Why are farmers afraid of cloudless nights? And how does a thermometer manage to measure a temperature of 39 degrees in ten seconds?
Newton's equation states that the amount of heat that passes through convection (one of the three ways to transfer heat) is directly proportional to the temperature difference. Since it is an algebraic difference, the temperature can be placed in degrees Celsius or Kelvin (the Kelvin scale indicates the absolute temperature of the body). When we stand outside our body only loses heat through convection. The cold wind blows on the skin and carries our body heat with it. Conduction does not exist, because air is a poor conductor of heat. However, this description does not correctly reflect reality. There is a third heat transfer mechanism and it is radiation. When we are in a big city like Tel Aviv, whose buildings surround us, we are less exposed to traffic because the obstacles make it difficult for the wind to flow in the summer. In addition, we absorb a large amount of heat in radiation. The radiation mechanism is usually not noticeable, because most of the bodies that surround us are at similar temperatures. The equations of heat transfer by radiation use the absolute temperature of the body and to the fourth power! That is, if our body emits radiation at a temperature of 37 degrees Celsius, which is 310 Kelvin (273+37) and the building in front of us is hot at a temperature of 323 degrees Kelvin (50 degrees Celsius), then in the heat exchange equation between us and the building, the temperature will be subtracted by a fourth degree. Instead of a difference of 13 for the convection, the result will be 1650. That is, for the same area, the radiation will be very significant compared to the convection. Let's go back to the original question for cold nights. As mentioned we exchange radiation with everything we see. If he is hotter than us, he will radiate more on us than we on him, and vice versa. When there are clouds we exchange radiation with them, when the temperature at this altitude is about minus 10 degrees for medium altitudes. When there are no clouds we exchange radiation with the outer universe, and it is known to be at absolute zero temperature (-273 Celsius). That is, in the calculation of the radiation we give to the sky, there will not be a term of sky temperature because it will be zero Kelvin temperature. If it is zero, there is nothing to offset the emission we send to the sky, and the lost energy will only include our body temperature to the fourth degree. Farmers on such nights would do well if they cover the crops.
The explanation for the rapid activity of thermometers is that the thermometer uses the radiation we emit to diagnose our temperature. Every body, no matter what it is and what its temperature is, emits radiation. If the body is warmer, its radiation will be more energetic and at a higher frequency. We as humans emit radiation in the infrared range, which makes it easier for example to identify human bodies at night. Devices that absorb this type of radiation see the bodies according to their heat - yellow and white for hot, and blue for colder. Our sun is yellow because it is boiling. It boils so much that it emits radiation in the visible range, and even above that (ultraviolet). By the way, the claim that you can't sunbathe inside the car with tinted glass windows is wrong. Yes, just slower. Frequencies in the 350 to 400 nm range still pass through without interference. Back to the question, the thermometer notices the frequency of the radiation we emit and accurately and quickly determines the heat of our body.
10 תגובות
First, I have no doubt that you understand heat transfer, but let me say that you expressed in quite a few words what can be reduced to two sentences. A body hotter than its neighbor has molecules that move with more momentum, and these transfer momentum to their nearest neighbors. This. You said more and less correct things. In convection, the body loses heat at a constant rate only if it is in equilibrium with itself. That is, at 37 degrees you produce a constant amount of heat and if there is a strong enough wind you also lose exactly this amount or more. But if you ran then on the drive you will lose fewer and fewer watts, because the temperature difference between you and the environment decreases. And transportation in a closed room is free, that is, driven only by the difference in densities. The air around you heats up and rises or falls depending on the density of your surroundings. That's all the difference. Second, heat transfer has nothing to do with concentration difference, but I will paraphrase your correct words (I know you just paraphrased incorrectly) in this way: there is a factor in physics called movement distance between particles. This is a factor that enters, for sometimes unclear reasons, into all fields of science. The more energetic the molecules in the gas around you are, the further apart they are, then each high-momentum molecule has to travel a greater distance until it meets a low-momentum molecule. In fact, we have created partial insulation, and we can talk about "concentration" of heat. The concentration of heat decreases in a given volume because there are fewer molecules. And finally, the difference between convection and conduction: associated conduction for a stationary state of molecules. If you realized that the body does not lose fewer watts in conduction because it produces the same amount - you will apply the same rationale to conduction. Both can be constant or variable, all depending on the environment's ability to remove the heat you emit.
Ahhh, a bit petty of you, Sabdarmish, isn't it? :) You made me laugh.
To the ivory, I really appreciate your work here, it deserves to be cherished. My question for this article is - what is the difference between heat conduction and its transport? Nature, subject to its various laws, strives for balance and comparison of concentrations. For example, when a hot body is in front of a cold body and there is nothing around them (and only they are there), they reach a temperature balance with each other through radiation alone, when the hot body radiates on the cold body. The temperature of the hot body gradually decreases as the temperature of the cold body increases.
In the case of a system such as a closed room with cold air inside and a body warmed by the air, heat conduction is also carried out which is a process in which the "hot" molecule vibrates faster which causes it to hit its cold neighbor which first vibrated slowly and now "got a few pushes" and moves faster until they are identical in properties this. But in practice they are not enough to compare the vibration property, because the cold one that has become slightly warmer, also transfers part of the movement it received to its neighbor who is still cold, and this is how the process unfolds, and the whole system heats up and gets closer to the temperature of the warm body. The molecules of the warm body come into contact with molecules that also have their own vibrations at a level that approaches that of the molecules of the warm body, and there is less transfer of this kinetic energy. This way the body actually loses less heat over time. If the room was open and a cold wind was blowing, then the same process would take place between the molecules of the warm body and the molecules of the cold air, only in this case the air that had just warmed up a little continued on its way and in its place came new, cold air that goes through the same process. It turns out that the warm body loses heat at a constant rate. The pace does not slow down as in the case of the closed room.
This is how I see the reason that the blowing cold wind and the standing air, both reduce your heat, when you are hot from them, in different ways, but according to one and only method, call it convection or conduction, I don't see the difference.
your answer?
One of the reasons for the mess in the article is that I failed to upload formulas properly, probably due to my poor understanding of the file upload system.
To the question: when you come to calculate the amount of heat removed from a body by convection, you multiply the convection coefficient in the area and by the temperature difference between you and the environment. In radiation, you multiply the area by an area coefficient (which I won't go into now what that means because the issue is complicated anyway - we'll consider it as one) and multiply by the expression: (your temp to the fourth minus the temp of the environment to the fourth), but in Kelvin degrees! In convection it doesn't matter which degrees you use because this difference is in the first degree and the unnecessary terms are reduced, but in the fourth degree you have to kelvin and then the difference is huge.
Hello Jonathan!
In the example you gave, the number 1650 was received. I did not understand what you brought up in the fourth that this number was received. 13 in the fourth it is not 1650.
Sabdarmish Yehuda
I did not understand your response in two ways. First of all, if you will be inside a sealed room, it does not mean that there is no transportation. There is no forced transport, but there is free transport, which originates from the temperature difference between you and your surroundings. This transport can be seen if someone runs 5 km fast on a cold day. When it stops there will be a column of hot air rising from it.
The second thing I did not understand from your response is how your words are interpreted from the article. After all, radiation is not convection, and on the contrary, it is much more significant than convection when there is no flow. If you will be in a cold room which is empty (!) then there is no transport at all. For example, in space there is no air to drive anything and there is no conduction either. So how do you cool down? in radiation
It appears from the article - if you are in a cold room but closed with no air circulation, you will not feel so cold because on the one hand the air does not transfer heat well and on the other hand there is no convection and this does not agree with reality.
The subject of the clouds is very true but the example is a bit far. Anyone who lives in Northern Europe will tell you that the coldest days in winter are the days with blue skies - no clouds.
I will surprise you: I completely agree with you. The power mechanism is intuitive. I hit the wall and he hits me. Radiation heat exchange between 2 or n bodies is not enough that is not clear to understand, but certainly to explain to others. And since I'm not gifted with extraordinary skills when it comes to clarity, I messed up a bit. Cora, and thanks for the understanding and the constructive comment. I agree with you.
It is not written so clearly. I happened to take a whole course on black body radiation (Prof. Zvi Dubinsky) so I can understand what it is about. But the explanation is not clear. And the most piquant part that amazes students is that even ice emits heat. Even liquid nitrogen. Every body without exception emits heat in sigma times its temperature in the fourth.
As for the heat meter, again, in my opinion the answer is incomplete and does not explain to the reader how this rapid measurement really occurs.
Sorry for the negative feedback, my intentions are good and I really like your section, Yonatan Shenhav, but this time I think it is hasty to publish a partial and slightly confused article.
Ami
A blanket prevents air from taking the heat of our body. It prevents transportation. Clouds prevent heat loss by radiation. But by and large you are absolutely right.
Can you say that the clouds serve as a blanket that keeps the air warm?