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

Q1. A strange question: does the air smell? By graduates in a daze

A:

Taste is a human feeling. If we use the reflex arc of middle school biology to describe the physiological structure that produces this feeling, the receptors, afferent nerves and nerve centers are all needed to produce taste. When the receptor is stimulated, the action potential produced by energy consumption is transmitted through the nerve to the brain to form a sensation. However, the receptors are immersed in the air all the time, and the ordinary air means that there is no threat, there is no food, everything is normal, and the receptors are unnecessary and impossible to stimulate the air, let alone consume energy for it. Therefore, the air will not lead to the production of action potential, so there is no feeling, no smell.

By Jing Yun

What are the advantages and disadvantages of Q2, round, square and triangular manhole covers? By Anonymous

A:

Manhole covers, which can be seen everywhere in daily life, as the basic components of urban management, play a role in protecting the city and promoting urban management. Manhole covers are generally round and square, while other shapes are relatively rare.

Image source: in engineering and science, the circular manhole cover is made of ferroalloy, which is easy to grind, and the circular manhole cover is better than a square or triangle, and the circular force is more uniform, and it is not easy to collapse or tilt under greater pressure. and the arch structure in the well can effectively resist the earth pressure. In contrast, the square manhole cover can be very large, if a square manhole cover is not enough, it can be made into two or more combined square manhole covers, but the circular manhole cover can only be made into one piece. In terms of safety, the biggest feature of the circle is that the radius is equal, and the circular manhole cover will not fall to the bottom of the well. The diagonal length of the square is greater than that of any side, the height of the triangle is less than the length of the side, and both the square and triangular manhole covers may fall into the bottom of the well. Squares or triangles with edges and corners are not only easy to warp up, but also easy to wear and tear.

Figure source [1] in terms of maintenance, the circular manhole cover is easy to carry, because the manhole cover is a heavy piece of iron, if the manhole cover is damaged and needs to be replaced, the maintenance workers only need to roll the manhole cover to move it to one side. The space for the builders to move in the triangular well is not as large as that in the circular well, the range of movement is blocked, and it is not convenient for maintenance. But the engineering manhole cover had better choose the square one, because the square manhole cover area is relatively large, and the working space under the manhole cover is more abundant than that under the circular manhole cover.

Reference:

[1] so why is the manhole cover round?

By floor sweeping monk

Q3, is this true? By convergent light

A:

The news is true, but it doesn't have to be cancer.

World Health Organization (WHO) claims that the sweetener aspartame "may cause cancer" and will add it to the list of carcinogens from next year. According to World Health Organization (WHO) 's classification criteria of carcinogens, carcinogens were divided into four grades, and carcinogenicity was divided into grade 1, grade 2A, grade 2B and grade 3 from high to low. Grade 1 carcinogens have sufficient evidence of carcinogenicity to humans, including tobacco, alcohol, betel nut, etc.; grade 2A carcinogens have sufficient animal carcinogenicity, but the evidence of carcinogenicity to humans is limited, beef and mutton is in this level; grade 2B carcinogens have limited evidence for human carcinogenicity, and animal carcinogenicity is insufficient, such as mobile phone radiation; grade 3 carcinogens have only potential carcinogenicity, including cola, tap water and so on.

The classification of aspartame is not clear, but it is generally believed that it will not be higher than level 2B. Research on the carcinogenicity of aspartame has been going on for decades, but no hard evidence has been found. These disputes are also an important reason why World Health Organization (WHO) did not put it on the list of carcinogens. [1]

For now, the carcinogenicity of aspartame does not need to be worried. Not staying up late, playing with your phone or eating hot pot may be more effective against cancer than avoiding aspartame. But I really don't recommend diet cola with aspartame, because it's not as good as the one with sugar.

[1] study on carcinogenicity of aspartame

By Jing Yun

Q4. Why does your stomach ring when you are hungry? By Gu Chicken

A:

This onomatopoeia derived from Greek is indeed quite interesting, and the reason is caused by the activities of people's digestive system.

We know that the gastrointestinal tract is a smooth muscle system that extends from the mouth to the anus. There is a gastric juice in the stomach that breaks down food. When the stomach is filled with gastric juice, the gastric wall contracts periodically, which is called gastric peristalsis. Peristalsis is a kind of muscle contraction to transport food downwards, which is caused by the rhythmic fluctuation of potential in smooth muscle cells, which we call basic electrical rhythm (basic electrical rhythm,BER). People's autonomic nervous system and hormones can regulate this BER.

About two hours after the stomach and small intestine are emptied, receptors on the stomach wall sense the loss of food, causing the intestinal nervous system to reflex electrical activity waves that send out a migration myoelectric complex (migrating myoelectric complexes,MMC), which spreads along the stomach and small intestine, leading to hungry contractions. The gurgling sound is caused by the peristalsis of the stomach and intestines in hollow tubes with gastric juice, air, leftover food and bacteria.

Of course, there are also "grunts" when you are full and "rumbling" caused by uterine contractions for similar reasons. In addition, if there are other uncomfortable symptoms, it is recommended to seek medical help in time!

Reference:

[1] Stomach rumble

By Gyoku

Q5. Can I use cola to make carbon dioxide? By chen

A:

Yes, but it's not necessary.

Coke does dissolve a lot of carbon dioxide, which can escape by shaking or heating it. Considering that there are no other easily escaping gases in Coke, if you really want to use Coke to make carbon dioxide, you can pass the bubbles from heated Coke through a dry bottle. The problem may be that the production quantity is relatively small and the production rate is difficult to control, so it may not be easy to clean the air in the production device. If you heat it slowly with a large bottle of Coke, it will alleviate these problems.

However, there are very mature and cheap methods for making carbon dioxide, many of which can be done at home; on the other hand, Coke is made by pressurizing pure carbon dioxide into sugar water and now using Coca-Cola to make carbon dioxide. It's not worth it, after all, there is no need to use the carbon dioxide extracted from Coke to make Coke (manual dog head).

By Jing Yun

Q6. Why is the rotation period of pulsars so stable? By Anonymous

A:

Pulsar is a kind of neutron star, which has a strong magnetic field and a short rotation period, ranging from 1.4 milliseconds (PSR-J1748-2446) to 11 seconds (PSR-J1841-0456). Yes, by the time you finish reading this sentence, the pulsar has already turned several times. The reason why it rotates so fast is that when the star collapses into a neutron star, the radius decreases greatly, and so does the moment of inertia. In order to maintain the conservation of angular momentum, the angular velocity has to increase accordingly.

The rotation axis of a pulsar generally does not coincide with the magnetic axis, just as the magnetic poles of the geomagnetic field do not coincide with the north and south poles in the geographical sense. When the pulsar rotates, it rotates with the magnetic axis, and the radiated electromagnetic waves are swept periodically. On Earth, it seems that it will receive a short and stable pulse signal, hence the name pulsar.

To return to the question, the rotation period of pulsars is very stable, which is actually relative to the human time scale. After all, it is only 56 years since Hugh Ish and Bell first discovered pulsars in 1967. In thousands of years of human civilization, we all think that the rotation cycle of the earth is very stable. But paleontology shows that the earth was 20.8 hours a day in the Cambrian 500 million years ago, indicating that the earth's self-transformation was slow. We now think that the rotation of the moon slows down the rotation of the earth.

For pulsars, they are losing energy and radiating electromagnetic waves all the time, and their rotation periods should also change. We say that it is "very stable". In fact, it is more or less "unknown to the bacteria, and the oratosquilla does not know the spring and autumn". And pulsars are also affected by the gravitational pull of the surrounding celestial bodies. For example, Taylor Hills, who won the Nobel Prize in physics in 1993, discovered a certain fluctuation in the pulsar's orbit, thus discovering the first pulsar binary star, PSR1913+16. [2,3]

Pulsar binaries refer to the systematic references of a pulsar and a celestial body of similar mass orbiting each other:

[1] shocked that there are 424 days in a year! Institute of Geology and Earth, Chinese Academy of Sciences

[2] Hulse, R. A. (1994). The discovery of the binary pulsar. Reviews of Modern Physics, 66 (3), 699710.

[3] Lee Chong Wei. Pulse binaries PSR1913+16-1993 Nobel Prize in Physics [J]. College Physics, 1994, 13 (3): 6

By sheep herding

Q7, the principle of the remote control? Can infrared and coding sequences be explained in detail? By rrycx

A:

First of all, infrared (light), infrared (infrared light,IR) is a kind of electromagnetic radiation, which is usually recognized as a wavelength range from about 1mm (300GHz) to about 700nm (430THz). Almost all blackbody radiation from objects close to room temperature is at infrared wavelength. As a form of electromagnetic radiation, infrared propagates energy and momentum and has the characteristics corresponding to waves and particles (photons).

The main technology used in infrared remote control is infrared (IR) light, which uses infrared light pulses to transmit information.

When we press the button on the remote control, the light emitting diode (LED) built into the pointing end of the remote control emits an infrared pulse stream. The infrared LED is controlled by the transmitter's microprocessor (MCU) modulation signal, which can help the receiver to distinguish the desired signal from other infrared noise sources. The receiver in the receiving equipment recognizes the emitted mode, converts infrared light into current using photodiodes, converts current into voltage across resistance amplifier, and voltage passes through gain amplifier and filter before demodulation. During demodulation, the carrier signal is stripped. The demodulated signal can be directly connected to the receiver's microcontroller (MCU) for decoding, and then the device can make a corresponding response.

Infrared modulation and demodulation | Image source [3] Infrared remote control modulation and coding theory modern infrared remote control design adopts digital modulation. Two basic digital modulation technologies are amplitude shift keying (ASK) and frequency shift keying (FSK). ASK uses two different carrier frequencies to represent these logic levels (FSK is not widely used because the use of two frequencies increases the complexity and cost of demodulation). The coding principle is still complicated. Here we list several coding methods in some common ASK modulation systems: pulse position coding, pulse distance coding, pulse width coding, Manchester coding (biphase coding).

In addition, there are many infrared protocols in the industry, among which the pulse distance protocol and Manchester protocol (RC5) are the most popular: the pulse distance protocol adopts ASK modulation and pulse distance coding, the carrier frequency is 38kHz. RC5 protocol is introduced by Philips, using ASK modulation and Manchester coding, and the carrier frequency is 36khz.

Reference:

[1] Infrared

[2] Remote control

[3] MSP430 C C. Infrared Remote Control Implementation With MSP430FR4xx [J]. 2014.

By Gyoku

Q8. What does spin mean in quantum physics? Why does a particle with a spin of 1 stroke 2 have to rotate twice to be the same? if it rotates only once, what is the difference between it and the original particle? By OrlandoLe

A:

Spin is a basic intrinsic property of particles. For bosons, spins are integers; for fermions, spins are half integers. The student who raised this question, the editor seriously suspected that he had also read Hawking's "A brief History of time". The book used playing cards as a metaphor to describe the spin scene and the confusion brought to the editor is still fresh in his memory. It is mentioned in the book that, for example, a particle with a spin of 1 can be turned into the same as before, and a particle with a spin of 1 and 2 must rotate twice to be the same. But the image, which takes two turns to be the same, is obviously not so easy to find in the real world.

And this image of turning a few circles actually originated from group theory. Group theory is a mathematical tool that can be used to describe symmetries. First, let's look at examples in three-dimensional space. Suppose you hold a pen in your hand and rotate freely in the air on the axis of one end of the pen, so that the other end of the pen can draw a sphere in space, which is the image of the invariance under the rotation transformation of real space-the length of the pen is the same. This kind of rotational transformation is called three-dimensional special real orthogonal group, namely SO (3) in group theory. In the SO (3) group, the transformation matrix is + 1 after the rotation of 2 π angle, so the original physical quantity remains unchanged.

Next, let's look at the electron, which is a spin 1ap2 electron, whose spin rotation transformation also satisfies a special symmetry, but unlike SO (3) just now, to describe this symmetry requires the use of a more complex group, SU (2). Under the control of this group, after rotating the electron spin wave function at an angle of 2 π, unfortunately, a negative sign appears in front of the wave function. Naturally, we can turn it around again (a total of 4 π angles), so that the two negative signs produced by the two rotations cancel each other out and eventually return to the original electron wave function. However, such a rotation actually takes place in a place called spinor space, and at the same time, since the phase in front of the wave function (in most cases) does not directly affect the observable, it is difficult to find an intuitive counterpart to the description of nature in the real world, 🧐.

By Callo

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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