The photon discussion a few weeks back got me reading about cavity radiation. I'm puzzled. Perhaps somebody can point me in the right direction. For those who don't already know, cavity radiation is a surprising phenomenon which required quantum theory to model. Metals radiate energy in the form of light. Each metal has a characteristic "radiance" for each temperature, the amount of energy it radiates. If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!) My book suggests that this is intuitively sound because if cavity radiation had different values, a violation of the second law of thermodynamics would occur. Set up two cavity radiators of a metal X and a different metal Y like so: XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXX XXX YYY YYYYYYYYYYYYYY XXXXXXXXXXXX XX YY YYYYYYYYYYYY XXXXXXXXXXX YYYYYYYYYYY XXXXXXXXXXXX XX YY YYYYYYYYYYYY XXXXXXXXXXXXXX XXX YYY YYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY If X and Y have the same temperature and Y has a higher level of cavity radiation, then X will get hotter and Y will get cooler. In other words, you would have a perpetual motion machine. So the radiance of each cavity must be the same. So far so good, but this argument should also apply to surface radiance, which we know to be different: XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY If Y has a higher level of surface radiance than X, then one would expect X to grow hotter, also making possible a perpetual motion machine. Clearly, something went wrong somewhere. Can anybody clue me in?
One of the ways to model a 'black body' radiator is using this technique. The point being that the energy of the photons emitted through the hole have a statistical energy distribution that matches (at least close enough for experiments) the radiation emission of a black body at a characteristic temperature. On Fri, 17 Aug 2001, Anonymous wrote:
The photon discussion a few weeks back got me reading about cavity radiation. I'm puzzled. Perhaps somebody can point me in the right direction.
For those who don't already know, cavity radiation is a surprising phenomenon which required quantum theory to model.
Actually statistical mechanics, not quite the same thing as quantum theory. But you can't talk about QT w/o SM.
Metals radiate energy in the form of light. Each metal has a characteristic "radiance" for each temperature, the amount of energy it radiates.
That should probably be the amount of energy it radiates at particular frequencies.
If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!)
My book suggests that this is intuitively sound because if cavity radiation had different values, a violation of the second law of thermodynamics would occur.
Set up two cavity radiators of a metal X and a different metal Y like so:
XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXX XXX YYY YYYYYYYYYYYYYY XXXXXXXXXXXX XX YY YYYYYYYYYYYY XXXXXXXXXXX YYYYYYYYYYY XXXXXXXXXXXX XX YY YYYYYYYYYYYY XXXXXXXXXXXXXX XXX YYY YYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
If X and Y have the same temperature and Y has a higher level of cavity radiation, then X will get hotter and Y will get cooler. In other words, you would have a perpetual motion machine. So the radiance of each cavity must be the same.
Actually not, as Y cools its emissions drop (looking at your example in reverse). At some point X becomes hotter than Y and the net travel of photons goes the other way. It's worth remembering that what is actually happening is that both bodies are emitting radiation continouly, only the statistical 'average' of the results model this 'one way' flow. Eventually they'll reach a point where the average energy emitted by one, and absorbed by the other, equals. From that point both objects will emit radiation into their surroundings and both will cool until that exchange balances.
So far so good, but this argument should also apply to surface radiance, which we know to be different:
XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
If Y has a higher level of surface radiance than X, then one would expect X to grow hotter, also making possible a perpetual motion machine.
Clearly, something went wrong somewhere. Can anybody clue me in?
The flat radiation of the surface isn't the same as with the hollow spheres. For the hollow spheres the greatest part of the radiation is re-absorbed by the relevant body (and re-emitted, and re-absorbed, ...). This re-adsorption is what leads to the black body behaviour. It acts as an averaging mechanism. The plates just emit into space and the majority of radiation is lost, not reabsorbed. -- ____________________________________________________________________ natsugusa ya...tsuwamonodomo ga...yume no ato summer grass...those mighty warriors'...dream-tracks Matsuo Basho The Armadillo Group ,::////;::-. James Choate Austin, Tx /:'///// ``::>/|/ ravage@ssz.com www.ssz.com .', |||| `/( e\ 512-451-7087 -====~~mm-'`-```-mm --'- --------------------------------------------------------------------
Quoting Anonymous (nobody@paranoici.org):
If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!)
This is actually a phenomenal result. If one makes a large enough apparatus so that the cavity is very, very large with a correspondingly small port is drilled, you've then got the makings of a great beam weapon. However, aiming a multi-cubic parsec block of metal may limit effectiveness somewhat. Regards, Steve -- ``If religion were nothing but an illusion and a sham, there could be no philosophy of it. The study of it would belong to abnormal psychology.... Religion cannot afford to claim exemption from philosophical enquiry. If it attempts to do so on the grounds of sanctity, it can only draw upon itself suspicion that it is afraid to face the music.'' -- H. J. Paton, "The Modern Predicament"
On Thu, 16 Aug 2001, Steve Thompson wrote:
Quoting Anonymous (nobody@paranoici.org):
If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!)
This is actually a phenomenal result. If one makes a large enough apparatus so that the cavity is very, very large with a correspondingly small port is drilled, you've then got the makings of a great beam weapon.
You're missing the parameter where the walls have to be kept at a temperature T. This implies that your environment around the ball is controlled. In a lab environment you'd wrap the cavity in some sort of heater blanket. To help you get a better grasp of the scale of the effect, consider the odds of a single photon entering the cavity port from the outside. What is the odds that that photon will be re-emitted? Almost nill. It effectively absorbs radiation at all frequencies (hence the 'black' term). Now, consider the inverse, what is the odd that a photon of a given wavelength will be emitted? The odds that a perfect black body absorber would radiate a photon from inside the cavity to the outside environment is also low. So, to make a black body radiator into a beam weapon implies so much energy keeping the cavity at the temperature required that it would be simpler to use that energy directly instead of trying to create a high energy photon stream from a very(!) inefficient emission process. The Dynamics of Heat Fuchs ISBN 0-387-94603-9 -- ____________________________________________________________________ natsugusa ya...tsuwamonodomo ga...yume no ato summer grass...those mighty warriors'...dream-tracks Matsuo Basho The Armadillo Group ,::////;::-. James Choate Austin, Tx /:'///// ``::>/|/ ravage@ssz.com www.ssz.com .', |||| `/( e\ 512-451-7087 -====~~mm-'`-```-mm --'- --------------------------------------------------------------------
On Thursday, August 16, 2001, at 04:54 PM, Steve Thompson wrote:
Quoting Anonymous (nobody@paranoici.org):
If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!)
This is actually a phenomenal result. If one makes a large enough apparatus so that the cavity is very, very large with a correspondingly small port is drilled, you've then got the makings of a great beam weapon.
No you don't. Read up on the Stefan-Boltzmann laws and figure out what frequencies (energies) a black body radiator emits. Get real. --Tim May
Quoting Tim May (tcmay@got.net):
Get real.
You think I'm not being real? Puhleeze. Regards, Steve -- ``If religion were nothing but an illusion and a sham, there could be no philosophy of it. The study of it would belong to abnormal psychology.... Religion cannot afford to claim exemption from philosophical enquiry. If it attempts to do so on the grounds of sanctity, it can only draw upon itself suspicion that it is afraid to face the music.'' -- H. J. Paton, "The Modern Predicament"
(The physics cited by Anonymous is hand-waving. I regret spending even 10 minutes composing this reply. First, the physics is naive. Second, this has nothing to do with any conceivable topic of interest. Third, I expect Choate will be jumping in with his Choate Prime version of physics.) On Thursday, August 16, 2001, at 05:03 PM, Anonymous wrote:
The photon discussion a few weeks back got me reading about cavity radiation. I'm puzzled. Perhaps somebody can point me in the right direction.
For those who don't already know, cavity radiation is a surprising phenomenon which required quantum theory to model. Metals radiate energy in the form of light. Each metal has a characteristic "radiance" for each temperature, the amount of energy it radiates.
Planck solved the black body radiation problem by figuring out that energy levels are quantized (heading off the "ultraviolet catastrophe," which we knew wasn't happening, but not why it wasn't).
If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!)
It is not correct to say the radiance is either higher or lower than some other material: the radiance approximates that of a perfect black body. These are not mystical objects. I used them in my lab a few decades ago. "Integrating spheres" is the lab supply store name. Looked at from another point of view, a hollowed-out sphere with a small hole. Light entering the sphere bounces around and is absorbed, reflected, bounced around, etc., until it "thermalizes." The integrating sphere thus "integrates" the light hitting the hole. (A more mundane example is the ordinary house with a glass window. Nearly all of the light passing through a house window is "thermalized.") Take the above physics and play it BACKWARDS IN TIME: the light exiting the hole in an integrating sphere is the result only of the TEMPERATURE of the sphere. The sphere behaves pretty much like a perfect black body radiator. Geometry is the key. Nothing mystical at all.
So far so good, but this argument should also apply to surface radiance, which we know to be different:
XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
If Y has a higher level of surface radiance than X, then one would expect X to grow hotter, also making possible a perpetual motion machine.
Clearly, something went wrong somewhere. Can anybody clue me in?
The "perpetual motion" part is a non sequitor. There are many cases where a material of higher radiance, e.g. the surface of the earth, is "looking at" (in the sense of the drawing above) a material or thing of lower radiance, e.g., deep space. And guess what: the earth radiates more energy toward deep space than deep space radiates toward the earth. This is one reason deserts get so cold so fast at night. Absent other sources of energy input (heating by the sun, geological energy, etc.), the earth would indeed eventually reach thermal equlibrium with deep space. So? Nothing surprising, and no perpetual motion. You need to spend about an hour reading up on some basic thermodynamics, especially about why all objects in a furnace (a good approximation to the cavity radiator) look to be the same "color" no matter their composition. Think in terms of the time-reversed model, with light going into a furnace and bouncing around, if it helps you to see the intuitive reason for the equilibrium solution. Why would someone ask for help on a physics problem using anonymous methods? I suspect a troll. --Tim May
At 10:38 PM 8/16/2001 -0700, Tim May wrote:
These are not mystical objects. I used them in my lab a few decades ago. "Integrating spheres" is the lab supply store name. Looked at from another point of view, a hollowed-out sphere with a small hole. Light entering the sphere bounces around and is absorbed, reflected, bounced around, etc., until it "thermalizes." The integrating sphere thus "integrates" the light hitting the hole.
One can perform some interesting experiments using an evacuated "integrating" sphere. Instead of a pinhead sized opening use something a bit larger with a quartz window. Suspend a small high temperature refractory ball in the center of the sphere (tungsten) and line the interior of the sphere with PV "cells" (e.g., overlapping of standard rectangular cells) overcoated to efficiently reflect frequencies above or below the band gap. Sunlight from a concentrator is focused on the sphere bringing it to incandescence. The combination of the ball and the reflective overcoat functions to "remix" the solar energy limiting the "leakage" of photons not in the bandpass from exiting the system. The vacuum prevents convective coupling of the ball and the cells although cooling the exterior of the sphere with a water jacket may be advised. System conversion efficiency can exceed 35%. steve
participants (5)
-
Anonymous -
Jim Choate -
Steve Schear -
Steve Thompson -
Tim May