How a Microwave Oven Works?

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How a Microwave Oven Works?

• By Emmy Mak

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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.

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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 lt 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 gt1 mm
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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

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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

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• 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

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• 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.

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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.
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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

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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
• Ex. the lens of the eye exposure to high levels
  of microwaves can cause cataracts.
• 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
• Still experimenting
• Best to stay a way (an arms length) in reducing
  exposure to microwaves

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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/h
  base/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.