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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 as they attempt 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.
Let's get 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 electromagnetic spectrum! Perhaps we may want to reconsider the saying “Seeing is believing”.
On the other side of the spectrum are x-rays and gamma rays. I won't even try to wrap your mind around how short a wave that is.
The microwave oven's 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 stimulate any molecules receptive to this vibration.
This magic box of electromagnetic radiation has been widely used since the 1950s. But is it safe? “If everyone was jumping off a cliff would you?” Well in response to that question I would usually ask, “How many people jumped because it looked like fun, and how many survived?” Containing such a powerful force for the sake of fast meal preparation may seem foolish. But the fact is that microwave radiation can't leak out like gas or water because it is a wave. And just because we can't see it doesn't mean that it's small. In the fact is that microwave radiation is physically unable to leave the microwave oven cavity because of its size. Microwaves are much larger than the electromagnetic radiation that we can see. Even if the microwave's glass or plastic window were removed microwaves would still be contained within the small grid of metal that is imbedded within the door like a mini-maximum security prison. So enjoy your frozen pizza and fear not!
Engineers will often use a platter or turntable that rotates food through the microwaves for more even heating. Oven models without a turntable have a small spinning blade at the end of the wave-guide to thoroughly distribute the microwaves created by the magnetron.
A microwave oven must have some moisture in its oven cavity to work properly. Running a microwave oven empty or regularly nuking extremely dry foods such as popcorn can greatly reduce your microwave's life span, causing internal components to overheat. Placing a small amount of water in a container with your buttery treat will help to cool the microwave's components and extend its service life. Contrary to popular belief, metal can be placed into a microwave. However, the shape of the metal is the most important aspect in generating the entertaining but also dangerous arcing effect that a ball of foil or a forgotten fork can cause. Negatively charged electrons moving through the oven cavity can collect on a metallic point, such as a fork. When the charge has built up to a point that the host material can no longer contain the charge will jump causing the all to familiar arcing effect. Metal is often used in our favorite microwave foods to reflect back heat that is radiating away from the food for crisping and browning your Hot Pocket.