Shulou(Shulou.com)11/24 Report--

Sun Yingrui

Graduate students of Grade 2021, University of Chinese Academy of Sciences

Training unit: Institute of Physics, Chinese Academy of Sciences

A few days ago, a friend asked me, "I heard that microwave ovens don't leak radiation, so why do mobile phones still have a signal in the microwave oven?" For a moment, I was at a loss. Never mind whether the mobile phone will have a signal, but why does the mobile phone appear in the microwave oven?

However, looking at his sincere eyes, I decided to answer this question with a shu feed (w é n). According to the theory, the metal shell of the microwave oven and the shielding net of the furnace door will shield the microwave in the furnace chamber and will not leak. In this way, mobile phones that use microwaves as signals will be "isolated" from the rest of the world. But as an experimental physicist, the first step in answering questions is to throw your cell phone into the microwave oven in the lab. The last outrageous experiment in a closed box was Schrodinger's cat that was both alive and dead. My mobile phone is about the same as Schrodinger's cat, well, I am Schrodinger, is there anything wrong with me (manual dog head).

I invited two friends from Shanghai and Beijing to call the cell phones in the microwave oven. For the first time, the phone of a friend in Shanghai got through, but the line was busy for a long time before it was connected; however, the 4G network did not pop up Wechat message before opening the microwave door. Is the busy line too far away or is the shielding effect of the microwave oven better? We continued the experiment. Then, the phone call from a friend in Beijing could not be dialed, but I received a message from Wechat under the 5G network. Up to now, I still regard the received signal as an experimental error, and the statement that "the microwave oven can't block the cell phone signal" is not false, is it? But the next experiment really hit me in the face. The second call in Shanghai was connected at the normal speed; the second call in Beijing was connected; the third call in Beijing was also connected, and even friends stressed on Wechat that "it was connected very quickly"-well, Wechat, which received the 4G signal. Then, Shanghai Wechat was received on both 4G and 5G networks. It seems that the first few calls that could not be connected properly were the experimental error, and the screen door of the microwave oven really could not block the cell phone signal. Is there something wrong with the theory in the textbook? Or is the microwave oven actually the radiation demon around us?

What exactly is the story of radiation? it starts with radiation.

Although radiation sounds scary, it is really not a strange thing. Radiation is just a general term for all kinds of electromagnetic waves and particle flows. The alpha and beta rays emitted by nuclear reactions are certainly a kind of radiation, but sunlight, rainbows (and ponies) are also radiation, soybean milk, Youtiao, and tea eggs are all sources of radiation, and all electromagnetic waves-- from X-rays, ultraviolet rays, visible light, to infrared, microwave and even radio on radio stations-- are also radiation. Radiation can be roughly divided into two categories: ionizing radiation, which is harmful to people and touching as little as possible, and non-ionizing radiation that is relatively harmless and can be paid attention to. The difference between ionizing and non-ionizing radiation is whether it can ionize atoms into nuclei (or atoms) and free electrons. The following picture shows the ionizing power of various electromagnetic waves (that is, electromagnetic radiation). On the right side of the red vertical line is non-ionizing radiation.

Electromagnetic spectrum [1]

Ionizing radiation carries a high amount of energy, which can ionize the electrons in the atom and destroy the DNA more easily. Fortunately, we do not come into contact with it casually in our daily life. After all, it's under the big triangle warning board.

Behind the lead shielded door.

It was so tightly blocked that there was no escape at all.

Compared with the strong ability of ionizing radiation to directly block carbon and oxygen, non-ionizing radiation is much milder. It only changes the motion energy of molecules and atoms, and its effect on people is mainly reflected in the thermal effect. In other words, it's not a problem as long as you don't feel hot in the face of non-ionizing radiation.

Microwave oven: from magnetron to screen door, the common structure of microwave oven is not complicated, and the hardware mainly includes magnetron, wave guide tube, cavity and furnace door. The old microwave oven would have a rotary fan to scatter the microwave evenly into the furnace chamber. Modern microwave ovens use more intensive components to achieve the corresponding functions. Part of the microwave in the furnace chamber is radiated directly on the food, the other part is reflected again against the furnace wall, and finally most of the microwave energy is absorbed by the food, which heats up the food.

The structure of the microwave oven indicates that the microwave used in the microwave oven is actually a kind of electromagnetic wave, and the common frequency is 2450MHz. Although electromagnetic waves are driven by electric and magnetic fields to move forward, it is the electric field that plays a major role in the interaction between electromagnetic waves and matter (heating food). The size (or direction) of the electric field changes over time. If a charge is placed on the propagation path of the microwave, it will move back and forth with the change of the electric field-oscillating with the frequency of the electromagnetic wave. In this way, as the charge is forced to vibrate, the energy is transferred from the electromagnetic field to the charge.

Linearly polarized electromagnetic waves signal [3] in the specific application of microwave ovens, microwaves transmit energy to other food molecules by forcing water molecules in food to vibrate constantly, thus heating the whole food. Although the water molecule is not charged as a whole, it turns an angle so that the positive and negative centers no longer coincide. This forms the electric dipole in physics. With it, the electric field can make the water molecules rotate to get more energy like twisting the bottle cap. This special configuration of water molecules allows us to heat food conveniently in the microwave oven. Let's say: thank you, water molecules!

Water molecular structure diagram [4] in the electromagnetic waves scattered by the rotary fan, a considerable part of the electromagnetic waves will hit the microwave oven door, when the metal mesh interlayer in the glass door will play a role. The mesh of these metal meshes is only a few millimeters in diameter, but there are plenty of wavelengths of microwave, which is too large compared to the mesh. Microwave encountered such a fine mesh, can not directly pass through, and even the diffraction effect becomes very weak. It is like a repair dog stuck in a fence, facing an iron plate rather than a net with holes.

Image source reference [5] however, electromagnetic waves are not stupid dogs after all, even in the face of "iron plate", electromagnetic waves also hold on to their own unique temperament.

When electromagnetic waves interact with matter, they will reflect and transmit on the material surface-the transmitted wave propagates in a different direction from the original incident wave and is called refraction in optics. The reflection angle θ'is equal to the incident angle θ, while the refraction angle θ'is less than θ, which is the optical knowledge learned in junior high school and is also applicable to general electromagnetic waves.

The reflection and refraction of electromagnetic wave on the surface of the medium, k, k 'and k' respectively represent the direction of incident wave, reflected wave and transmitted wave [6] electromagnetic waves with different frequencies. When they encounter different media, the energy distribution of reflection and transmission is also different. Some are "transparent" because of their high transmittance, while others have high reflectivity and are as bright as mirrors. Most metals, when they encounter an electric field, will induce a layer of charge on the surface, repelling the electric field out of the body-- this is the content of electrostatic shielding in high school-- when faced with constantly changing electromagnetic waves, the response of the charge will not be so timely, but it can also make the electromagnetic waves enter only a little bit of the thickness of the metal, and most of the energy is reflected on the metal surface.

Through some deductions that are not complicated (but can only be remembered until the night before the exam), we can calculate the reflectivity of the 2450MHz electromagnetic wave used in the microwave oven to the copper plate, that is to say, if the microwave oven shielding door is regarded as a complete metal plate, then almost all the microwave energy irradiated on the shielding door is reflected back into the furnace chamber, and only about 4/10000 passes through the barrier of the shielding door. Even the 4/10000 is theoretically absorbed by the screen door itself. The energy that can really come to the outside of the microwave oven to have an impact on the human body will be less. However, we can not observe the heating of the microwave oven door in our daily life, so we will ignore the absorbed energy for a while.

According to the above calculation, microwave ovens with less than a kilowatt power in daily use will theoretically leak less than 0.5 watts of radiation at most. The surface area of a common 23L microwave oven is about-- calculated to be less than the power density of the leakage, which is only 1% of the upper limit of radiation leakage stipulated in the national standard [7].

So, does the leaking radiation value stipulated in the national standard do any harm to the human body? The human body's ability to withstand non-ionizing radiation mainly depends on the thermal effect. When the whole body is exposed, it can withstand four watts of radiation per kilogram of body weight, and when only a small piece of skin is exposed to non-ionizing radiation, because the blood circulation takes away a large part of the heat, so the radiation tolerance of this small piece of tissue is 100 watts per kilogram of body weight [8]. As a result, even 10g skin tissue can withstand 1 watt of microwave radiation, and the area that these skin can probably cover-according to the upper limit of the national standard, they will only be exposed to a total of 0.1 watts of qualified microwave ovens. Therefore, the theoretical radiation value of the microwave oven is far less than the upper limit of the national standard, which is far within the bearing range of the human body. Even if manufacturers "cut corners" a little bit and replace the iron plates used in our theoretical calculation with metal mesh, as long as their products can still be normally out of the (zhuqin) factory (Qi á n), it will still not have any impact on human safety.

Mobile phone: there is no radiation source. Let's get off the subject for a moment and take a look at the backplane of the desktop computer. Among the various jacks, the three-hole plug is the power cord, which is responsible for supplying electricity and lighting up the mainframe. Other USB and HDMI interfaces are connected to the keyboard and mouse monitor to transmit signals. The current can carry either energy or information. It's the same with electromagnetic waves. The electromagnetic waves used in microwave ovens are "ten meetings with one force drop", holding molecules to accelerate them to become hot, while mobile phone signals are "four or two dialing thousands of pounds", relying on changes in waveforms to transmit information.

There are two kinds of radiation related to mobile phones: the signal strength received by mobile phones and the transmission power transmitted by mobile phones. The communication protocol used in our mobile phone specifies the transmission power of the mobile phone under different signal strength. The weaker the signal, the weaker the connection between the phone and the base station, so the transmission power of the phone will be greater. The signal strength is reflected in the menu bar at the top of the phone, which is what we usually call "how many bars of the signal are there". More precisely, the unit of signal strength is dBm, and the formula is. If the phone receives radiation from the base station 1mW, the signal strength is 0dBm; if it receives 1W radiation, the signal strength is 30dBm.

Comparison table of mobile phone signal strength and radiation power [9] in cities, the reference strength of mobile signal is-90dBm, that is, the mobile phone is qualified if it receives radiation from the base station. Even if it is lower, the call quality can be generally guaranteed [10]. At this time, the radiation intensity emitted by the phone is-- and this is the upper limit of the intensity that the phone can emit, so the phone is really safe.

In the case of ensuring the quality of daily calls, the signal strength of the mobile phone can fluctuate in a wide range. My phone has experienced the highest-61dBm and lowest-125dBm signal fluctuations in a short period of time-the conversion success rate is 1 million times the difference. What if you are a mobile phone, singing while eating hot pot, talking comfortably on-60dBm and suddenly falling into-100dBm or even-110dBm? If the environment doesn't make you respectable, give yourself decency. After a lot of hard work with full power, it's okay to talk about Wechat.

Cell phone signal strength | Image Source: why on earth can I answer the phone in the microwave oven that I cut off by myself? Having said so much, we can finally answer the first question. The reason why a mobile phone locked in the microwave oven can receive the signal is not because the microwave oven leaks radiation, but because the phone is "too sensitive". Although the microwave oven shields off most of the radiation, leaving only a few thousandths of power, the range of signals that a mobile phone can talk normally spans five to six orders of magnitude. Outside the furnace door, the mobile phone can talk smoothly; inside the furnace door, although the signal is very weak, the mobile phone can still cope with it. In daily life, as long as we normally use qualified electrical appliances without stuffing our heads in the microwave oven or taking X-rays as beauty cameras, we really don't have to worry about radiation harm to our health.

Reference:

[1] how to correctly distinguish electromagnetic radiation from ionizing radiation? can long-term electromagnetic radiation affect people's normal health?

[2] what is the scientific knowledge about microwave ovens?

[3] how to establish the propagation model of electromagnetic wave?

[4] Springer Material

[5] Chai Dog

[6] Guo Shuohong. Electrodynamics (third Edition). Higher Education Press

[7] GB4706.90/2014

[8] Radio waves, risk and regulation

[9] find out how much radiation is from base stations and mobile phones.

[10] what is the signal strength of mobile phones?

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Tibetan idiot

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