Transcript How a Microwave Oven Works?

‫?‪How a Microwave Oven Works‬‬
‫‪By: Emmy Mak‬‬
‫קובץ זה נועד אך ורק לשימושם האישי של מורי הפיזיקה ולהוראה בכיתותיהם‪ .‬אין לעשות שימוש כלשהו‬
‫בקובץ זה לכל מטרה אחרת ובכלל זה שימוש מסחרי; פרסום באתר אחר (למעט אתר בית הספר בו מלמד‬
‫המורה); העמדה לרשות הציבור או הפצה בדרך אחרת כלשהי של קובץ זה או כל חלק ממנו‪.‬‬
What are Microwaves?

Microwaves are a form of electromagnetic energy, like light
waves or radio waves

Microwaves are used extensively in communications

such as to relay long-distance telephone signals, television programs and
computer information across the earth or to a satellite in space.

Good for transmitting information because it can penetrate
haze, light rain and snow, clouds, and smoke.

Also used in radars and in detecting speeding cars.

Microwave has become most familiar as the energy source
for cooking food.
Electromagnetic Spectrum


Electromagnetic
radiation exists in a
range of frequencies
called the
electromagnetic
spectrum.
Each frequency has a
specific wavelength and
as the frequency
decreases, the actual
length of the wave gets
longer.
Table 1: Frequency and Wavelength Range of Each Radiation
Type in the Electromagnetic Spectrum.
Radiation Type
Frequency Range (Hz)
Wavelength Range
Gamma rays
above 3 x 1019
< 10-12 m
X-rays
3 x 1017 - 3 x 1019
1 nm - 1 pm
Ultraviolet Radiation
7.5 x 1014 - 3 x 1017
400 nm - 1 nm
Visible Spectrum
3.8 x 1014 - 7.5 x 1014
750 nm - 380 nm
Infrared Radiation
1011 - 3.8 x 1014
25 um - 2.5 um
Microwaves
108 - 1012
1 mm - 25 um
Radio waves
104 - 108
>1 mm
History




Invented accidentally by Dr. Percy LeBaron Spencer
While testing a magnetron during work, he discovered
the candy bar in his pocket melted
Experimented with other food products (popcorn and
eggs), and realized microwaves can cook foods quickly
At 1947, 1st commercial microwave oven produced
(called Radarange)


Mostly used by restaurants, railroad cars,
ocean liners and military
Improvement and refinements made ~
by 1967, 1st domestic microwave oven
produced
How the Oven Works



Electricity from the wall outlet travels through the power
cord and enters the microwave oven through a series of
fuse and safety protection circuits
These circuits include various fuses and thermal
protectors that are designed to deactivate the oven in the
event of an electrical short or if an overheating condition
occurs
When the oven door is closed, an electrical path is also
established through a series of safety interlock switches
• Sensing that all systems are set to go, the signal activates triac, producing a
voltage path to the high-voltage transformer.
• The high-voltage transformer along with a special diode and capacitor
arrangement increases the typical household voltage from ~115 volts to ~3000
volts
•
•
•
•
The magnetron converts the high voltage in to the microwave frequency for
cooking
The microwave energy is transmitted into a waveguide
The waveguide feeds the energy to the stirrer blade and into the cooking area
When the door is opened, or the timer reaches zero, the microwave energy
stops.
Magnetron
Electrons from a hot filament would travel radially to
the outside ring if it were not for the magnetic field.
The magnetic force deflects them as shown and they
tend to sweep around the circle. In doing so, they
“pump” the natural frequency of the cavities. The
currents around the resonant cavities cause them to
radiate electromagnetic energy at that resonant
frequency.
How Foods Get Cooked


The microwaves that penetrate the food have an electric field
that oscillates 2.45 billion times a second, a frequency that is
well absorbed by polar liquid molecules such as water, sugars,
fats and other food molecules.
Water interacts with the microwave:




flipping its orientation back and forth very rapidly
bumping into one another and producing heat, cooking the food.
Glass, paper, ceramic, or plastic containers are used in
microwave cooking because the microwaves pass through
them
Metal reflects microwaves

Unsafe to have metal pans/aluminum foil in oven, may damage oven
Health Hazards


It is known that microwave radiation can heat body tissue the
same way it heats food.
Exposure to high levels of microwaves can cause a painful
burn



Microwave oven used low level of microwaves, within the
region of non-ionizing radiation
Still uncertain in the effects of humans from long term
exposure to low level of microwaves


Ex. the lens of the eye ~ exposure to high levels of microwaves can
cause cataracts.
Still experimenting
Best to stay a way (an arm’s length) in reducing exposure to
microwaves
References
1.
Dalhousie University, Halifax, Nova Scotia.
http://www.dal.ca/~ehs/files/microwave.safety.pdf
2.
Homepage of Central Valley Christian School.
http://home.cvc.org/microwaves/
3.
Nave, C. R. Hyperphysics. Georgia State University.
http://hyperphysics.phy-astr.gsu.edu/hbase/waves/magnetron.html
4.
Microtech: The Complete Microwave Oven Repair and Information
Network.
http://www.gallawa.com/microtech/index.html
5.
Wright, Michael and Mukul Patel. Scientific American: How Things
Work Today. New York: Marshall Editions Development Ltd., 2000.