How A Microwave Works
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DO NOT REMOVE A MICROWAVE OVEN CABINET UNLESS YOU ARE VERY COMFORTABLE WITH WHAT YOU ARE DOING!
Most foods contain water molecules. Each molecule of H2O has a positively and negatively charged end, just like the north and south poles of a magnet. The electromagnetic field of a microwave orients all of the positive ends in one direction. However the magnetic field reverses 4.9 billion times each second. This causes the molecules to flip back and forth attempting to remain balanced. This microscopic flipping motion creates friction-producing heat. Glass, ceramic and plastic containers are water-free and thus remain cool, although heat transfer from the exited food gives the impression that microwaves are heating them as well.
To understand what a microwave is we need to look at the electromagnetic spectrum. A microwave is a large radiation wave about 1 centimeter in length slightly shorter than ultra high frequency radio waves. To give a general picture, an electromagnetic wave length between 1 meter and 10 kilometers (about 6 1/4 miles) is used for radio broadcasting. Visible light waves are between 300 and 650 nanometers that's about 3.5x10 to the negative 26th power percent of the spectrum, that's a lot of zeros! On the other side of the spectrum are x-rays and gamma rays. I wont even try to wrap your mind around how short a wave that is.
The microwave ovens transformer, diode and capacitor raise your household electrical supply from 120 volts to about 3,000 volts. The magnified voltage energizes a relatively small wire within the magnetron called the cathode filament. As the filament heats up it emits negatively charged electrons. Large magnets generate a magnetic field that causes the cloud of electrons to revolve. As it does, spokes or arms are created that pass positively charged plates. Each passing spoke provides a negative charge to the cavity between the plates, which then falls off until the next spoke arrives. The rise and fall creates an electromagnetic field in the cavities that oscillates at 2.45 gigahertz. And that's how microwaves are born. The newly created microwaves move through a waveguide into the cooking chamber, where the waves reflect off metal walls and stimulating any molecules receptive to this vibration.