Microwave heating principle

Using microwaves in microwave ovens to cook food has become very popular now. Since microwave cooking has the advantages of fast heating, energy saving, no pollution to the environment, and good freshness preservation, etc. Microwave ovens are widely promoted and used in our country.

1. Principle of microwave heating

The basis of microwave cooking is the heating of medium by microwave. According to physical theory, media molecules can be divided into two categories: polar molecules and non-polar molecules. The centers of positive and negative charges of polar molecules do not overlap. There is a distance between them, which can be equivalent to an electric dipole (such as water). Under the action of an external electric field, the originally chaotic polar molecules are turned along the direction of the external electric field, resulting in steering polarization (the positive and negative charge centers of the non-polar molecules coincide, and under the action of the external electric field, the positive and negative charge centers in the molecules are made to move along the direction of the external electric field. The direction of the electric field only produces displacement polarization). If the external electric field is alternating, then the steering of the polar molecules will continue to change direction as the electric field changes. In this process, due to the mutual collision between molecules, electrical energy will be converted into kinetic energy of molecules, and then converted into thermal energy, causing the temperature of the object to increase. It can be seen that for objects composed of polar molecules (such as cooked food), the alternating electric field can easily heat them.

The physical quantity that characterizes the degree of polarization of a medium under the action of an external electric field is called the dielectric constant. (Under the action of an alternating electric field, the dielectric constant of the medium is a complex number, and the imaginary part reflects the loss of the medium). In fact, the dielectric constant is not a constant number, and its dielectric constant is also different under different conditions. For example, the dielectric constant and loss of water under microwave conditions are much larger than those of ordinary substances, so it is easier to absorb microwave energy and be heated.

Microwave is a kind of electromagnetic wave with extremely high frequency, which will be totally reflected when irradiated on the surface of ideal conductive metal. When irradiated on the surface of a medium, a small part is reflected, while most of it can penetrate into the interior of the medium, where it is gradually absorbed by the medium and converted into heat energy. Its penetration depth is mainly determined by the dielectric constant of the medium and the frequency of the electromagnetic wave. At microwave frequencies, the penetration depth of ordinary objects can reach several centimeters.

Microwaves also have a biological effect on living organisms. Under certain conditions, they can inhibit and kill cells and bacteria.

2. The difference between microwave heating and ordinary heating

It can be seen that microwave heating is different from ordinary heating methods. The usual heating method is to have a high-temperature heat source, which first heats the surface of the object through radiation and conduction, and then gradually heats the depth of the object through conduction and convection, so the heating speed is very slow; while microwave heating uses magnetic The control tube (at the top of the furnace) generates microwaves, and then irradiates the microwaves into an empty box (also called a resonant cavity) made of metal on six sides. The food is placed in the box, and the microwaves are reflected back and forth on the walls of the box. It penetrates into the food being cooked in all directions to heat the food. The box wall does not absorb microwaves, and only the containers and food in the box are heated, so it is highly efficient and fast. Because the heating speed is fast, it does little damage to food nutrition (that is, it has good freshness preservation).