All Questions
Tagged with thermal-radiation thermodynamics
669 questions
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Event Horizon vs. "Interior" of a black hole from perspective of distant oberserver
From the perspective of a faraway observer watching an object fall into a black hole, they will see the object approach the black hole's horizon at a slower and slower rate until eventually "...
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What is the purpose for the blackbody radiation graph to be graphed using the below parameters?
If you observe the above graph, for y axis, "intensity per wavelength" is used as the parameter. I am aware we use "per wavelength" because it is hard to measure and graph isolated ...
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Physics behind gold+silver space blanket placement
In the first aid and mountaneering, "space blanket" or "astro foil" is used for protection against cold enviroment or as first aid after burns. There are many different versions of ...
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Can nonimaging optics concentrate heat to a higher temperature than the source? [closed]
Practical goal:
I'd like to concentrate the low-grade heat from the back of solar panels, which I've measured is in the range of 20-50F greater than the ambient temperature throughout the day, and ...
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Planck's law in terms of flux [duplicate]
The Stefan-Boltzmann law describes blackbody radiation in terms of radiant flux per unit area i.e. how much energy passes through a 2D surface per unit time. I find this very simple and I understand ...
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3
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Can small solid bodies emit radiation with wavelength larger than the body itself?
It's known that EM waves cannot pass the aperture with a diameter much smaller than the wavelength. How is it for an opposite situation? If, for example, a solid body is a $100 \ \text{nm}$ sphere, ...
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Spectral energy density of black body radiation [closed]
$u(\nu,T)$ is known as the amount of energy emitted per unit frequency $\nu$ per unit volume $V$. It is the function of $\nu$ and $T$. In case we would like to know the amount of energy density at ...
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How is radiation emitted from a small hole in a box, if the hole is small enough to prevent radiation from escaping in the first place?
So I totally understand how a hole in a box works as a good approximation for a perfect absorber of all incident radiation (a black body). However, a black body is also a perfect emitter of radiation ...
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Why does not Earth radiate more energy into the space while it gets warmer? [closed]
The earth is getting warmer. By Stephan's law, it should radiate more energy into the universe.
On the other hand, the energy from the sun is a constant. So how to achieve a balance?
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Matter made of neutral charges does not radiate?
We know that any blackbody that has a certain non-zero temperature radiates. This happens due to the jiggling of charged particles (protons and electrons), and accelerated charges create ...
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Do black holes convert 100% of their mass into energy via Hawking radiation?
And if so, does that mean that we do not need matter/antimatter annihilations for 100% mass to energy?
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Underlying mechanism for black body radiation [duplicate]
I understand that every body of some temperature above the absolute zero radiates EM waves. When we heat it up enough it enters the visible spectrum and than proceeds to go upwards to higher and ...
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How can the air be warmer than both the floor and the glass ceiling of an aluminum foil-lined box in the sun?
I've constructed a styrofoam box, with the insides spray-painted black, on top of which I placed five layers of borosilicate glasses, air-gapped with a bit of cork. It is diagrammed below:
The box ...
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How to estimate thermal conductivity of a sample?
There is a rectangular sample whose emissivity is known, and I want to estimate its thermal conductivity. A heater with a known heat flux is applied to one end of the sample and a fixed temperature is ...
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Is a hotter heat source always necessary for an object to be heated?
Heat always moves from a high-temperature object to a low-temperature object. Therefore, a low-temperature object cannot become hotter than the relative high-temperature object from which it is ...
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How to build a reverse greenhouse?
So basically, I stumbled across this concept of radiative cooling. There are a couple of YouTubers who posted videos on this topic.
What I understood, was that in passive daylight radiative cooling, ...
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Trouble understanding the classic approximation of a black body as a hole on a cavity
While studying the Rayleigh-Jeans attempt to explain the spectral energy distribution of black bodies I have trouble understanding the concept of a black body as a small hole on a cavity.
We define a ...
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Regarding the absorption property of a blackbody
Consider a blackbody of surface area $S_b$ and at temperature $T_b$. It is placed inside an evacuated chamber (to neglect all the effects of convection), with walls of chamber at temperature $T_c$ and ...
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Cooling properties of trees and thermal radiation
A question about trees, shading and thermal radiation: Will the shaded area under trees have lower average air temperature than adjacent non-shaded areas?
On the one hand, I've read that the real air ...
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Two interacting blackbodies (one inside another) - when will thermal equilibrium be attained? [closed]
As I understand it, an ideal blackbody absorbs (and subsequently starts emitting) all incoming radiation. In typical setups like determining a planet's temperature given its albedo and distance from a ...
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Thermal spectrum of a warmer body in a colder room
Here are a few questions about heat that I've been wondering about.
Suppose I had a piece of glass (just as an example) at room temperature, let's say $T_0 = 293$ K, and I moved it into a dark room ...
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Does Kirchhoff's law of thermal radiation fail for optically thin sources?
A box made of any material with a small hole in it will give blackbody radiation. According to my textbook by ARNAB RAI CHOUDHURI, If you place an optically thick source of same temperature as the box ...
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Why is a cavity with discrete modes necessary in the derivation of Planck's law?
In my readings, I have come across the concept that a finite cavity allows for the quantization of electromagnetic modes, leading to discrete energy levels. However, it's not clear to me why this step ...
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What do we mean when we say the CMB has a temperature and how do we measure it?
I have read this:
An object without any internal degrees of freedom, like a single photon, can't really have a temperature. But an ensemble of photons can have a temperature. If you put an ensemble ...
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Should I hold a baby formula bottle to cool it down faster?
This is an interesting and somewhat surprising physics problem - holding a hot object in your hand will cool it down faster, even if the air around is colder. I guess that 90% of people would be ...
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In a universe with no photons, will everything necessarily be at absolute zero temperature?
Imagine a universe governed by the same physical laws as ours, i.e., the same fundamental forces, with the only caveat that there are no photons, hence no electromagnetic radiation in this universe. ...
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Radiation power emmited by a material with two different temperatures [closed]
Let's consider a cylindrical sample of a solid material surrounded by air. From $0 \leq r \leq r_1$ the temperature of the material is $T_1$ and from $r_1 < r \leq R$, $T=T_0$ which is also the ...
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How do differently colored objects reach the same temperature if they are completely isolated together?
Imagine a perfectly reflective container, filled with vacuum and two bodies with different colors, i.e. they have different emissive and absorptive properties. The bodies don't touch each other or the ...
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Thermal emissivity for cool pavements
I have a very basic question that I'm struggling to find a clear answer to as I do research about cool pavements.
The literature says cool pavements should have high albedo, high thermal emissivity. ...
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Why are planets cold?
Excuse me for me ignorance, I'm just fiddling around this question for quite a while.
The question of course is not limited to planets. Boiling down to, why isn't the universe heated up by now to ...
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Linearization of long wave radiation equation
I am basing my reduced order building model on the equations proposed in the article “A methodology for generating reduced-order models for large-scale buildings using the Krylov subspace method” ...
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Does this system involving blackbody radiation violate the second law of thermodynamics?
Consider the surface of rotation shown in heavy lines in the accompanying sketch. It is made up of parts of the surfaces of two confocal ellipsoids of revolution, and that of a sphere. The inside ...
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Fundamental principles for simple radiative heat transfer problems
The picture below illustrates what is intended to be a very simple one-dimensional textbook-style radiative heat transfer problem. It is meant to be a pedagogical tool for explaining the greenhouse ...
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Unintuitive observation by Rumford
I was reading this paper on the work of Count Rumford in developing the modern view of heat as a form of energy and not some kind of fluid. In that I encountered this experiment.
Say you had two ...
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How the range at which the radiation is emitted get affected with changing temperature?
If the emitting body is blackbody, then the range at which radiation is emitted won't change, because a blackbody is a blackbody whether the temperature is 4000K or 120000K, meaning that, by ...
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Would a hypothetical perfect reflector emit thermal radiation?
The question says it all. I believe a hypothetical perfect reflector is what's referred to as a "white body", but I might be wrong. From what I understand such a hypothetical perfect ...
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Black Body Radiation similarity to Hawking Radiation
Do black body radiation of a 40-Earth mass osmium planet with radius of Earth which was just formed and has a temperature of 10000 degrees Celcius emit photons not just near it but even at infinity ...
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Second law of thermodynamics does not seem to hold under radiation
The Second law of Thermodynamics can be stated as heat always flows from hot to cold.
Consider the following system described in the image, where we have two parts A and B, at some time $t = 0$ we ...
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Is the blackbody intensity from a hot object always higher than from a cold object?
This is a conceptual question I can't quite wrap my head around.
Take two blackbodies with temperatures $T_{hot} > T_{cold}$. Both should have a spectral intensity described by Planck's law
$$I(\...
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Electromagnetic entropy maximum in Planck's black-body radiation law
I am reading Planck's work on black-body radiation.
In the paper on the page 19 it is said that the expression
$$R_\nu=\frac{\nu^2}{c^2}U\tag1$$
where $R_\nu$ is the intensity of a linearly polarised ...
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Can people feel the low heat radiation from very cold surfaces?
Here's a thought experiment about the way that heat is transferred through radiation.
Humans can physically feel when a hot object radiates heat on them, such as a campfire or an infrared-based space ...
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Thermal radiation in hollow sphere
I have been asked to calculate the time taken for a highly conducting hollow sphere to cool down from a certain temperature say $\theta_1$ to a temperature $\theta_2$ ($\theta_1, \theta_2 > \...
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How do I calculate the rate of heat loss from an enclosure when I have a plot of temperature vs time and the enclosure's material details? [closed]
I have an enclosure running an ON/ OFF temperature control system. The below graph shows internal air temperature over time. I have the raw data providing actual values.
I have the dimensions of the ...
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Heat transfer vs. Volume transfer [closed]
Apologies in advance if this has been addressed before... it's something if a daily puzzle during my morning ablutions and I haven't gotten a handle on it yet.
Consider the hot water pipe from my ...
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Why does blackbody radiation formula work for radiator?
I'm just trying to understand this from a purely thermodynamical standpoint. I'm comfortable with the scenario of a blackbody cavity's interior being in thermal equilibrium with the photon gas ...
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Can a body be in thermal equilibrium at a different temperature from surroundings?
As per my knowledge bodies attain constant temperature (thermal equilibrium with surroundings) when they absorb and emit energy at equal rates.
Let us say temperature of surroundings is T1.
We have a ...
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Do all small-large AdS black hole phase transitions have swallow tail like behaviour for the Free Energy v/s Temperature plot?
In the literature the swallow tail like behaviour is prominently seen for small-large AdS black hole phase transition for the Free Energy vs Temperature Plot. Recently I was trying to reproduce the ...
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Thrid law of thermodynamics and quantum mechanics
Third law of thermodynamics says that it is impossible to reach absolute zero temperature in finite numbers of operations. According to quantum mechanics, every system has energy levels and ground ...
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Why are the hottest stars appear mostly in blue or blue-white? [duplicate]
Does it have to do with anything regarding wavelengths and frequencies..
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265
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Thermodynamics explaining the cold wall sensation
Let's consider a 16 m² room (4m x 4m x 3m height cube) containing air at 20°C. Let's say the walls of the cube are at a constant temperature $T_{w}$.
There is a well-known effect called "cold-...
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