Is it 40°C for water to mix at 30°C and 50°C in volumes? | No.356

常温常压下50摄氏度的水和30摄氏度的水以相同体积混合兑出来的水是40度吗      

Q1

Answer:

If an object with a rest mass M moves at velocity v in a reference frame, its mass will expand to , and its scale will shrink to the original in the direction of motion, does this mean that the object may collapse into a black hole, the answer is no. The reason is that it is always possible to pick another viewer moving at a uniform speed, so that the object appears stationary to the viewer, due to the covariance of the general relativity field equations, the space-time geometry in different inertial reference frames only differs by one global coordinate transformation, there is no gauge singularity in one reference frame, and there is no gauge singularity in all reference frames, so an object cannot be made a black hole just by accelerating it (slowly) in a certain direction.

Q.E.D.

Q2

Answer:

Of course there are limits. The so-called "water with gravity of one ox can produce buoyancy greater than one ox" because buoyancy is "the gravity of the water" and not the "gravity of water". For example, the cylindrical object below sinking at the bottom of the sink is subject to, not the gravity of the water.

It should be noted that the law of buoyancy is a law of classical physics, in fact, this law shows that buoyancy is closely related to the pressure of liquids at different depths: where S(h) is the cross-sectional area of an object at depth h. Putting forward the part of the above equation that has nothing to do with integrals, it becomes that the weighted integral of the cross-sectional area over the pressure at different depths of the liquid is buoyancy.

Above we naturally use the concept of liquid pressure, which shows that the outer surface of the object should have at least a macroscopic thin and microscopic thick liquid film - friends who have studied undergraduate electromagnetism may be familiar with this statement. Macro thinning is to compress the liquid film thickness as much as possible; The microscopic thickness ensures that enough liquid particles hit the object due to thermal motion at any given time, and the number of impacts does not change much with time. Assuming that this thickness is the R-r above, and take 1μm, and then take r = 10cm, then 1N heavy water can probably become a deep liquid film, corresponding to the buoyancy of about.

Above we imply the assumption that both the outer wall of the sink and the inner wall of the object are absolutely smooth. Of course, the processing accuracy in reality can only be on the order of one trace (0.01mm). In order to meet the requirements of "liquid can fill the gap of the wall of the device and there is still surplus", we take the thickness of the liquid film R-r = 0.1mm (10 wires), then the corresponding buoyancy is about 500N.

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Q3

by anonymity

Answer:

This is indeed an idea for scientists to design the "invisibility cloak", but there is still some distance between it and Harry Potter's invisibility cloak.

Pendry et al. proposed a "stealth device" in 2006, the structure of which is shown in the figure below. The blue area is the "invisibility cloak", while the circular area in the middle is the object to be hidden. The blue "invisibility layer" deflects light that would otherwise hit the object, directing the light around the object and returning to its original trajectory.

The principle of this model is to use the method of optical transformation to link the virtual space and the physical space through coordinate transformation, and only have a "stealth" effect on a single wavelength of light, so once the wavelength of the incident light changes, the objects in it have nowhere to hide.

The closest material to light deflection to realize this idea is a new type of artificial material, named metamaterial. This material is composed of many crystallites smaller than the wavelength in a specific arrangement and has many unique properties, such as negative refractive index materials. Unlike traditional materials, where incident and refracted light are on either side of the normal, this material has incident light and reflected light on the same side of the refractive index.

This unique property has some interesting performance in optics, and the use of this material to design the optical path can achieve "invisibility" to a certain extent. Of course, the study of metamaterials faces many challenges, and there is still a long way to go to achieve true three-dimensional stealth.

Resources:

[1]J. B. Pendry et al. ,Controlling Electromagnetic Fields. Science312,1780-1782(2006).

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Q4

Answer:

Streaming: Not strictly equal, but not much error.

First of all, we assume that all water constitutes an isolated system, that is, only the energy exchange between waters, without dissipating heat into containers, air and other environments, nor considering the work done by the outside world on water during the mixing process. In this way, we only need to compare the sum of the internal energy of two glasses of water at 30°C and 50°C and the internal energy of two glasses of water at 40°C. This problem needs to consider the density and specific heat capacity of the water at different temperatures. The density and constant pressure specific heat of 30°C, 40°C, 50°C are shown in the figure below

Source: References[1]

Internal energy. Suppose both cups of water are V mL in volume, then

the internal energy of a glass of 30°C water;

the internal energy of a glass of 40°C water;

The internal energy of a glass of 50°C water.

That is to say, the total internal energy of the same volume of 30°C water and 50°C water under normal pressure is lower than that of twice the volume of 40°C water, and the temperature of the mixed water should be slightly less than 40°C. Of course, this is a very rough calculation, and to accurately calculate the internal energy, the specific heat should be integrated from absolute zero to the actual temperature value.

It is worth noting that the above equation calculates that the difference is very small, only a few hundredths. This difference can even change qualitatively due to changes in significant digits. In addition, the polynomial expansion method used in the reference to find specific values may also cause a larger error than this difference of a few percent (a miserable signal-to-noise ratio). From this point of view, it can also be considered that the mixing is equal to 40 °C.

Resources:

[1] Thermodynamic properties of water

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Q5

by anonymity

Answer:

Ah what? Do people really have such a thing as a sense of direction? The editor, who often can't find North, expresses doubts, especially in unfamiliar places.

But indeed, in general, in daily life, we cannot do without the perception of our own movement to act, and we cannot do without the memory of the surrounding environment and space to plan the route, let's talk about some physiological foundations of people's sense of direction.

When we move forward, do we know that we are moving forward? Haha, this seems to be a nonsense, in fact, this process is not so ordinary, on the one hand, the change of perspective seen by the eyes can provide information about their own position, on the other hand, it is also necessary to provide information on their own speed and acceleration with the help of the body's own "gyroscope" - the auricular vestibule, as shown in the figure

The vestibule and cochlea (really like a snail) are located in the inner ear of the person, and the vestibule includes three semicircular canals and a chamber connecting the cochlea, these structures not only convert vibrations into bioelectricity so that we can hear sound, but the physical changes stimulus receptors in them can also provide acceleration information in the head [1], which is important for us to perceive orientation. If it is too sensitive, it can also cause characteristics such as motion sickness.

Of course, I believe that everyone, like the editor, can grasp the direction in a more familiar place, which talks about the memory ability of the human brain to space. In fact, most of the research on the brain is based on rats, and the relevant research also won the 2014 Nobel Prize to the Mosers, showing that the hippocampus-entorhinal cortex system in the brain, especially the location cells and grid cells in it, provide the ability of rats to form a map of environmental cognition, and the hippocampus of rats and humans are structurally very similar. In addition, people have also found other cells with important roles, such as head facing cells, boundary cells, etc., interested students can learn more.

And what about in unfamiliar places? What is it like to have a sense of direction in an unfamiliar place? To talk about the sense of direction, the strongest is birds, often need to migrate long distances but rarely get lost, they can comprehensively use celestial bodies, the earth's magnetic field, mountains and other signals to find the direction, and these abilities are born built into the body, which is enviable. Using celestial bodies and mountains to judge direction, people can use it through acquired learning, so can humans use the earth's magnetic field to directly perceive direction? There are constant attempts to explore this, such as [2], which probe the brain's perception of magnetic fields. But the current location, related microscopic processes and whether actual humans have this ability should belong to unsolved mysteries, and the editor with a general sense of direction subjectively doubts ~

Resources:

[1] Bao Hugjiletu, Wei Guojian, Wei Junwen. Journal of Inner Mongolia Normal University(Natural Science Chinese Edition),1997(01):80-84.)

[2] Wang C X, Hilburn I A, Wu D A, et al. Transduction of the geomagnetic field as evidenced from alpha-band activity in the human brain[J]. eneuro, 2019.

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Q6

Answer:

First of all, it needs to be clear that "on fire" is not a specific disease, but a general term for a type of physical discomfort in daily life, and there is no term "on fire" in biology and Western medicine. In general, the uncomfortable symptoms of "getting on fire" (including but not limited to) are: swollen gums, sore throat, mouth sores, constipation, etc. Fried foods do have the potential to cause these symptoms.

During the frying process, starch under the action of high temperature and oil, forms a delicious "crispy shell", although delicious, but when we chew and swallow, this relatively hard brittle shell has the potential to cut tiny wounds on the oral mucosa, gums and respiratory mucosa. And fried food seasonings will be used more, resulting in increased local osmotic pressure of the mucosa, local dehydration, may cause inflammation, manifested in the mouth is mouth ulcers, in the respiratory tract is sore throat, foreign body feeling. Most of the fried food is meat, easy to stuff teeth, not cleaned in time, bacteria breed and cause gum inflammation; And its rich in a large amount of protein, in the absorption of metabolism need to consume a lot of water, resulting in thirst, if not timely replenishment of water will lead to dry stools, decreased urine output. Compared with steamed food, due to the sufficient water content of the cooking process, the surface of the final product is relatively soft, compared with fried food, it is indeed easier to "catch fire". But food with a crispy surface that can't stand high fat and calories is more delicious! (Drooling .jpg)

The above does not constitute medical advice, if you are unwell, please consult a professional doctor in time

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Q7

By middle school students who have questions about the world

Answer:

Wormhole is a space-time structure that is envisaged to exist in the real physical world under the framework of Einstein's general theory of relativity, which can connect black holes and white holes, allowing travelers to travel from inside the black hole event horizon, through wormholes, to the white hole area, and then leave the white hole to another distant space-time region.

It's directly a way to travel faster and cheaper, and a lot of people's imagination about using wormholes is focused on breaking through the speed of light to travel between stars.

But since the classmate asked this question, he obviously wanted to hear some different answersSo today, the editor will give this classmate a different job

"What would the world look like if humans mastered wormhole technology?" ——That is, of course, to open a courier company ah what in the future (6) (1) (8), Double Eleven, chop hands don't wait, just place an order here, the merchant over there will open a wormhole between the two of you, this express delivery will be delivered to your home in a second, isn't it beautiful? And the editor even designed the company's brand and trademark, the name is WEXPRESS, taking the meaning of Wormhole Express, the trademark is naturally pinched by the editor himself, this classmate can take it away for free without thanks

Q.E.D.

Editor: Shepherd