The microwave oven is a household appliance very present in our kitchens. It is generally used to quickly heat or reheat food, by stirring the water molecules it contains under the effect of microwave radiation. The microwave is therefore a metal box inside which microwaves are generated, which heat the material.
How does a microwave work?
Microwaves are electromagnetic waves. These are disturbances of the magnetic and electric field. However, the water molecules (present in any food) are sensitive to these disturbances which vary with a very high frequency, which increases the temperature of your food. This is why microwave ovens have become one of the most used cooking methods in our kitchens: it’s quick, economical, and practical.
The microwave, which appeared in kitchens in the 1970s, does not use heat to cook food but microwaves, electromagnetic waves produced by a magnetron and conveyed inside the oven. It looks like a Faraday cage, a metal box in which the microwaves are reflected. When the food is cooked in the best microwave oven, the microwaves get into foods that contain water molecules (H2O).
Focus on the action of the microwave oven on water molecules
However, H2O molecules have the property of aligning with the magnetic field. That generated by the magnetron changes polarity 2.45 billion times per second. The friction generated by the very rapid movements of the water molecules produces heat that cooks food or heats it evenly. The water molecule, chemical formula H2O, is made up of two hydrogen atoms and one oxygen atom. In the normal state, the water molecules are in the disorder that is to say that they do not respect any particular order of orientation.
When water molecules are subjected to a continuous electric field, it tends to attract the orientation of the positive poles of water molecules. However, the electric field that makes up the waves of the microwave oven is rotating; the positive poles of the water molecules therefore rotate around the axis of symmetry of each molecule. A sufficient frequency, such as around 1 GHz, allows heat to be released from the absorption of wave energy in the food, thanks to the very small size of the water molecules which do not require too high a frequency. Only the phase shift between the oscillation of water and the oscillation of waves allows what is called a dielectric loss, generating heat, due to the phenomenon called “relaxation” of water molecules.
On the other hand, if the frequency is too high, the wave would be entirely absorbed at the surface of the food because phase oscillation would be impossible, and the dielectric losses would only be located at the surface. The choice of the frequency of the microwave oven therefore results from a compromise between the heating of the food and the penetration of the wave in it. Finally, note that the explosion of certain foods (the skin of tomatoes, sausages, eggs) is due to the volume of water vapor produced by water molecules in evaporation following the release of heat. However, only the relative abundance of water in any food can allow all food to cook properly with the microwave.