| MadSci Network: Chemistry |
Greetings: Although the total electrical charge in molecules is neutral, the positive and negative charges do not occur at the same location within the molecule. The amount of separation between the positive and negative charges is called the dipole moment of the molecule. Some molecules have small dipole moments and some molecules such as water have large dipole moments. The microwave energy passing through a material placed in a microwave oven rapidly alternates between positive and negative voltage 2450 million times per second (2450 Megahertz). The alternating electric field causes the charges of the dipoles to try to align the molecules with the microwave field, plus to minus and minus to plus. This causes the molecules to rapidly rotate generating heat by friction like forces. There are two reasons why plastics heat more then ceramics in a microwave oven. First, plastics can trap water within the material during the manufacturing process. The very high temperatures required to manufacture ceramics drives all the water out of the material. The trapped and possible absorbed water molecules cause the plastics to heat more than ceramics when exposed to a microwave field. The second reason that some plastics heat more than others in microwave fields is that plastics are formed from long changed hydrocarbon molecules that have there own dipole moments which can also cause heating of the plastic. Some ceramic glazes used for coloring also can also cause extra dipole heating in ceramics; these ceramics are usually not suitable for microwave cooking purposes. You can find more detail with animation of the microwave cooking process on the following web site: http:// www.gallawa.com/microtech/howcook.html Best regards, Your Mad Scientist Adrian Popa
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