The temperature-dependant emission of electromagnetic waves. Combine this tag with [tag:thermodynamics] for a macroscopic view or [tag:quantum-mechanics] for a microscopic explanation.

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579 views

Why would different metals glow red at different temperatures?

According to everything I've been taught about incandescence and black-body radiation, and some quick Googling to confirm I'm not crazy, just about everything, regardless of composition, should start ...
1
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2answers
232 views

Thermal radiation of a nitrogen sphere

Let's imagine a : sphere of 100% pure nitrogen (N2), (edit: 1 m diameter) with a constant volume (edit: using a kind of "magic forcefield") (edit : at 1 bar) in the void far from any light source ...
3
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1answer
130 views

What does a hot, optically thin gas *look* like?

In another question I tried to answer what a sample of the Sun's photosphere or core would look like, if it could be brought into the lab. Here is a broader question - if I have a small inert ...
3
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1answer
354 views

Conservation of energy when focusing black body radiation on another black body

This question about whether or not it is possible to focus black-body radiation to make something hotter than the radiation's source was answered mostly negative: the second law of thermodynamics ...
2
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1answer
49 views

Counting modes Rayleigh-Jeans

In the derivation of the Rayleigh-Jeans Law, we count the number of EM modes in a square cavity. After calculating the number of allowed modes due to boundary conditions, we multiply it by a factor of ...
2
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1answer
336 views

Calculating new temperature of an object when air temperature changes

I'm trying to calculate the new temperature of an object when the air temperature around it changes, given a period of time. Basically I get periodic readings from an air temperature sensor in a ...
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1answer
56 views

How to use Planck's law of Black Body radiation?

On the wikipedia page for Black Body radiation, it states that: where $I(\nu,T)$ is the energy per unit time (or the power) radiated per unit area of emitting surface in the normal direction per ...
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1answer
34 views

Why do high altitudes have larger diurnal temperature variation than lower altitudes?

It seems like the lack of atmosphere should not be playing a role in the diurnal temperature variation because that's what makes it colder. Mountains are not that dry, usually.
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1answer
45 views

Ideally black plates

Let's consider $m$ ideally black, thin, infinite plates. Let $T_i$ be the temperature of the $i$-th plate. For given $T_1 > T_m$, why is the radiant flux between any two adjacent plates the same ...
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1answer
18 views

How can I apply Planck's law to calculate the photon energy density of a warm room?

I am trying to calculate the rate at which photons of any particular frequency will pass through a volume in a room illuminated by black body radiation only. I've found a couple of starting points ...
0
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1answer
61 views

light effects of electric current

In german high school textbooks and curriculua, the distinction of following main sorts of effects of electric current is usual: magnetic, heat, light and chemical. My question is, whether the glow ...
0
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1answer
76 views

How does the density of states for black-body radiation change with geometry?

If I have a hollow conducting cylinder with another conducting cylinder inside it (as with a coaxial cable), would the density of states of the photons/radiation between the two cylinders be any ...
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1answer
50 views

What would happen to the Earth's atmosphere if all the solar radiation was in the extreme ultraviolet?

According to this, our Earth's atmosphere is completely opaque to radiation with wavelengths less than 100 nm as this radiation has enough energy to ionize the air. Since the surface temperature of ...
0
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1answer
156 views

Work done in adiabatic reversible process

I was solving a problem on turbine.the steam works on turbine adiabatic reversibly .Is change in enthalpy or is it change in internal energy which equal this work?
5
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0answers
208 views

Gravitational redshift of Hawking radiation

How can Hawking radiation with a finite (greather than zero) temperature come from the event horizon of a black hole? A redshifted thermal radiation still has Planck spectrum but with the lower ...
5
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0answers
104 views

Photon pumping in Laser

Let's consider a ring laser where the laser must pass through the gain material before it is sent toward a partially reflective surface $\ R=1-T $. The other mirrors are perfect reflectors with $\ ...
4
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0answers
40 views

Temperature modelling

I have to find a mathematical model for the temperature vs. time to study the temperature of the environment next to a lamp. This lamp is made off and on on, let's say, a daily basis The lamp is ...
4
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0answers
1k views

How do you estimate colour temperature based on the position of the sun in the sky?

I'd like to estimate the colour-temperature of sunlight (as applied in photography) based on the position of the sun in the sky for a mobile phone app I'm working on (app link from a more appropriate ...
3
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0answers
76 views

Chemical Potential and Blackbody Radiation

What is the physical significance of the fact that the chemical potential of the photons in a box of volume V is zero ?
3
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0answers
133 views

black body simulation

black body radiation is typically understood from Planck's argument of light resonance in a box, from which the density of states is computed. Now, suppose I want to simulate a black body ...
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0answers
70 views

A problem concerning the change of temperature and spectrum of a filament

The spectrum of a filament has been given before, the left one having the lowest temperature, the middle with a medium temperature and the right one with the highest. My question is this: Why does ...
2
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0answers
41 views

Car Heating due to the sun: comparison of different contributions

Let's imagine that we need to choose a car in order to minimize the Temperature that the inner part of the car will reach after some hours under the sun. We know that two factors (if we suppose that ...
2
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0answers
24 views

In theory, Is it possible irradiate a blob of matter -for instance- with microwaves in order to that matter emit more energy than has received?

Suppose that you want coerce a block of matter to lose his energy. I mean thermal energy. This should be done by irradiation of more energy. The final thermal energy should be lower that the original ...
2
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0answers
401 views

Radiative cooling time for a black body

Okay, so this is confusing me a bit. How can I calculate the time it takes to cool a perfect black body from an initial temperature $T$ down to equilibrium temperature (say, 3 K for space)? I know ...
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0answers
23 views

Radiation collapse to black hole

I want to find the temperature at which radiation in AdS will collapse to form a black hole. I have even found a reference that gives the answer but I cannot understand it: ...
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0answers
33 views

At the lambda point, why does specific heat capacity tend to infinity?

The specific heat capacity is the energy required to raise the temperature of unity mass by 1K, if at the lambda point all the bosons occupy the lowest quantum state, shouldn’t the specific heat ...
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0answers
57 views

When can we assume a body to behave like a black body?

In an experiment I estimated the temperature of a Tungsten bulb filament measuring the resistance of it, and tried to verify whether the power of the filament is proportional to the fourth power of ...
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0answers
21 views

How to understand/derive Eq 5.5 in Geiner's Quantum Mechanics - An Introduction?

Geiner's Quantum Mechanics - An Introduction has an unnumbered equation above Eq. 6 in section 2.4 discussing density -- not sure if it is energy density -- of radiation: ... $$dE/dV = E/V =a ...
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0answers
74 views

Radiation heat transfer

I'm looking at the radiosity equations for heat transfer http://en.wikipedia.org/wiki/Radiosity_(heat_transfer)#Radiosity_method Specifically, I'm hesitant to accept the equation: $$ \dot Q_i = ...
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0answers
29 views

Radiation pressure at high temperature

I am reading a book on the history of the hydrogen bomb. The author describes the Teller-Ulam design and explains that the liquid deuterium charge was compressed by the radiation pressure generated by ...
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0answers
46 views

What would Planck's Radiation Law look like if there were only two modes of oscillation, $n = 0, 1$?

I have an idea that the law (since it's based on average energy, number of modes per volume, and a constant) would be shortened to something that would eliminate the summation terms in average energy. ...
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0answers
397 views

Does coffee with cream cool slower than black coffee?

At Modernist Cuisine, Nathan Myhrvold states that "Coffee with cream cools about 20% slower than black coffee". He then gives three reasons, two of them related to radiation and the third one to the ...
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0answers
201 views

Intensive radiative heat transfer in very hot gas ( >5000K, gas core nuclear reactor related )

The question: At temperatures above ~5000K are not stable any solid or liquid materials or even more complex molecules (such as fullerenes and Polycyclic aromatic hydrocarbons) which emit/absorb ...
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0answers
49 views

Please help me with this doubt from thermodynamics

My textbook says that net rate of heat transfer due to radiation is $\epsilon \sigma A(T^4-T_o^4)$ but i couldn't understand it. Rate of emission is $\epsilon \sigma A T^4$ and rate of absorption is ...
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0answers
25 views

On Rayleigh-Jeans black body distribution derivation

When trying to derive the Rayleigh-Jeans distribution function, all authors say that in k-space each solution of the electromagnetic wave functions(of waves inside a cubic black body) represents an ...
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0answers
44 views

Does nature really follow the heat equation?

I think the heat equation says that the first derivative of temperature with respect to time in a stationary solid varies as the negative of the second derivative of temperature with respect to ...
0
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0answers
19 views

Heat Losses to furnaces

I am looking in to the thermal efficiency of a furnace vessel. I am wanting to get a full, or at least better, understanding on the calculations needed to look at the thermal efficiency. So I know ...
0
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0answers
26 views

Why does vertical polarizer absorb the vertically polarized waves?

They explain that vertical wires serve as the secondary transmitter: the waves induce since oscillations of electrons in the wire effectively absorbing the wave. Ok, let's believe that wave is ...
0
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0answers
28 views

Why does an optical pyrometer read low in the open?

An optical pyrometer relies on the fact that the brightness of heat radiation from a body depends on its temperature. I have come across the fact that an optical pyrometer invariably reads low when ...
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0answers
41 views

Can anyone explain Wien's distribution law

I searched the internet for this but could find only the equation associated with the law. I want to know what this law states, what are its significance and why is it called as "distribution" law.
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0answers
37 views

Can thermal states of light be monochromatic?

As one can read from the quantum optics textbooks each mode of light of a certain frequency, spatial configuration and polarization can be considered as a quantum harmonic oscillator. One can imagine ...
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0answers
26 views

Continuous vs. Line Spectra

A hot solid or liquid produces a continuous emission spectrum while a hot rarefied gas produces a discrete emission spectrum. (And a hot dense gas produces a continuous spectrum with absorption ...
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0answers
30 views

Is there an acoustic Black Body Radiation?

Imagine a cavity in a infinitely extended solid filled with another type of solid in thermal equilibrium. Are there statistics for phonons equivalent to the Black Body Radiation by Planck?
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0answers
37 views

Am I very wrong with my grey body calculations?

I'm trying to calculate the maximum heat that an object on the moon can reach with the energy provided from Sunlight. I've got a total power output of the sun per square meter at the distance of the ...
0
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0answers
25 views

how can I compute the temperature of a metal (iron / steel) rod when a current is applied?

I would like to compute something like $T(j,l,R)$ where $T$ is the temperature of the rod, $j$ the current, $l$ the lenght of the rod and $R$ the radius of the rod. there is any difference if the rod ...
0
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0answers
26 views

Polarisation states in 1d?

I am working through a derivation of the spectral energy density in a 1d cavity. The derivation says that the number of modes (per unit volume) in a frequency interval $dv$ is given by: $$g(\nu)d\nu ...
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0answers
98 views

What kind of thing did Lummer and Wien use as a 'black body'?

When I was reading about the experiments on the black body, I was wondering what such a black body would look like. When I google for the black body used during these experiments by Wien and Lummer, I ...
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0answers
128 views

How do I map color to temperature?

Following up to Do burning red coals and red-hot iron have the same temperature? and Are Colors Emitted at Specific Temperatures?, how do I determine the temperature of the object by the color of the ...
0
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0answers
56 views

Can we make any substance that will produce ions at low temprature

I am talking about thermal ionization, Is to possible to make any material or is there any element that can be ionized at let say 20 degree centigrade Temperature? I am a computer engineer and into ...
0
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0answers
95 views

Landauer's principle vs Rayleigh–Jeans law

Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of $kTln2$? If it is so, can we also ...