The heating principle of microwave oven

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In the Modern Family Kitchen, the microwave oven has become the indispensable cooking equipment. Compared with traditional heating methods, microwave heating has the advantages of high speed, high efficiency and simple operation. Behind this unique heating is the physics of how electromagnetic waves interact with matter. Today, we’re going to uncover the secrets of the microwave oven, and see how this seemingly ordinary appliance uses invisible microwaves to turn food from cold to hot in a short amount of time.

1.Generation of microwave

Microwave is an electromagnetic wave in the electromagnetic spectrum between radio waves and infrared radiation. Its frequency range is usually defined as 300 mhz to 300 ghz. The operating frequency of the household microwave oven is strictly limited to 2.45 ghz. The generation of microwave depends on the magnetron which is the core component. Magnetrons convert electrical energy into microwave radiation through the cyclotron motion of electrons in a strong magnetic field. The efficiency of this conversion process is usually between 50-70% , and the rest of the energy is dissipated in the form of thermal energy, which explains the heating of the microwave oven during operation. The generated microwave is introduced into the furnace cavity through the waveguide system, and the standing wave field is formed under the multiple reflections of the metal wall, which provides uniform energy distribution for food heating.

2.Interaction between microwave and food molecules

The key to microwave heating is pinpointing the water molecules in the food. As polar molecules, water molecules are like tiny magnets, positively charged at one end and negatively charged at the other. When the microwaves are turned on, the direction of their electric field changes at an astonishing rate of 2.45 billion times per second, forcing water molecules to spin wildly in the fast-moving field. This violent molecular movement causes a lot of intermolecular friction, which in turn efficiently converts the energy carried by microwaves into heat. Because of this, high water content of vegetables, soups and other foods can quickly heat up, and dry biscuits, nuts and other heating effect is not good.

3.Energy conversion and heat transfer

The energy conversion process of microwave heating presents a typical”Electromagnetic energy-molecular kinetic energy-thermal energy” conversion path. When microwaves penetrate food, their electric field component does work on polar molecules, giving them kinetic energy. According to the Molecular dynamics theory, these directed motions are then transformed into random thermal motions by molecular collisions, which dissipate energy. It is worth noting that the heat transfer mechanism of microwave heating is essentially different from that of traditional heating: traditional heating relies on heat conduction and convection, and the direction of heat flow is from the outside to the inside; while microwave heating generates a volumetric heat source through dielectric loss, and the volume heat source is generated by microwave heating, internal and external synchronous heating can be realized.

4.Control of heating uniformity

The control principle of microwave oven heating uniformity is mainly based on multi-aspect collaboration. On the one hand, using advanced sensor technology, it can sense the temperature of different areas of food in real time, and convert temperature information into electrical signals and transmit them to the control system. On the other hand, using intelligent frequency conversion technology, the control system dynamically adjusts the microwave transmitting power and frequency according to the sensor feedback. Changes in power and frequency can affect the distribution and penetration of microwaves in the oven chamber, allowing them to act more evenly on food. In addition, the special furnace cavity structure design, with the reflection material, let the microwave in the furnace multiple reflection, diffusion, all-round coverage of food, reduce the heating dead angle, so as to achieve uniform heating.

Conclusion

The invention of the microwave oven has greatly changed the way we cook. It uses the wonderful characteristics of electromagnetic waves, so that food molecules”Spontaneous heating”, to achieve unprecedented heating efficiency. From the microwave emission of magnetron, to the microscopic movement of water molecules, to the process of energy conversion and heat transfer, the heating principle of microwave oven embodies the wisdom of physics and engineering. With the development of science and technology, microwave heating will evolve in the direction of higher efficiency and more precise control.