a poor absorber of microwave energy. Other
factors determining the effectiveness of microwave cooking are the shape and
material of the container used. Round containers generally have better cooking result
as food tends to be overheated at sharp corners and edges.
Metallic Objects
Microwave radiation can pass through plastic and glass, but not metallic objects. This
is why the glass window of the door of microwave oven is laced with metal wiring;
the metal wiring keeps the microwaves from leaving the cooking chamber. If you put
a metal object, such as a fork, into the oven, microwaves hitting the fork will get
reflected back to the source. The extent to which microwaves are reflected varies from
the types of metal. Certain metal composites actually absorb microwave energy.
Let us take a closer look at what happens when microwaves come into contact with
metallic objects. When microwaves are reflected from a metallic object, it produces a
so-called arcing effect (Arcing is a microwave term for sparks in the oven). Arcing is
resulted from the build-up of excessive charges in metallic materials. In microwave
ovens, this can occur when two or more metallic objects, e.g., a metallic food
container and the inner metallic wall, are placed close to each other causing the air
between them to ionize, producing sparks. The arcing effect can also take place when
a cooking utensil with sharp metal edges or burnt food are come into contact with
microwaves inside the oven. If the arcing object is further bombarded by microwaves,
the temperature of the object rises sharply and being overheated and eventually
causing a fire in the oven.
Superheating
A phenomenon known as superheating is known to occur when a cup of water is
heated in a microwave oven. The water is referred to as superheated when its
temperature increases above its normal boiling point (the boiling point for water at
standard atmospheric pressure is 100 0C). The superheated state is an unstable state.
When water is superheated, the presence of an external agent such as a spoon or even
milk powder can cause the water to boil vigorously into explosion. If one litre of
water is superheated by only 1 °C, it can produce about 3 litres of steam. You can see
how dangerous it is.
Safety of Microwaves
The safety of microwave radiation in microwave ovens is still a subject of intense
research. From the above discussions it is clear that we should not put metallic objects
or boiled water into microwave oven. Other tests have revealed that nearly all
microwave ovens in the market emit microwaves whilst in operation. In general, the
amount of microwave emission decreases with the distance. The official
recommended value of microwave emission for all domestic ovens is about
10mW/cm2.
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As cellular or mobile phones are getting increasingly popular in recent years, the
effect of microwaves on our health has also been a subject of study. The main concern
is the effect of heat produced by microwave radiation on our body, particularly the
brain. Despite the fact that the recorded temperature rise on our body caused by
microwave radiation emitted from mobile phones was so low, just a fraction of a
degree Celsius, some researchers believe that radiation emitted from mobile phones
could increase the risk of brain diseases such as the Alzheimer's disease.
In addition to microwave radiation, micro-waved foods are also subject to
investigation. Some tests have revealed that the molecular structures of nutrient in
vegetables, such as carrots and broccoli, are being deformed at cellular level by high
frequency microwaves.
[About the author: Professor Tai Kai NG received his BSc degree from the University
of Hong Kong in 1981. He completed his PhD degree at Northwestern University in
1987. He joined the Physics Department of HKUST in 1991. In 1993, Professor Ng
received an Honorable Mention in the 1993 Achievement in Asia Award of the
Overseas Chinese Physics Association, for his contribution to the understanding of the
non-equilibrium mesoscopic Kondo effect. He is a Fellow of the American Physical
Society since 2000. He is awarded the Croucher Senior Research Fellowship for
2002-2003. Prof. Ng also participates actively in Hong Kong's secondary and primary
school science education. He is a (Hong Kong) representative in the joint Hong KongMainland expert working groups on Science Education.]
[About the author: Mr. C W Chan is a Research Assistant, Department of Physics,
HKUST.]
Keywords:
Microwaves, Electromagnetic waves, Electric field, Friction, Dielectric constant,
Superheating
Related Topics in the Syllabus:
Electricity and Magnetism, Heat and Friction
Extensions (from the syllabus):
Bring out ideas: Microwave as part of electromagnetic spectrum, Heat production via
friction of polar molecules
Warm-up discussion:
o
What is electromagnetic wave?
Electromagnetic waves (EM waves) are transverse waves. They are the
oscillations of electric field and magnetic field in space. EM waves can
propagate in vacuum or media. The speed of EM waves in vacuum c=
is a constant. It is the fastest speed in the universe. Visible light,
microwave and gamma ray are examples of EM waves.
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o
What is the function of a magnetron?
Function of magnetron: It can convert electrical energy into microwave
energy. And the microwaves produced are then used for microwave
cooking.
o
What is the function of the metal wiring in the glass window of the
door?
It keeps the microwaves from leaving the cooking chamber.
o
Why water molecules are referred to as polar molecules?
Water molecules are polar because one end is dominated by a negative
charge (Oxygen atom) and the other by a positive charge ( 2 Hydrogen
atoms).
o
How does food substance get cooked in a microwave oven?
The magnetron of the oven produces microwaves to oscillate the water
molecules inside the food. The water molecules get pulled back-andforth at the rate of about 2.5 billion times per second. This rapid backand-forth motion between the water molecules creates friction, and
hence heat. Typically, microwave can only penetrate about 3.5 to 5 cm
into the food. The centre of the food is mainly cooked by heat
conduction.
o
Why should we avoid putting metallic objects inside a microwave oven?
When microwaves are reflected from a metallic object, it produces a
so-called arcing effect (Arcing is a microwave term for sparks in the
oven). Arcing results from the build-up of excessive charges in
metallic materials. In microwave ovens, this can occur when two or
more metallic objects, e.g., a metallic food container and the metallic
interior of the wall, are placed close to each other causing the air
between them to ionize, producing sparks. The arcing effect can also
be produced when a cooking utensil with sharp metal edges or burnt
food are come into contact with microwaves inside the oven. If the
arcing object is further bombarded by microwaves, the temperature of
the object rises sharply and overheats and eventually causing a fire in
the oven.
Points for further discussion:
o
What happens if non-polar molecules are radiated by microwaves?
No reaction.
o
What happens if an egg is heated in a microwave oven?
The egg will bomb.
o
How to prevent “superheating” when a cup of water is heated in a
microwave oven?
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Eg. To add a wooden spoon in the cup.
o
How the specific heat capacity c of the food may be estimated by the
microwave oven?
By Pt = mc
o
Compare the cooking process of steaming with a wok and microwaving with a microwave oven in terms of energy transfer.
-Steaming: Water is heated by conduction through the wok. The
internal energy of water increased further when it changes to steam at
100 C. When the steam touches the food, it condensates and releases
the latent heat to the food. The food is hence cooked.
-Micro-waving: In the microwave oven, electrical energy is converted
to microwave energy by magnetron. The microwave is emitted to
everywhere inside the cooking chamber. When the microwave reaches
the food and gets absorbed by the water molecules there, the food is
heated by radiation. Once the surface of the food is heated, the internal
of the food (about 3.5 – 5 cm below the surface) where microwave
cannot reach is heated by conduction.
Activities:
o
Teachers can bring their students to the H.E. room to present this
interesting topic and demonstrate microwave cooking.
- Ask students to heat something up to find out the specific heat
capacity of the food by assuming that no water is running away during
heating.
- Compare the traditional cooking method (steam) to that of the
microwave oven to see their difference in the H.E. Room.
o
Study the concept of “interference” mentioned in the section “Cooking
with Microwave”.
What is interference and how does it occur?
Approach:
- Study wave with a slinky spring.
-Show the relation between vibration of medium and wave
propagation.
-Show the adding and subtraction of wave pulse.
(Introduce the principle of superposition.)
-Show interference by adding of two trains of waves (incident and
reflected waves) – introduce nodes and antinodes and their relations
to energy.
-Show the interference of light with laser pointer and a double-slit
slide.
Related web sites:
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o
The History of the Microwave Oven
This site contains a brief history of the Microwave Oven.
http://www.gallawa.com/microtech/history.html
o
The Magnetron TubeStructure and Operation
A brief introduction of the “heart” of microwave oven---the magnetron.
http://www.gallawa.com/microtech/
magnetron.html
o
eBuyGuru.com - Microwave Oven(working)
A brief note on the operation of microwave oven. It also introduces the key
components of a microwave oven and their function.
http://www.ebuyguru.com/BuyingGuide.asp?
categoryID=23&articleID=37
o
Physics2000 - Microwave Ovens
This site contains many simple animations and simulation programs to reveal
how microwave oven works.
http://www.colorado.edu/physics/2000/
microwaves/index.html
o
Microwave Oven Experiments
These 2 sites contain many videos of untried experiments using microwave
ovens. These experiments are so dangerous that it is not recommended to try
them without teacher’s guidance and sufficient safety measures.
http://home.earthlink.net/~marutgers/fun/
microwave/microwave.html
http://www.eskimo.com/~billb/weird/
microexp.html#exper
o
Food and Environmental Hygiene Department - Microwave Oven
Cooking Hygiene and Safety
This site contains some hygiene and safety tips in using microwave oven.
http://www.info.gov.hk/fehd/safefood/library/
microwave/micro.html
o
Sciscape 新聞 - 微波爐的新用途 (Chinese version only)
This site discusses the latest applications of microwave oven in chemistry.
http://www.sciscape.org/news_detail.php?
news_id=989
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