If you are holding a mobile phone all the time, you will find that when you enter the basement or enter some high-rise buildings, the mobile phone signal often has a large attenuation; when you enter the elevator, there is usually no signal at all. Seeing this, I think you all remembered a similar experience.
Perceptual cognition tells us that the signal seems to wear out when it passes through the wall, and it seems to have something to do with the thickness of the wall. This is probably the reason why the signal is not good in the basement or in some buildings. Using this "theory" can also barely explain why there is no signal in the elevator. However, when I tell you, using a thin layer of tin foil to wrap the phone can completely block the signal. This phenomenon seems to be unexplained by the theory of “thick wall block signal†under our perceptual cognition.
I believe everyone knows that the spread of mobile phone signals is carried out using electromagnetic waves that vary according to a certain frequency. In the vicinity of the mobile phone, the greater the amplitude of the electromagnetic wave carrying the signal, the better the signal of the mobile phone. If we come to a weak signal, it means that only a small part of the electromagnetic signal can reach here. Life experience tells us that the amplitude of electromagnetic waves does decrease after passing through the wall, causing the signal to decay. But what is it that hinders or even blocks the spread of electromagnetic waves, so that the phone has no signal?
To solve this problem, we must start with a physical phenomenon.
The propagation of electromagnetic waves in the medium follows the "skin effect". As the name implies, it is the phenomenon that electromagnetic waves tend to the surface. Looking at the definition, we know that when there is alternating current or alternating electromagnetic field in the conductor, the current distribution inside the conductor is not uniform, and the current is concentrated in the "skin" part of the conductor, that is, the current is concentrated on the thin layer of the outer surface of the conductor. Close to the conductor surface, the higher the current density, the smaller the actual current inside the wire. As a result, the resistance of the conductor is increased, so that its power loss is also increased. This phenomenon is called the skin effect. What is the relationship between this effect and the spread of cell phone signals?
It turns out that when electromagnetic waves carrying signals pass through some media, they are restricted by the skin effect, so that some electromagnetic waves stay on the surface of the medium and cannot completely pass through the medium. The better the conductivity of the medium, the more obvious the effect of blocking electromagnetic waves.
Now let's assume that there is a wall through which electromagnetic waves will pass. If the wall is completely insulated, there is no such wall in the electromagnetic wave. It can pass very happily without any loss; but if the wall is electrically conductive, for example, the wall is metallic, in the electromagnetic wave It seems that this is an insurmountable high wall, it is almost impossible to pass through, which is why a thin layer of tin foil can make the phone have no signal, and it is for this reason that the signal is particularly bad in c. (The elevator is equivalent to a metal box).
At this point, we know that materials with strong electrical conductivity (metal or various substances capable of conducting electricity) can have a serious impact on the propagation of electromagnetic waves. At this time, some people may ask, is the wall of the building not electrically non-conductive? Why does it hinder the electromagnetic wave propagation and let the cell phone signal attenuate?
As said before, if this is a "pure" wall, there is almost no loss of electromagnetic waves passing through it. However, we know that in order to enhance the load-bearing capacity of the wall, reinforced concrete structures are widely used in the construction industry. That is to say, the interior of the wall not only has insulating materials such as cement, but also conductive steel bars, and of course, various wires buried in the wall. This makes the wall have a certain hindrance to electromagnetic waves. So when we are in the underground space, the signal will be weakened to varying degrees until it disappears completely.
Examples of skin effect blocking signals are widely available in various fields. We know that sea water is a good conductor of electricity. In the military, in order to transmit simple signals such as "up", "down", "forward", "stop" to underwater submarines, large antenna arrays must be used to transmit high-intensity electromagnetic signals. However, even in this case, communication of up to 30 meters underwater can be achieved, and once this distance is exceeded, communication becomes extremely difficult. However, everything has its two sides, and the skin effect does bring us a lot of inconvenience, but there are also some beneficial applications. For example, using the skin effect to create an electrostatically shielded area (similar to a Faraday cage), perform some precise experimental measurements.
At this time, someone must ask, is there no way to eliminate or avoid this effect, so that the signal can spread smoothly? Because this is the basic nature of electromagnetic waves, it is indeed difficult to eliminate them, but there are many ways to avoid this effect. For example, many elevators have a small signal repeater installed to make the elevator full of signals. Similarly, many subway stations have some signal repeaters installed along the line, so that you can still be underground more than ten meters deep. Can play mobile phones unimpeded.
Physics tells us that there are physical principles behind all kinds of phenomena in life. The physicists summed up the simple and profound laws of physics through various phenomena.
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