The Microwave Oven

Our experience tells us that we need something warmer to heat something else (e.g. a cup of milk), so that the energy from the hotter body (flame, cook top…) can flow to the colder one and heat it as a result. But the energy can also be conveyed without direct contact between the bodies through electromagnetic radiation. The electromagnetic waves emitted by the sun, for example, warm our skin and the earth as well.

The transmission in a microwave oven is similar to this transmission of energy between the sun and the earth. The microwaves are generated (in the magnetron) and directed into the cooking compartment. There, the electromagnetic waves penetrate the food.

The radiation causes the individual water particles in the food to move faster (rotation). The temperature of the water molecules rises, since higher temperature is equivalent to faster movement of the smallest particles (atoms, molecules). Now the water molecules transfer that motion to other components of the food through friction, and thus the moisture of the food gradually heats the whole food.

	ð		ð
Motion (rotation)		Friction		Temperature increases

Microwaves have two noteworthy properties:

• They vibrate with a specific frequency and therefore only heat the water. When heating a cup of milk, the milk is thus considerably warmer than the cup itself. Other materials like ceramic, glass or plastic are hardly heated as well. That it nevertheless becomes warm after some time is a result of the warmth flowing from the hot water to the cold cup.
• A second characteristic of an electromagnetic wave is its inability to penetrate metal, as it is reflected. So it makes no sense to try warming food in a closed metal pot. Furthermore, metallic objects may damage the microwave.