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.
24
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3answers
846 views
The Pioneer anomaly finally explained?
Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
19
votes
4answers
389 views
Paradox?: What is the form of radiation experienced by a harmonically accelerated observer?
Theory predicts that uniform acceleration leads to experiencing thermal radiation (so called Fulling Davies Unruh radiation), associated with the appearance of an event horizon. For non uniform but ...
17
votes
6answers
3k views
Is fire plasma?
Is Fire a Plasma?
If not, what is it then?
If yes why, don't we teach kids this basic example?
UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify ...
13
votes
2answers
268 views
Why aren't gas planets and stars fuzzy?
The edge of Jupiter looks very sharp.
Even more bothersome, the edge of the sun looks sharp, aside from kind of a soup of particles floating above it.
The sun's surface has an incredibly low ...
13
votes
4answers
869 views
Entropy of radiation emitted into space
In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a ...
11
votes
6answers
616 views
What are the various physical mechanisms for energy transfer to the photon during blackbody emission?
By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium ...
11
votes
1answer
284 views
Quantum uncertainty of particle falling in black hole
A stationary observer at infinity sees a particle of mass m falling in a supermassive Schwarzschild black hole. He observes an increasing redshift and sees the particle ceasing to progress when it ...
8
votes
1answer
143 views
Do gases reflect some IR radiation?
The usual definition given for a greenhouse gas is that it absorbs infrared radiation. Of course, then the gas emits its own thermal radiation, and it does so without preference for direction ...
7
votes
2answers
687 views
Why do we use Planck's constant?
I have been trying to reason why energy packets (i.e. photons) are assumed to be quantized. I know this originated from Max Planck, but may someone explain why energy couldn't be emitted continuously ...
7
votes
3answers
1k views
Would wearing clothing that is black on the inside and white on the outside keep you cooler?
The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get ...
7
votes
3answers
892 views
Quantum mechanics of thermal radiation
What are the quantum mechanisms behind the emission and absorption of thermal radiation at and below room temperature? If the relevant quantum state transitions are molecular (stretching, flexing and ...
6
votes
3answers
3k views
Why do lightbulbs continue to glow after the light is turned off?
I've noticed that whenever I turn the lamp off in my room at night, the lightbulb seems to continue to glow for a minute or so after that. It's not bright though; the only way I even notice it is if ...
6
votes
2answers
473 views
Deriving Planck's radiation law from microscopic considerations?
In the usual derivation of Planck's radiation law, the energies or frequencies $\omega$ of the oscillators depend on the measurements $L$ of the black body. The model is such that the only ...
6
votes
1answer
614 views
What’s the relationship between thermal radiation and Johnson thermal noise?
All objects above absolute zero emit radiation due to random collisions between the atoms they are made of. The spectrum of radiation emitted varies according to the temperature of the object, I ...
5
votes
4answers
1k views
Light emitted by an object according to its temperature
According to this picture
the light emitted by an object depends on its temperature.
That makes perfect sense when we heat a metal. As its temperature raises we see it red at first, then orange, ...
5
votes
4answers
2k views
What causes hot things to glow, and at what temperature?
I have an electric stove, and when I turn it on and turn off the lights, I notice the stove glowing.
However, as I turn down the temperature, it eventually goes away completely. Is there a cut-off ...
5
votes
4answers
198 views
what is the basic form of the 'fire'? [duplicate]
Possible Duplicate:
Is fire matter or energy?
What is the basic form of fire? physics defines every entity by a basic form either solid or liquid or as a gas,
example: water is liquid,
ice ...
5
votes
1answer
53 views
Temperature of glowing materials
As I understand it, Stars emit visible light, OBAFGKMRNS, in the range of $10^3 - 10^4 K$.
Yet materials such as steel emit similar frequencies at much lower temps; red is around 800K.
Why the ...
5
votes
1answer
557 views
What temperature can you attain with a solar furnace?
A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your ...
4
votes
2answers
267 views
Greenhouse gases
A post (below) on the Bishop Hill blog relating to climate change asserts that no warming effect can be attributed to CO2. I don't know whether the author is really a physicist but it sounds ...
4
votes
1answer
52 views
Bunsen Burners and the Sun
Why do Bunsen burners burn blue in the center? What element is being burned?
Why does the sun glow yellow, and not blue-a Bunsen burner is much cooler and yet it burns blue. Is it because the ...
4
votes
1answer
85 views
How is the index of refraction dependence in Planck's law compatible with thermodynamics?
In various formulae for black-body radiation where $c$ appears, there is an implicit index of refraction dependence, since $c=c_0/n$, where $c$ is the speed of light, $c_0$ is the speed of light in ...
4
votes
2answers
148 views
Radiation pressure on a Dyson sphere
To find the outward pressure from the sun's light on an enveloping spherical shell (Dyson sphere), one can simply divide the insolation by $c^2$. Using the entire system, we can specify the power of ...
4
votes
1answer
251 views
Historic derivation of Wien's law
Every book I've read, including a lot of websites, Wikipedia, etc, say that Wien derived this:
$$\rho_\nu(T)=\rho(\nu,T)=\nu^3f\left(\frac{\nu}{T}\right)$$
Being $\rho_v(T)$ the spectral enegy ...
4
votes
1answer
223 views
How to estimate condensation from air?
How to estimate the amount of water condensing from air on a surface, given the air's temperature and relative humidity and how they change over time, the surface temperature, material's thermal ...
4
votes
0answers
55 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 ...
4
votes
0answers
132 views
How do I measure the temperature of a tiny water droplet?
How do I accurately (+/- 0.1 degrees Celsius or better) measure the temperature of a small (5 to 50 microliter) water droplet without noticeably affecting its temperature?
The mass of a thermistor or ...
3
votes
5answers
1k views
Will adding heat to a material increase or decrease entropy?
Does adding heat to a material, thereby increasing electrical resistance in the material increase or decrease entropy?
Follow up questions:
Is there a situation were Heat flux ie. thermal flux, ...
3
votes
3answers
807 views
Does radiation change entropy?
Are there entropy changes associated with the transmission of energy from the sun to the earth? Does radiation differ from other modes of heat transfer with respect to the entropy changes?
Are the ...
3
votes
1answer
149 views
Hawking Radiation: how does a particle ever cross the event horizon?
The heuristic argument for Hawking Radiation is, that a virtual pair-production happens just at the event horizon. One particle goes into the black hole, while the other can be observed as radiation.
...
3
votes
2answers
293 views
How to interpret Stefan-Boltzmann's law?
The Stefan-Boltzmann equation states $e=\sigma T^4$, but how do we interpret this?
Is this completely wrong: A body of size $s^2$ generates the radiation/temperature $T^4$ for a given size and a ...
3
votes
5answers
164 views
Will a warm body naturally slowdown?
Suppose a warm body moving in an empty space with high speed.
The body emits radiation based on its temperature. The protons emitted forwards of the body will have higher energy due to Doppler shift ...
3
votes
1answer
244 views
Will this poor-man's single band pyrometer work?
One of the unsolved problems I still have is measuring temperature in furnace at 700-1100C range (I need some +-0.1% repeatability and +-1% precision).
Obviously, contact thermometers are quite rare ...
3
votes
2answers
255 views
How might a resonant antenna and black body radiation interact?
How does an antenna behave when it is cooled so that its black-body radiation is emitting energy at its resonant frequency?
Edit: To clarify, its not how they're related in general, but how might ...
3
votes
1answer
572 views
How do we see different colours?
Why do different wave lengths cause electrons to behave(?) differently, causing us to see different colors?
What is happening at the quantum level which causes the colour black to absorb all of the ...
3
votes
0answers
42 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 $\ ...
2
votes
2answers
246 views
Why does Planck's law for black body radiation have that bell-like shape?
I'm trying to understand Planck's law for the black body radiation, and it states that a black body at a certain temperature will have a maximum intensity for the emission at a certain wavelength, and ...
2
votes
3answers
199 views
Why aren't the graphs for black body radiation straight lines?
We know that a wave which has greater frequency will have low wavelength and high energy. So, by decreasing the wavelength, the frequency and consequently energy (intensity) of that wave will increase ...
2
votes
1answer
2k views
Planck's Law in terms of wavelength
I am drawing a blank when it comes to equation transformation. Wikipedia gives two equations for the spectral radiance of black body:
First as a function of frequency $\nu$: $$I(\nu, T) = \frac{2 h ...
2
votes
2answers
245 views
Are Newton's law of Cooling and Stefan's law related?
Many of Indian school textbooks claim a proof of Newton's law of cooling from Stefan's law of black-body radiation.
As far as I am aware of, Newton's law is based on cooling from convection currents ...
2
votes
3answers
459 views
How is Planck's law defined?
Now, I found three different definitions of Planck's law:
$$
P_1(\nu,T) = \frac{8 \pi}{c}\frac{h \nu^{3}}{c^2} \frac{1}{e^{h\nu/kT}-1}
$$
$$
P_2(\nu,T) = 2\frac{h \nu^{3}}{c^2} ...
2
votes
1answer
127 views
Find temperature of surface (Blackbody Radiation)
An astronomer is trying to estimate the surface temperature of a star with a radius of $5 \times 10^8\ m$ by modeling it as an ideal blackbody. The astronomer has measured the intensity of ...
2
votes
1answer
215 views
What really is Planck's constant and what are its origins?
In the physics texts I have read and other info online, they says Planck's constant is the quantum of action or that it is a constant of the ratio of the energy of a particle to its frequency. Im ...
2
votes
1answer
104 views
What is the diameter of the sun as a function of wavelength/frequency (around 10GHz)?
I have this vague recollection of being told that the diameter of the apparent surface of the sun is a function of what band you observe it in. I'm looking for a model of this that works for bands in ...
2
votes
2answers
621 views
How thermal imaging cameras work
As far as I see from wiki, 'consumer'-grade(non-cryogenic) thermal imaging cameras use microbolometer sensors to get integrated IR intensity over some 5-12um range. But I had an impression, that ...
2
votes
1answer
99 views
Commercial Infrared lights
I purchased an infrared light. It's a 100 W Philips infrared lightbulb. Says it's infrared, but I haven't done any spectrum analysis so I don't know for sure if it's just red or really infrared.
As I ...
2
votes
2answers
284 views
Possible colors of fire?
I have learnt that depending on the various gases those are involved in the reaction that produces fire, different colors (yellow, red or blue) of flames become visible.
I have a question .. what are ...
2
votes
4answers
880 views
Which is more efficient, heating water in microwave or electric stove?
So our propane tank in the kitchen ran out again today.
Which is more energy efficient, boiling water in a microwave on an electric stove? All things being equal i.e. starting temperature and mass ...
2
votes
0answers
141 views
Classical blackbody radiation 'solution'
I never understood how the equipartition theorem was applied electromagnetic waves inside the metallic blackbody. As hyperphysics puts it ...
2
votes
0answers
42 views
A problem concerning the change of temperature and spectrum of a filament
http://www.flickr.com/photos/83373487@N04/7636968446/in/photostream
The spectrum of a filament is given before, the left one have the lowest tempreture, the middle has the medium and the right one ...
