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Question:

If you put a CD in a microwave oven, it will

do nothing.
burn up the microwave oven.
burn up the CD.

Observations About Microwaves

Microwave ovens cook food from inside out
They can cook foods unevenly
They don’t defrost foods well
You shouldn’t put metal inside them?!
Do they make food radioactive or toxic?

Electromagnetic Spectrum

Long-wavelength EM waves: Radio & Microwave
Medium-wavelength: IR, Visible, UV light
Short-wavelength: X-rays & Gamma-rays

Water Molecules

Water molecules are unusually polar
An electric field orients water molecules
A fluctuating electric field causes water
molecules to fluctuate in orientation

Microwave Heating

Microwaves have fluctuating electric fields
Water molecules orient back and forth
Liquid water heats due to molecular “friction”
Ice doesn’t heat due to orientational stiffness
Steam doesn’t heat due to lack of “friction”
Food’s liquid water content heats the food

Effects of Microwaves

Non-Conductors: Polarization
Mobile, polar molecules orient and heat
Immobile, polar molecules do nothing much
Non-polar molecules do nothing much
Conductors: Current flow
Good, thick conductors reflect microwaves
Poor conductors experience resistive heating
Thin conductors experience resistive heating

Interference

Identical waves that overlap can interfere
Interference is when the fields add or cancel
Adding fields are constructive interference
Canceling fields are destructive interference
Reflects cause interference in a microwave
Interference causes uneven cooking
Good microwaves “stir” waves or move food

Generating Microwaves

Magnetron tube has tank circuits in it
Streams of electrons amplify tank oscillations
A loop of wire extracts energy from tanks
A short ¼-wave antenna emits the microwaves

For more

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[attachment=15277]
How a Microwave Oven Works?
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.
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
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.
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
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 arm’s length) in reducing exposure to microwaves