- #1
RGann
- 12
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It is perfectly possible to derive, for instance, the Maxwell distribution of speeds in a heuristic way with only two things:
1. The Gibbs hypothesis, which says that to count the number of states at speed v you take all the points in phase space between v and v+dv in velocity space. This makes the "number of ways", g, proportional to v2.
2. The Boltzmann factor
Then you can write the distribution of speeds by [tex] P(v) = A g(E) P(E) = A v^2 e^{-m v^2/2 k T},[/tex] and normalize the distribution. The idea, of course, is that there are more ways to have a given speed v than a smaller one, but the probability goes down from the Boltzmann factor, the competition of which gives the MB distribution. This is not rigorous, but it's intuitively satisfying.
I wonder whether the same is not possible for the blackbody spectrum (Planck's Law). The typical proof given depends on the body being in a cavity, so that the modes can be counted. This then allows for the calculation of the properties of a photon gas in equilibrium with the object, so that when you poke a hole in the cavity, the photon gas leaks out and you see the spectrum.
Needless to say, I find this proof distasteful for conceptual reasons. The object radiating is not in a cavity, and I find it hard to believe that this construct is the only way of doing it. Moreover, I don't think that students have a hard time with discrete energy levels of an atom. So I wonder if an argument couldn't be constructed, with a toy model, as follows:
In a substance (of any phase), where the atoms are in equilibrium, the way the equilibrium is mediated (e.g. phonons) can give rise to promotion of electrons in the atoms to higher energy levels. Presuming that the outer electrons are in equilibrium with the substance, the average level they can be promoted to is indicated by the Boltzmann factor, e^{-E/k T} where the energy is given by n h f. Then, the average energy of the excited atom would be
[tex]E_\text{avg.} = \frac{h f}{e^{h f/ k T}-1}. [/tex] (This can be computed by finding the partition function, which is a geometric series, and either differentiating -ln Z with respect to beta, or finding the probability by (1/Z) times the sum of E e^{-E/kT}.) These electrons then decay to the ground state giving rise to a spectrum of photons.
You can see on http://en.wikipedia.org/wiki/Planck%27s_law" , where they reach the same. However, the density of states, g, is now needed to compute the internal energy. I'm having some kind of mental disconnect. Without use of the cavity, how does one say what the density of states is? Is it possible to state it in terms of the components of the wave vector?
The ultimate answer is the intensity (energy radiated per time per area), which is
[tex]I(f,T) = \frac{2 h f^3 }{c^2} \frac{1}{e^{h f/kT}-1}.[/tex]
I have scoured the web, Am. J. Phys, Found. Phys., and not found any treatment that gives one a feel for this problem. Any ideas?
1. The Gibbs hypothesis, which says that to count the number of states at speed v you take all the points in phase space between v and v+dv in velocity space. This makes the "number of ways", g, proportional to v2.
2. The Boltzmann factor
Then you can write the distribution of speeds by [tex] P(v) = A g(E) P(E) = A v^2 e^{-m v^2/2 k T},[/tex] and normalize the distribution. The idea, of course, is that there are more ways to have a given speed v than a smaller one, but the probability goes down from the Boltzmann factor, the competition of which gives the MB distribution. This is not rigorous, but it's intuitively satisfying.
I wonder whether the same is not possible for the blackbody spectrum (Planck's Law). The typical proof given depends on the body being in a cavity, so that the modes can be counted. This then allows for the calculation of the properties of a photon gas in equilibrium with the object, so that when you poke a hole in the cavity, the photon gas leaks out and you see the spectrum.
Needless to say, I find this proof distasteful for conceptual reasons. The object radiating is not in a cavity, and I find it hard to believe that this construct is the only way of doing it. Moreover, I don't think that students have a hard time with discrete energy levels of an atom. So I wonder if an argument couldn't be constructed, with a toy model, as follows:
In a substance (of any phase), where the atoms are in equilibrium, the way the equilibrium is mediated (e.g. phonons) can give rise to promotion of electrons in the atoms to higher energy levels. Presuming that the outer electrons are in equilibrium with the substance, the average level they can be promoted to is indicated by the Boltzmann factor, e^{-E/k T} where the energy is given by n h f. Then, the average energy of the excited atom would be
[tex]E_\text{avg.} = \frac{h f}{e^{h f/ k T}-1}. [/tex] (This can be computed by finding the partition function, which is a geometric series, and either differentiating -ln Z with respect to beta, or finding the probability by (1/Z) times the sum of E e^{-E/kT}.) These electrons then decay to the ground state giving rise to a spectrum of photons.
You can see on http://en.wikipedia.org/wiki/Planck%27s_law" , where they reach the same. However, the density of states, g, is now needed to compute the internal energy. I'm having some kind of mental disconnect. Without use of the cavity, how does one say what the density of states is? Is it possible to state it in terms of the components of the wave vector?
The ultimate answer is the intensity (energy radiated per time per area), which is
[tex]I(f,T) = \frac{2 h f^3 }{c^2} \frac{1}{e^{h f/kT}-1}.[/tex]
I have scoured the web, Am. J. Phys, Found. Phys., and not found any treatment that gives one a feel for this problem. Any ideas?
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