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

The clothes are worn out and wrinkled.

Don't worry, it will be smooth when pressed with an iron.

But you know why.

Can I iron my clothes with an iron?

Q1. How to keep the cup warm? What method can be used to strengthen the thermal insulation effect? By ~

A:

Usually rely on vacuum interlayer.

There are three ways of heat transfer, namely, heat conduction, heat radiation and heat convection. All three types of heat transfer occur as a glass of water is placed on the table and gradually cools. Hot water always radiates heat in the form of electromagnetic waves, which is thermal radiation; the hot air near the water surface is convective with the cold air in the distance, and the heat is carried away by the hot air; the heat of the hot water can also be transferred directly to the guide cup, and the cup is transferred to the table, which is called heat conduction. In general, the thermal radiation heat transfer efficiency is slow, the heat conduction efficiency is high, and the heat loss mainly comes from the heat conduction, but the heat conduction also depends on the thermal conductivity of the material, and the thermal conductivity of the metal is very good. If the enamel cup is filled with hot water, you will soon find that the cup becomes hot, which is the embodiment of metal heat conduction.

The vacuum interlayer of the insulation cup is to insulate the heat conduction, and the heat conduction also needs a medium, so there is no way to conduct the heat when pumping into a vacuum. The heat conduction medium in the heat preservation cup is only the cup itself, but although the inner wall of the insulation cup is stainless steel, the outer wall is generally a material with low thermal conductivity, which conducts much less heat. After the cup lid is closed, the thermal radiation and convection are also very weak, so it can be kept warm naturally.

As for strengthening the thermal insulation effect, I am personally satisfied with the thermal insulation effect of the thermal insulation cup, but since the reader has asked a lot, I will give you a trick.

According to the average age of our readers, we should all have seen another artifact with better thermal insulation, yes, the warm pot, the little fat green man who is more than a foot tall in the old TV series! Of course I didn't mean you were old.

Although this kettle is fragile, the heat preservation effect is really good. It is nice to keep boiling water in it for three days and still burn your mouth. But the principle of its heat preservation also depends on the vacuum interlayer. But it also has two small secrets: first, its inner wall is glass, and the thermal conductivity of glass is much worse than that of metal, so it conducts more slowly; second, it uses a silver mirror to react with a layer of silver on the inner wall, which reflects higher efficiency of silver to thermal radiation and further reduces heat loss, so the thermal insulation performance is better.

However, the above two steps do not seem to be easy to achieve, in fact, you can also consider pumping a high vacuum, but you do not seem to have a molecular pump, so just buy a thermos.

By Frost White

Q2. Why can the iron iron smooth the clothes? By, a curious high school student.

A:

Before answering this question, let's talk about why the clothes are wrinkled.

There are many kinds of fabrics for clothing, but they all contain polymer fibers, and these polymer chains maintain the stability of the overall structure through hydrogen bonds. If it is subjected to external stress, or if it is heated by water, the original hydrogen bond will be broken, and a new hydrogen bond will be produced, thus changing the overall structure, and there will be a strong interaction where the hydrogen bond is denser, resulting in wrinkles in the clothes.

So if you want to flatten the clothes, you have to let these hydrogen bonds exist in places that make the clothes flat, and by ironing, you can achieve this effect. The iron vaporizes the water in the sink by heating and ejects it through the hole at the bottom. When high temperature steam acts on clothing, it can not only reduce the number of hydrogen bonds in the fiber, but also make the fiber itself more relaxed. After ironing, the segments inside the fiber form new hydrogen bonds in the new equilibrium position and stabilize as the temperature decreases, so that the clothing can return to its previous flat state.

From this point of view, it is not difficult to see that the two most important conditions for clothes to return to a flat state are water (softening fiber) and temperature (changing structure), perhaps without an iron. using other appliances that can produce high temperatures (such as hairdryers) with water may have the same effect.

By Eric

Q3. Excuse me, if sodium chloride and potassium nitrate solids can be dissolved in pure water and then evaporated at a certain temperature (without destroying ions), can potassium chloride solids and sodium nitrate solids be obtained? Thank you for the answer. By zcx

A:

Yes, but not just these two solids. The details can be quite complicated.

And both substances are made up of ions. When they are dissolved in water, they exist in the form of ions. There are four kinds of ions in aqueous solution, and their amount is determined by the amount of sodium chloride and potassium nitrate. When evaporating the solution, these four ions can be combined into, and four substances (or ion combinations), all of which may precipitate. The specific precipitation order and amount depend on the ion concentration and crystallization temperature in the solution. The substance that is the first to reach solubility will be the first to precipitate. If you want to analyze such a complex example in detail, you need to refer to the phase diagram.

The source of the base map is reference [1], for example, the above phase diagram is the phase diagram of the aqueous solution system of four ions at 100 ℃, the transverse and longitudinal axes are the ion concentration, and the solid line is the phase boundary. At 100 ℃, the solution at D point will precipitate, and the solution at D point will precipitate and.

It should be noted that once the precipitation begins, the composition of the solution changes and moves to a new point on the phase diagram. And analyze it according to the group of this point. The composition is constantly changing and will cross a line on the phase diagram in the process of precipitation.

So, if the solid potassium chloride and sodium nitrate with a molar ratio of 1:1 are dissolved in water (phase diagram O point), sodium chloride crystal will be precipitated first (moving along the dotted line of OA to the phase boundary), then potassium chloride and sodium chloride (moving to the point along the phase boundary), followed by sodium chloride, potassium chloride and potassium nitrate (stop point) will be precipitated to evaporate. In this case, sodium nitrate crystals will not be precipitated. If the initial proportion is not 1:1 but other proportion, then the precipitation order, precipitation amount and even the type of precipitated crystals will change, which requires specific analysis.

Reference:

[1] if I dissolve both KCl and NaNO crystals together and evaporate the solution, is it KNO or KCl?

By is a Tibetan fanatic.

Q4. How does the machine used to detect liquids in the subway station work? By is ignorant

A:

In short, the machine makes use of the differences in physical properties of different liquids to achieve the purpose of security check without contact with liquids.

At present, the quasi-static computer tomography technology, which is based on the measurement of dielectric constant, is the most widely used in subway liquid safety inspection in our country. The dielectric constants of safe liquids and dangerous liquids are quite different, and the dielectric constants of different polar molecules are different, and the liquids with different dielectric constants will have different effects on the surrounding electromagnetic fields, such as changing the strength of the electric field, the capacitance between plates and so on. Using this point, we can detect the feedback signals (such as electric field strength, capacitance or complex impedance) of the measured liquid after the electric field is applied to analyze the polarity of the liquid molecule. For example, the polarity of oil molecules is weaker than that of water molecules, so it has a lower relative dielectric constant. Flammable and explosive dangerous liquids are mostly organic compounds (such as gasoline, kerosene, etc.), drinks and other safe liquids have high water content, so the polarity of dangerous liquids is generally weaker than that of safe liquids, so we can judge whether the liquid is flammable and explosive.

By depth

Q5. In the "three-body", why does the outer explosive of the nuclear bomb explode but the nuclear bomb does not explode? By du Yuxin

A:

Let's first look at the snippet from the original work:

"there is a way: shoot at that ball." The explosives expert whispered in Dashi's ear.

"it won't detonate?"

"it will only detonate the conventional explosives in the periphery, but it will break up the explosives, unable to produce accurate centripetal compression of the central nuclear explosives, and there will certainly be no nuclear explosion."

Chapter 25 of the first part of the three-body

In fact, Da Liu's idea is very obvious here, because the initiation of a nuclear bomb (set in the book as an atomic bomb) is to ignite the surrounding conventional explosives to produce an inward compressed shock wave, thus making the internal fissile material reach the critical state required for the reaction. According to this description, the nuclear bomb in the girl's hand should be detonated in the form of implosion, or convergence.

Comparison of two types of implosion detonation among the publicly available data of this kind of nuclear bomb, there is also a method of gun detonation. It divides the nuclear material into two or more parts. For example, the following diagram is divided into two hemispheres, each part of which does not exceed the critical mass of the nuclear reaction. When the conventional explosive on one side is detonated, each part of the nuclear material is impacted and squeezed together to form a whole with supercritical mass, thus initiating the nuclear fission reaction.

To sum up, the implosion type is to ignite the outer circle of explosives to squeeze the nuclear material inward, and the gun type is to ignite one side of the explosive to squeeze the nuclear material in one direction. Now you should be able to easily tell the detonation mode of "fat man" and "little boy" from the appearance.

May the world be peaceful.

By sheep herding

Q6. How did the vortex come into being? By Zi

A:

The vortex is the phenomenon that the fluid moves around the central axis of the cylinder. In nature, we can often see swirls, such as destructive tropical cyclones and swirls behind bridge piers in the water. The specific mechanisms of different eddies in nature are different, but they are basically formed under the action of geostrophic deflection force. The geostrophic deflection force is a kind of inertia force, which causes the moving body on the earth surface to be subjected to a force perpendicular to its direction of motion due to the rotation of the earth. We know that at low latitudes on the earth, it is far away from the rotation axis, so it moves at a higher linear speed from west to east around the rotation axis. When an object moves from south to north in the northern hemisphere, its linear velocity from east to west is higher than that of the surface at lower latitudes, so the trajectory of the object moves eastward relative to the surface, that is, it deflects to the right. When moving from north to south in the northern hemisphere, the trajectory of the object will move westward relative to the surface. Generally speaking, when the fluid moves due to convection and other factors, it is easy to move clockwise in the northern hemisphere to form a vortex. On the other hand, the fluid moving in the southern hemisphere will form a counterclockwise vortex.

In addition to these natural fluid vortices, there are many other kinds of vortices, such as superconducting flux vortices, which are common in superconductors. We know that superconductors have absolute diamagnetism, that is, their internal magnetic field strength is maintained at 0. When we apply a magnetic field to a superconductor, as the magnetic field approaches the critical magnetic field of the superconductor, there will be a flux vortex on the superconductor, which is a non-superconducting normal state inside the vortex, and the magnetic field passes through, as shown in the figure. Each bright spot in the picture represents a flux vortex, which reflects the spatial modulation of the order parameters in the superconducting body. The corresponding study of vortex dynamics is also a direction of condensed matter physics.

By chen

Q7. How is the speed of light in water measured? By has a good time.

A:

The measurement of light speed in water can be divided into direct measurement and indirect measurement.

Indirect measurement of the speed of light in water can be very simple, as long as through (this is the definition of refractive index) and the law of refraction (which can be directly derived from Fermat principle), the speed of light in water can be easily obtained by measuring the total reflection angle of the air-water interface.

Without the use of modern optoelectronic systems, direct measurement is a little more troublesome. Historically, Fizeau used the rotating gear method to measure the speed of light in the air in 1849, and the same method can measure the speed of light in water with little improvement.

Left: main view of light path; right: side view of light path

In Fizeau's experiment, a gear and a mirror were placed on the optical path, and the distance between them was L. The axial direction of the gear is the same as the direction of light propagation. Light hits the mirror through the gap between the upper teeth of the gear and passes through the gear again. When the gear does not rotate, the reflected light can pass through the gear from the same backlash. Turn the gear and the light will be blocked by the teeth. By changing the speed, at a certain speed, the reflected light can pass through the gear at the next backlash. Then speed up, the appropriate speed can reflect light through the next backlash. Record the speed difference, then there is, where N is the number of gear teeth. If a length of water is inserted between the gear and the mirror (assuming the container wall is transparent and very thin), the above formula is simply changed to. Given c, the speed of light in water is not difficult to understand.

Reference:

[1] how is the speed of light calculated?

By is a Tibetan fanatic.

Q8. Is an one-meter ruler as long as it is horizontal and vertical at an altitude of zero on the earth? if not, why? By Anonymous

A:

Too long to read the version: not the same length, the vertical ruler will be shorter than the horizontal one, but very few.

It can be understood that the real ruler itself has weight, so the lower part of the ruler will be compressed by the upper part of the ruler, thus shorter. However, due to the hardness of the ruler itself, the amount of compression is so small that it is usually negligible.

However, in the spirit of rigorous science, we can simply estimate how short the ruler is (physical warning ahead):

Set the total length of the ruler, the cross-sectional area is S, and the density of the ruler material is perpendicular to the ground. Establish a coordinate system on the uncompressed ruler as shown in the following illustration

I can't find such a long ruler. Please think that it has a small piece of a 1m long ruler to analyze. For the microelement located at the coordinate, the pressure is the gravity of the ruler with the total length above.

Young's modulus is a quantity that describes the longitudinal elasticity of an object, which is defined as the strain on the stress ratio, which is only related to the properties of the material itself.

Let's say that the length of this small piece of microelement is changed after compression, and its strain is, that is,

Finally, the total variable length of the ruler is obtained by summing the whole ruler.

Using this formula, it is estimated that Beijing is about 39 degrees north latitude, and you might as well take the acceleration of gravity with latitude 40 degrees and altitude zero.

If this is a steel ruler, the density of the steel, the Young's modulus of the steel, then the steel ruler can be shortened by only 1/5000000, and the length of the ruler is reduced to 0.9999998m (6 9s).

If it is a plastic ruler, the plastic ruler on the market generally uses relatively hard PS plastic, the density of this plastic is about, Young's modulus is about, the plastic ruler can be shortened, a little more than the steel ruler.

In short, although very few, the vertical ruler is indeed shorter than the horizontal time lost!

By childe

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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