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

After the mobile phone enters the elevator

The signal will drop suddenly.

This is because the metal elevator forms the Faraday cage.

So can we also achieve signal shielding with aluminum foil?

Q1. What is the principle of high voltage transmission? Why can high voltage transmission reduce the loss of electric energy? By Hu

A:

The loss in power transmission mainly comes from Joule heat. High-voltage transmission is to reduce the current in the power network, and then reduce the heat of the wire and reduce the loss. Let's explain briefly.

As we all know, Joule heat comes from resistors, satisfaction, note, here is a high school entrance examination knowledge point, this is the definition of Joule heat, other variants using Ohm's law are based on pure resistance circuits, we should pay attention to the scope of use. Looking at this formula, it is not difficult to see that if you want to reduce the Joule heat, you can either reduce the resistance of the transmission line or reduce the current in the transmission line. There is no need to say more about the resistance. For the current, we know that the power of the input in the transmission line is certain, which depends on the generating power of the power plant. It can be seen that as long as the voltage difference between the two ends of the transmission line is increased, the current in the transmission line can be reduced, so it is reasonable for high-voltage transmission.

However, the voltage in the lifting circuit will also face some problems, such as the insulation of power facilities, radiation problems, safety problems, and so on. Therefore, high-voltage transmission lines are often built in the wild where no one can reach, and towers are used to keep them safe.

By Frost White

Q2. Why is the iron block silver and the iron powder black? By Min Xiaodai

A:

First of all, the high reflectivity of the iron surface comes from its good conductivity. due to the ohmic effect, the electromagnetic wave attenuates when it propagates in the conductor, which can only penetrate a very shallow distance, which is also called the penetration depth. For elemental iron, the penetration depth of the visible band is much smaller than the wavelength, which means that the energy of the electromagnetic wave is reflected back almost undissipated, so that the smooth iron shows a silvery metallic luster.

However, iron powder with smaller particle size, such as reductive iron powder with a particle size of about 1 ~ 100 μ m, is often dark gray or black under visible light, for three reasons:

First of all, the iron powder has no smooth surface, and the visible light is diffusely reflected on the iron powder, so there is no shiny metallic luster; secondly, the iron powder has higher reactivity, and its surface is usually attached with an oxide layer with complex compositions. the dark oxide layer has a strong absorption rate of visible light. Finally, the iron powder has a large surface area and loose stacking structure, so that the incoming light usually has to be reflected many times before it can be emitted, which further reduces the reflectivity.

It is the comprehensive effect of the above three points that makes the iron powder dark.

By is happy in his heart.

Q3. Can you isolate the signal by wrapping the phone with aluminum foil? By Anonymous

A:

This is a good electrodynamic problem. When the electromagnetic wave irradiates on the metal surface, part of it is reflected and part of it can be transmitted into the metal, resulting in absorption.

Let's first look at the transmission part. Most metals have a good absorption effect on the electromagnetic waves entering the interior of the metal. For electrostatic fields, this effect is shown as electrostatic shielding learned in high school; for electromagnetic waves, that is, changing electromagnetic fields, this effect allows metals to attenuate the intensity of electromagnetic waves to 1 / e (about 37%) in a relatively short distance. This length is called the penetration depth of electromagnetic waves into metal. Where f is the electromagnetic wave frequency, the 5G signal of the mobile phone is about 2.5GHz, which is the magnetic permeability of the material and the electrical conductivity of the material. By substituting the relevant values of aluminum into the above formula, the calculated penetration depth is about 1.6 μ m, while the general thickness of aluminum foil is about 50 μ m. In other words, the aluminum foil has about 30 penetration depths, which can attenuate the electromagnetic wave to about the original.

In addition, a considerable part of the electromagnetic wave has been reflected off before it enters the aluminum foil. The electromagnetic wave is incident vertically on the metal surface, and the reflectivity is that for aluminum, only 2/10000 of the electromagnetic wave can enter the interior of the aluminum. The situation of oblique incidence is relatively complex, but there will be no difference in magnitude, so we will not expand here.

Considering reflection and absorption, wrapping the phone with household aluminum foil can weaken the electromagnetic waves received by the phone by 17 to 18 orders of magnitude. But the signal strength range of the mobile phone itself can only span 7 orders of magnitude. As a result, aluminum foil wrapped phones can theoretically block cell phone signals-of course, this experiment can be easily carried out at home. If you are interested, you might as well come and have a lively test.

Finally, I hope this question is meant to explore, not to worry about cell phone radiation-after all, there is really nothing to worry about. The shielding ability of metals to electromagnetic waves, cell phone signal strength and radiation safety related to mobile phone radiation. In this article, we all talk about answering the phone in the → microwave oven. It's a little outrageous, but don't be afraid.

Reference:

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

By is a Tibetan fanatic.

Q4, why not myopic people wearing myopic glasses will feel that the object is getting smaller? By Dongfangshuo

A:

Myopic glasses are originally "reduced" mirrors.

Myopic glasses are machines used to correct myopia. Myopia is a kind of ametropia. The lens of the human eye has the ability to refract the light that enters the eyeball, which is called refraction. The refracted light converges on the focus and the focus falls on the retina to get a clear image. However, if due to old age, disease and other factors, the focus falls on the front of the retina or behind the retina, there will be a blurred image, which is ametropia. Among them, the focus in front of the retina is myopia, and what falls behind the retina is hyperopia. If you want to correct myopia, you must make the image move backward and fall on the retina. This is the principle of myopic glasses.

Simple geometrical optics tells us that the concave lens has a divergent effect on the light. After the divergent light passes through the lens, the focus of myopia that originally falls in front of the retina can be shifted backward. By adjusting the focal length of the concave lens, the focus can just fall on the retina, thus achieving the purpose of correcting vision. Therefore, we use concave lenses to make myopic glasses.

Because the concave lens can diverge the light, its focal length is negative, that is, the focus is on one side of the object, so if you look at the object through the concave lens, a reduced virtual image will be observed on the side of the lens object, as shown in the following figure:

Concave lens imaging light path diagram, the blue one is the object, the red one is like

On the other hand, presbyopic glasses, which is used to correct hyperopia, is actually a magnifying glass through which you can observe a nearby object (within one focal length) and observe a magnified upright virtual image, but if you are looking at an object in the distance, you may also get a reduced real image of handstand, so you might as well try it.

By Frost White

Q5, why there is water vapor on the glasses to see the light source will be a circle? By Sirius coach

A:

I speculate that it should be formed by multiple refraction and reflection of light in water vapor. It's actually a rainbow.

The picture comes from the Internet. I think it's a similar effect.

The principle of this phenomenon is similar to that of rainbows, which has been fully introduced in previous articles, that is, sunlight forms a circular strip after two refractions and one reflection in small water droplets, so we will not repeat it here. Interested students can check out the previous articles No.164 Q6 and No.271 Q1.

In this topic, water vapor covers the entire mirror, after the light from the light source is incident, it will also experience two refractions and a reflection, and finally the light from different water droplets forms a circle around the light source, which is actually the same as the rainbow, in fact, the complete rainbow is also a whole circle, you can see No.119 Q8 specifically.

The next question is, why is it colored? in fact, it is the same as a rainbow. Because the refractive index of each monochromatic light in water is different, the refraction angle of each monochromatic light is different when it is refracted by water droplets. The path is different, so eventually there is such a color distribution. Of course, for the artificial light source such as light, it can also reflect the color composition of the light source.

By Frost White

What is the state of fire? By zinero114514

A:

In fact, we can judge that fire is not solid or liquid by our intuition of solid, liquid and gas. Then the most likely form of fire is the gaseous state. However, many readers must have seen the saying that fire is plasma. Let's focus on this question: is fire gaseous or plasma?

Fig. 1 relative temperature relationship of four common states of matter | the reason why the picture originates from [4] may be confused, because gas and plasma do have a lot in common, for example, the shape and volume are not fixed and will change according to the container. However, there are fundamental differences between the two: (1) composition: gas is basically composed of neutral molecules, while plasma is composed of positive and negative ions (including electrons, protons, etc.) and neutral molecules. (2) conductivity: the gas conductivity is very low, while the plasma conductivity is very high. (3) Velocity distribution: the gas particles collide frequently, and the velocity distribution basically satisfies the Maxwell-Boltzmann distribution. The particles with extremely high velocity are few, but the collisions of plasma are not frequent, and there will be a certain proportion of extremely high-speed particles. (4) interaction: the interaction between gases is mainly the collision of two particles, and the plasma can interact collectively through the long-range action of electromagnetic force to produce waves and other organized motions.

Fig. 2 Velocity distribution of some inert gases at room temperature | the picture is from [5]. So, how can the gas transition to plasma? It generally requires an electric field strength of more than 3 × 106V / m or a high temperature of more than thousands of degrees Celsius, because only under such extreme conditions can gas molecules lose some or all of their electrons to form positive and negative ions. For air, it needs to be heated to about 14000 degrees Celsius at standard atmospheric pressure before it can be converted into plasma. However, our common flame temperature is shown in the following table:

Combustion material flame temperature (°C) charcoal flame 750 to 1200 methane (natural gas) 900 to 1500 propane blowtorch 1200 to 1700 candle flame to 1100 (main), hot spot may be 1300-1400 magnesium 1900 to 2300 acetylene blowtorch to 2300 oxygen-acetylene to 3300 ordinary lighter flame

It can be seen that the temperature distance of most flames makes the air molecules become plasma. Therefore, most of the flames we see are gases that are burning violently. A collision between gases or molecules of solid combustibles such as firewood. The reaction itself is exothermic, which in turn accelerates other gas molecules and continues to collide, which is exactly what happens in the flame during combustion. At the same time, the flame will emit a certain color of light, depending on the composition of the combustion medium, but also related to the temperature.

Fig. 3 the flame of Bunsen burner under different oxygen supply conditions | the picture is from [1] of course, this does not mean that plasma can not exist in the flame, and some high-temperature flames may contain trace amounts of very low ionization plasma.

Reference:

[1] Yan-Wikipedia

[2] is fire plasma?

[3] Plasma-Wikipedia

[4] Plasma-the third edition of the Encyclopedia of China

[5] Maxwell-Boltzmann Distribution-Wikipedia

By clouds open leaves and fall

Q7. What if there is pure oxygen in the air without other impurities? By thinking

A:

The fresh air in the mountains and forests (with slightly higher oxygen content) tends to make people more relaxed and happy. Oxygen plays a vital role in the ecosystem, and the increase of oxygen concentration is of great significance in the history of biological evolution. On the one hand, it promotes the formation of an ozone layer that can effectively block ultraviolet rays, on the other hand, it promotes the emergence of aerobic respiration and greatly improves the efficiency of biological energy acquisition [1]. It also makes us wonder whether the higher the oxygen concentration, the better. Can we even change the air to pure oxygen?

If we assume that the earth's air is suddenly replaced by pure oxygen in the same volume. In fact, this is not a wonderful thing. The molecular weight of oxygen will lead to an increase in air pressure, which will bring discomfort and even danger to the lungs; the increase in oxygen concentration will also accelerate the metabolism of animals and accelerate aging; too high oxygen concentration for a long time will actually lead to oxygen poisoning in animals, because oxygen is actually an oxidizer, and there should be sufficient levels of reducing agents in the body to match it. In other words, human beings will not be able to survive if they do not take urgent measures immediately. In addition, without carbon dioxide, plants will not be able to carry out photosynthesis and therefore can not continue to grow, can only maintain a balance between respiration and photosynthesis, and can no longer provide energy for the entire ecosystem. In addition, the nitrogen cycle is also an important part of the ecosystem, and the loss of nitrogen can also lead to terrible consequences. Of course, anaerobes also express great displeasure. What is more troublesome is that because oxygen is an accelerant, high oxygen concentrations will make hill fires extremely easy, and several thunderstorms may lead to the destruction of forests around the world.

The variation of oxygen content in the Phanerozoic atmosphere estimated by different biogeochemical models | the figure is from the reference [2]. Perhaps this assumption is not very reasonable, how can it be suddenly replaced, and can it be replaced slowly in a way that adapts to the ecosystem? In fact, there have been two "great oxidation events" in the history of the earth's ecological evolution, in which the proportion of oxygen content has increased sharply in a short period of time, which is considered to have promoted the evolution of organisms in terms of environmental factors. It is worth noting that during the Carboniferous period, oxygen content reached an all-time high of 35% [2], when the ecology was very interesting, such as the existence of a variety of giant insects.

Carboniferous giant dragonfly, wingspan can be close to 1 meter. Although the cause of the great oxidation event is still controversial, it is reasonable to think that high oxygen concentration promoted the emergence of this giant insect. it also makes us wonder if the increase in oxygen concentration in today's world will also lead to the emergence of giant organisms. But it is also because of the high concentration of oxygen that these giant insects become extinct in a fire. Alas!

In general, it's crazy to change the air to pure oxygen. It would be acceptable to increase oxygen concentration by just a few percentage points, but the impact on the ecosystem would still be huge and would need to be carefully assessed.

Reference:

Luo Genming, Zhu Xiangkun, Wang Shuijiong, Zhang Shihong, Jiao Chaoqun. Genetic mechanism and climatic ecological effects of early Proterozoic great oxidation events [J]. Chinese Science: Geoscience, 2022,552 (09): 1665-1693.

Zong Pu, Xue Jinzhuang. Synergistic evolution of atmospheric oxygen content and biodiversity in geological history [J]. Biology Bulletin, 2015. 50 (04): 1-5.

By Xiao Fan

Q8. As in Liu Cixin's Mirror, if you simulate two identical cosmic beginnings, will the results really be the same?

Not necessarily the same, the uncertainty principle of quantum mechanics makes all evolutionary processes random and can only be described by probability distribution. We have popularized this point many times, and we strongly recommend our article on Bell inequality [1] published on Tuesday, which introduces in popular language how scientists have experimentally tested that there are no local hidden variables in quantum mechanics. it also shows that it is true random, not pseudo-random caused by the complexity of variables.

Here we introduce a model of the Norton dome, which shows that even in the framework of classical physics, the same initial conditions may not necessarily lead to the same results.

A small ball rolling off the Norton dome [2] in 2003, Norton imagined such a magical surface to let a ball fall from its top, and its falling height h was related to the falling distance r. Now we put the ball still on the dome, according to Newton's second law and the initial conditions of rest:

You will find that the solution of the evolution equation has an uncertain constant C, that is, the ball will fall at the time of tweak C, but we have no way to know exactly when. You can also think of it the other way around: a ball rolls up along the dome from below, and when it reaches the dome, it stops steadily and stays still for an infinite period of time. Looking back along this line of thinking, the ball is uncertain when the dome changes from stillness to fall.

In fact, in the final analysis, this is because Newton's law of motion corresponds to the second-order differential equation in mathematics, and we generally think that it can determine a unique solution by adding the initial conditions of initial position and initial velocity. But strictly speaking, this requires the differential equation to satisfy the Lipschitz condition, which is a more stringent condition for the smoothness of the function, and the dome constructed by Norton does not satisfy this condition.

Reference:

[1] want to figure out the Nobel Prize theory that Einstein got wrong? Let's have a quantum gamble!

[2] John D. Norton. The Dome

By sheep herding

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

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