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Hello, everyone, this is Radio Frequency School.

Today we are going to learn one of the most commonly used microwave instruments in our daily life-microwave ovens. When it comes to "microwave oven", everyone is familiar with it, that is, it can heat food. So how on earth do microwaves heat food? Is the microwave oven harmful to the human body? Maybe a lot of RF people don't know how to explain it. The young carpenter encountered this problem once. Today, let's all figure it out.

How did Tuyuan Pexels microwave oven come from? There are many ways to heat food. Our ancestors learned to heat food with fire a long time ago. Drilling wood to make fire, the appearance of flint makes it convenient for human beings to use fire. How did microwaves come from? Students who follow the radio frequency school know that when Mr. Maxwell summed up the great "Maxwell equations" on the basis of summarizing his predecessors, this system of equations directly predicted the existence of electromagnetic waves. Hertz's predecessors' experiments proved the existence of electromagnetic waves, so people opened the era of the use of electromagnetic waves, Marconi transatlantic communications, wireless telegraphy, mobile communications, to the present 5G era. Can not do without electromagnetic waves, the best carrier of information. Microwave is a kind of electromagnetic wave, whose frequency is from 300MHz to 300GHz.

Normally, it's hard for scientists to imagine using microwaves to heat food, and they don't have much interest in doing it. After all, with such a precious electromagnetic wave, what can we do to replace the function of fire? But it is such a coincidence that the microwave oven was invented, but as a by-product. In 1946, Percy Spencer, an RF engineer at Raytheon, was working on a radar project. In the course of the experiment, he discovered a wonderful phenomenon: the candy in his pocket melted. He was just testing a magnetron, and the temperature in the room was not hot enough for the candy to melt. Apart from being annoyed, Spencer suddenly realized that the electromagnetic wave had melted the candy. Curiosity prompted him to continue the experiment, he put high-density electromagnetic fields in a metal box, he continued to test other foods, such as eggs, which would explode and cover his face. The next day, he brought corn kernels for testing. The magnetron microwave blew them all up. Everyone in the office where he works wants to use their food for the invention of Spencer. So the microwave oven was born. The picture below is Spencer and his invention of the microwave oven.

The world's first microwave oven was thus developed by Percy Spencer, a radio frequency engineer from Raytheon. On October 8, 1945, Raytheon applied for a patent for the microwave oven and chose a beautiful name-Radarange.

In 1947, the first commercial microwave oven was put on the market, weighing 340kg, 1.8m high and costing about $3000. The cost of a small radar at that time was about $5000. So, whether you can afford it or not, I can't afford it. It wasn't until 1967 that a truly household microwave oven was made, it could be placed on the table, and the price was reduced to $495.

This new method of heating food has been sought after by people. Just as today's high-tech likes to bring "quantum", the "microwave" at that time was the Internet celebrity among the Internet celebrities.

Up to now, the microwave oven has almost entered thousands of households, whether you use it or not, my home is very convenient to use-cheap, easy to use, good quality and low price.

Picture source Pixabay so how on earth do microwaves heat food? As we all know, if you want to make an object hot, there are two main ways, one is conductive heating, the other is overall heating. Conduction heating is to heat the surface of an object and then transmit heat to the interior of the object through the heat conduction of the object itself; but microwave heating uses the penetration of microwave to make the internal molecules of the object change according to the change of electromagnetic field, thus causing the internal molecules of the object to collide and friction to produce heat, which is a kind of overall heating.

From a molecular point of view, many molecules are electric dipoles, such as water, which is made up of two hydrogen atoms and one oxygen atom (H2O), which forms a whole with its external charge, and an oxygen atom is surrounded by two hydrogen atoms to form an electric dipole. When there is no external electric field around, the electric dipole of the water molecule is balanced, and when placed in the external electric field, the electric dipole turns with the external electric field, as shown in the following figure. The turning frequency of the electric dipole is the same as that of the electric field. That means that when the frequency of the electromagnetic field in the microwave oven is 2.45GHz, the electric dipole of the water molecule turns 2450000000 times per second. Water molecules are so confused that their collisions with each other will be very intense, and the energy generated by such collisions and friction will be released in a hot way.

This whole heating method has many advantages over conduction heating, such as fast heating, heat penetration, internal and external heating together. So some foods with shells, such as eggs, when heated, Chinese chestnuts will burst due to internal thermal expansion and cannot be heated directly in the microwave oven. Of course, 2.45GHz 's microwaves cannot penetrate metal. Metal containers can not be directly heated in it, one reason is that it cannot be heated, and it may cause air to break down and catch fire.

At this point, I have to mention that the human body contains a lot of water molecules, so the human body will also be heated by microwave, this kind of heat is heated together from the inside to the outside, be careful that your internal organs can't stand it. So forget the idea of using microwave to keep warm, we humans should use conduction heating, at least with skin protection.

This article is from the official account of Wechat: radio Frequency School (ID:RF_Center), author: RF Carpenter

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