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

The clothes were finally washed and dried. You happily took the clothes down, but you found out why the clothes had hardened. Hey! Don't worry, this issue will use physical methods to help you get rid of the trouble of hardening clothes.

Q1. Why is it windy in the subway? Is the seal not tight? By Star Shadow is boundless

A:

Both subway cars and subway stations need to be ventilated and air-conditioned. Among them, the subway station is still connected to the tunnel, and the train will be windy. In the carriage, the ventilation system is necessary. Adults have a ventilation capacity of about 510L / min in a relatively quiet state. If there is no ventilation, there will soon be a lack of oxygen in the crowded carriages. These ventilation systems bring about the flow of air.

By Jing Yun

Q2. Can cyclic decimals be added, subtracted, multiplied and divided? Are there any strict rules? By Avenue to Jane bj

A:

A cyclic decimal is a rational number, which must be written as a fraction of two coprime integers and can be treated as an ordinary fraction when calculating. Not to mention addition, subtraction and multiplication, there is nothing special about calculus, even if you take the limit. The specific operation methods can be found in the mathematics textbooks of each grade.

By Frost White

Q3. Why are the socks so hard after being washed and dried every time? how to make them soft or soft after washing? 🤔️? By Sweet and Sour Spare Ribs

A:

In fact, not only socks, many fabrics containing cotton fibers, such as towels, will harden after natural drying. The important reason for this phenomenon is that the bound water layer attached to the surface of natural cellulose makes the fibers cross-linked into a network by hydrogen bonding.

Fig. 1 bound water layer on the surface of natural fiber | Picture is from [1] because natural cellulose is polymerized by glucose, the molecular structure is rich in hydroxyl groups and can form a strong interaction with water molecules in the form of hydrogen bond. As a result, cotton can have up to 8% water content under the condition of 25 °C and 60% relative humidity. In other words, the normal drying of cotton socks, towels and other fabrics are actually bound in the form of water. This kind of water will connect different fibers into a network, so that the fibers are not easy to slide relatively and become hard macroscopically.

Fig. 2 the structural experiment of cellulose shows that if all moisture is forcibly removed by heating in a vacuum, the fabric will soften again, which is difficult to do in real life.

Fig. 3 comparison of cleaning method with final hardness | the picture is from [3], however, two operations can improve this problem. One is to shake and dry after cleaning, which should help to reduce the moisture content in the cotton fabric. The other is the addition of softener, whose main component is cationic surfactant, which has been proved to prevent bound water from forming cross-linking network.

Reference:

[1] Igarashi T, Hoshi M, Nakamura K, et al. Direct Observation of Bound Water on Cotton Surfaces by Atomic Force Microscopy and Atomic Force Microscopy-Infrared Spectroscopy [J]. The Journal of Physical Chemistry C, 2020, 124 (7): 4196-4201.

[2] Salem K S, Barrios N, Jameel H, et al. Computational and experimental insights into the molecular architecture of water-cellulose networks [J]. Matter, 2023, 6 (5): 1366-1381.

[3] Igarashi T, Morita N, Okamoto Y, et al. Elucidation of softening mechanism in rinse cycle fabric softeners. Part 1: Effect of hydrogen bonding [J]. Journal of surfactants and detergents, 2016, 19: 183192.

By clouds open leaves and fall

Q4. Why does C have a relative atomic mass of 12.0096? should it not have a relative atomic mass of 12 based on the standard of square C? According to the abundance values in nature, polar ²C accounts for 98.892%, and diamond ³C accounts for 1.108%. These two alone add up to 12.01108, not to mention the larger number of masses, but why does the Internet search say that 12.0107 of the relative atomic mass is not in line with the 2009 IUPAC conclusion 12.0096?

By Anonymous

A:

This problem has also perplexed me in my first year of high school! (the second year of senior high school also puzzled why the electronic arrangement of some elements of lanthanide actinides did not fully follow the rules.)

We will find that the relative atomic masses of most elements in the periodic table are not integers because they represent the average atomic weights of the various isotopes of the element.

The numerical value calculated by the subject is to calculate the relative atomic mass of the particle ³C as 13, but in fact it is not. After considering two factors, the relative atomic mass of the two factors is about 13.003355 and 14.003241 respectively. In the question is the early lower limit value. For microscopic particles, due to measurement errors, only a numerical range (C) = [12.0096, 12.0116] [1] can be obtained, with the progress of science and technology. Some parameters will be closer to the truth.

There is a small difference between the mass of protons and neutrons, one is 1836 times that of electrons, and the other is 1839 times that of electrons. One cannot think that one more neutron is exactly 1 more than the relative atomic mass; in addition, the combination of protons and neutrons will release nuclear energy, resulting in a mass smaller than the sum of the original masses of protons and neutrons, which is called "mass loss". This is not really less mass, but the mass is expressed in the form of energy. It should be noted that mass is equivalent to energy, and it cannot be said that mass is converted into energy, which is clearly reflected in the mass-energy equation.

Reference:

[1] Prohaska, Thomas, Irrgeher, et al. Standard atomic weights of the elements 2021 (IUPAC Technical Report) [J]. Pure and Applied Chemistry, vol. 94, no. 5, 2022, pp. 573600. Https://doi.org/10.1515/pac-2019-0603

By strawberry bear

Q5. Why do mobile phones consume electricity quickly in summer? When is the right time to charge your phone? By wo cold jade

A:

The mobile phone consists of a variety of hardware components and software applications, and both hardware and software applications will consume the phone's power.

In summer, the mobile phone will heat up due to the rising ambient temperature and the use of the mobile phone, and it is not easy to dissipate heat. The high temperature will lead to the acceleration of chemical reaction inside the battery and the rapid consumption of mobile phone power. In summer, when you are exposed to the strong outdoor sun, some mobile phone settings will also accelerate power consumption. If you turn on automatic brightness adjustment, the phone will automatically increase the brightness of the screen. In addition, in summer, some people who prefer outdoor activities use mobile data to run some applications, and people who prefer to stay indoors use their mobile phones to spend time using Wi-Fi to run some applications. Wait, these mobile phone operations will increase the power consumption of mobile phones.

When to charge:

At present, most mobile phone batteries use lithium batteries. Lithium-ion batteries have no memory effect, and the capacity generally does not change due to the state of charging, although maintaining a mobile phone battery between 30% and 80% can indeed reduce battery loss, but the help is limited. So it is possible to recharge at any time. Temperature is also a key factor in prolonging life and maximizing battery life. When charging a mobile phone, the ambient temperature should be kept between 16 and 22 degrees Celsius. When devices such as mobile phones are exposed to an environment above 35 degrees Celsius for a long time, the battery may be permanently damaged.

Reference:

[1] Phone battery draining fast 7 reasons and how to fix them. (chargerpedia.com)

[2] Maximize battery life with these essential smartphone charging tips (androidauthority.com)

[3] What'sUp,Home?-Doesyourphonebatterydrainfasterinsummertime?-ZabbixBlog

[4] Shu Jian. Mobile phone before the "old" battery first "decay" charging habits or behind the black hand [J]. Public electricity, 2021 Magazine 36 (12): 84.

[5] [classmate he] quickly charge the battery? 40 mobile phones have been tested for two years, telling you the best charging method _ bilibili

By floor sweeping monk

Q6. Why does the wind blowing from the lake bring a blast of heat when walking by the lake at night? By Anonymous

A:

The specific heat capacity of water is larger than that of sand, brick and stone on land, and it can store more heat, heating up slowly and cooling slowly, so the land temperature is higher than the lake temperature during the day; the lake temperature is higher than the land temperature at night-this is written in the middle school geography book.

Unlike the example of the sea and land breeze in the textbook, the general water body on the lake is not large enough to form a circulation of "rising air flow in the high temperature region-the flow of gas from the low temperature area to the high temperature area-circulation under the action of the pressure difference" as the ocean does. At night, the wind blowing from the shore to the center of the lake was heated above the lake and blew to the other side before it could rise. At this time, the gas is already hotter than the air on the shore of the lake itself, so what people feel is the hot wind blowing from the lake.

By Jing Yun

Q7. Is a gaseous planet a big pile of gas? By Anonymous

A:

No, the outermost layer of a gaseous planet is indeed made up of gas, but there is still liquid and solid matter inside it.

The gaseous planet is mainly composed of hydrogen, helium, neon and other light elements, and the outermost layer of the planet is the corresponding gas, such as hydrogen, helium and so on. But a layer of gas up to tens of thousands of kilometers thick produces a great deal of pressure (200GPa, about 2000000 times the Earth's standard atmospheric pressure) inside the gaseous planet. Under the pressure, the gas is compressed into a supercritical fluid, which exists in the form of an ocean of metallic hydrogen. Inside the gaseous planet, there may also be a solid core of ice and rock. However, because the current human probe is unable to detect the interior of the gaseous planet, its internal structure has not been determined, and more is only the consensus in the current research field.

Schematic diagram of the internal structure of Jupiter and Saturn-- Source NASA

Diagram of the internal structure of gaseous planets in the solar system-Source NASA as we can see from the image above, for Jupiter and Saturn, it consists of an outer hydrogen layer, an internal metallic hydrogen layer, and a central rocky core, while for Uranus and Neptune, it consists of an outer layer of hydrogen, helium, methane, an inner mantle layer of water, ammonia and methane, and a central rocky core. Therefore, gaseous planets are not entirely made up of gas.

By single young women

Q8. Is the magnet a conductor or an insulator? By is cold

A:

Simply speaking, it is not accurate to say whether a substance is an insulator or a conductor, because the object may have a phase transition with the change of temperature, such as various insulation-metal phase transitions or metal-insulation phase transitions. Here we only consider the situation at room temperature. However, the resistivity of many different types of conductors are in the same range, so we still give the resistance change in the whole temperature region, but only focus on the room temperature.

Common magnets include ferrite magnets and neodymium magnets. The resistivity at room temperature is of the order of magnitude, which falls in the semiconductor range. It can also be seen from the resistance temperature curve that it is a semiconductor.

The log (ρ)-1 / T curve of Fe "O" shows that the room temperature resistance of Fe "O" [1] is slightly different, but the Rmurt curve shows semi-metallic behavior.

The resistance-temperature curve of Nd "Fe" B [2] it doesn't matter what semiconductors and semi-metals are, their definitions need to be understood by studying quantum mechanics and solid-state physics. It is enough to know that at least at room temperature, the conductivity of both magnets is between a conductor and an insulator.

Reference:

VERWEY, E. Electronic Conduction of Magnetite (Fe3O4) and its Transition Point at Low Temperatures. Nature 144,327-328 (1939). Https://doi.org/10.1038/144327b0

[2] Alexander M. BOVDA, Vera A. BOVDA, Valerij V. DEREVYANKO, Vitaly A. FINKEL, Tatyana V. SUKHAREVA,Electrical and Magnetic Properties of Nd2Fe14B Magnet in Demagnetized and Magnetized States at Low Temperatures,Journal of Iron and Steel Research, International,13-1 (2006), 92-96. Https://doi.org/10.1016/S1006-706X(08)60165-X

By is a Tibetan fanatic.

Q9. What is the principle of NMR? By Anonymous

A:

To understand this, we need to understand three points: the nuclear magnetic moment, the magnetic moment will split the Zeeman energy level under the external magnetic field, and the two energy levels can resonate with the electromagnetic waves of the corresponding frequency.

Nuclei have magnetic moments, which visually means that some nuclei, like strip magnets, deflect under the action of an external magnetic field. The number of protons and neutrons in these nuclei cannot both be even, such as zero spin in the ground state and no magnetic moment. The nucleus we can use should have non-zero angular momentum, so that we can have a non-zero magnetic moment. The commonly used nuclei are H, C, N, F and P, and so on.

Now an external magnetic field is applied to these nuclei with magnetic moment, and after a calculation, it will be found that the nuclei with different magnetic moments point to split into different energy levels, that is, for the same nuclide, the energy level difference is proportional to the external magnetic field. This is called the Zeeman effect of the nucleus.

With split energy levels, you can naturally transition between the two energy levels. If an electromagnetic wave is added to the atom in the external magnetic field, resonance will occur when the frequency of the electromagnetic wave is equal to the Zeeman energy level difference. In the experiment, the resonance can be judged by applying the electromagnetic wave for a period of time and then removing it, and observing the transverse magnetic susceptibility signal of the sample.

A simple nuclear magnetic resonance device diagram, in which 1 and 2 are used to generate the scanning external magnetic field of 50Hz, and 4 are used to generate radio frequency electromagnetic waves. This effect can be used to detect the chemical environment in which the element is located, because different chemical environments produce different shielding to the external magnetic field, so that the magnetic field actually felt by the atom is not equal to the external magnetic field. If the frequency of the applied electromagnetic wave is constant, then the intensity of the external magnetic field required to achieve the resonance condition will shift. This shift is like the business card of a chemical group, which corresponds to one by one. For using H, we generally take the resonance magnetic field intensity of tetramethylsilicon (TMS) as the standard, and find out how much the magnetic field intensity of the sample resonates compared with it, we can know what group the H in this sample is in.

The nuclear magnetic resonance hydrogen spectrum of methanol, the Abscissa is the chemical shift, from which we can see that methanol has two kinds of H in modern medicine, nuclear magnetic resonance has become a very effective imaging method. Although many people will smell "nuclear" color change, in fact, nuclear magnetic resonance only needs radio frequency electromagnetic waves, which are lower than the frequency of microwaves used in communications, and the amount of radiation it converts is very safe compared with X-rays or CT. Of course, from the nuclear magnetic resonance signal to the image involves more than 100 million stories of image signal processing, generally speaking, it uses the gradient magnetic field to obtain the signals of different spatial frequencies, and then uses the inverse Fourier transform to calculate the actual image. This invention, which benefits all mankind, was also awarded the 2003 Nobel Prize in Physiology or Medicine.

Reference:

[1] Yang Fujia. Atomic physics. Edition 4 [M]. Higher Education Press, 2008.

[2] [Lao Qi] Why does MRI know-bilibili

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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