- #1
jostpuur
- 2,116
- 19
How to describe a harmonic oscillator defined by
[tex]
H(q,p) = \frac{p^2}{2m} + \frac{1}{2}kq^2
[/tex]
in a heat bath with some fixed temperature [itex]T[/itex]?
I suppose this question alone is not quite well defined, because it mixes classical and statistical mechanics in confusing manner, but I thought that one could make the question more rigor by assuming, that the oscillator frequency is notably larger than the frequency of instants when the oscillator interacts with the heat bath. In this case we could identify the ellipse trajectories [itex]q^2 + \frac{1}{mk}p^2 = r^2[/itex] with the indexes of the energy states used in statistical treatment. Right? The trajectories, on the other hand, can be identified with the radius [itex]r\in [0,\infty[[/itex]. The energy corresponding to each radius is [itex]E(r) = \frac{1}{2}kr^2[/itex].
The problem is that if we set the Boltzmann probability measure to be proportional to
[tex]
\exp\Big(-\frac{kr^2}{2k_{\textrm{B}} T}\Big) dr,
[/tex]
it would not give the correct energy distribution. This is, because this formula doesn't correctly take into account the density of the ellipse trajectories. The correct probability measure should be something like this:
[tex]
\exp\Big(-\frac{kr^2}{2k_{\textrm{B}} T}\Big) \rho(r) dr,
[/tex]
but I've been unable to figure out what [itex]\rho(r)[/itex] should be.
I thought that this could be a good postulate to start with: When the oscillator interacts with the heat bath, the position [itex]q[/itex] remains unchanged, but the momentum [itex]p[/itex] becomes thrown to some arbitrary new value, so that the probability of the new momentum would not be weighted by any particular density function. This would be a model of a collision with some particle from the heat bath. So if we assume that the new momentum has a probability measure proportional to [itex]dp[/itex] (this is not normalizable really, but it shouldn't be a problem, because normalizable factors arise later), it should be possible to solve what measure [itex]\rho(r)dr[/itex] follows for the trajectories.
Unfortunately I found this task too difficult. Anyone having any comments to this? Did I start into wrong direction with this problem, or can the [itex]\rho(r)[/itex] be solved from what I started?
[tex]
H(q,p) = \frac{p^2}{2m} + \frac{1}{2}kq^2
[/tex]
in a heat bath with some fixed temperature [itex]T[/itex]?
I suppose this question alone is not quite well defined, because it mixes classical and statistical mechanics in confusing manner, but I thought that one could make the question more rigor by assuming, that the oscillator frequency is notably larger than the frequency of instants when the oscillator interacts with the heat bath. In this case we could identify the ellipse trajectories [itex]q^2 + \frac{1}{mk}p^2 = r^2[/itex] with the indexes of the energy states used in statistical treatment. Right? The trajectories, on the other hand, can be identified with the radius [itex]r\in [0,\infty[[/itex]. The energy corresponding to each radius is [itex]E(r) = \frac{1}{2}kr^2[/itex].
The problem is that if we set the Boltzmann probability measure to be proportional to
[tex]
\exp\Big(-\frac{kr^2}{2k_{\textrm{B}} T}\Big) dr,
[/tex]
it would not give the correct energy distribution. This is, because this formula doesn't correctly take into account the density of the ellipse trajectories. The correct probability measure should be something like this:
[tex]
\exp\Big(-\frac{kr^2}{2k_{\textrm{B}} T}\Big) \rho(r) dr,
[/tex]
but I've been unable to figure out what [itex]\rho(r)[/itex] should be.
I thought that this could be a good postulate to start with: When the oscillator interacts with the heat bath, the position [itex]q[/itex] remains unchanged, but the momentum [itex]p[/itex] becomes thrown to some arbitrary new value, so that the probability of the new momentum would not be weighted by any particular density function. This would be a model of a collision with some particle from the heat bath. So if we assume that the new momentum has a probability measure proportional to [itex]dp[/itex] (this is not normalizable really, but it shouldn't be a problem, because normalizable factors arise later), it should be possible to solve what measure [itex]\rho(r)dr[/itex] follows for the trajectories.
Unfortunately I found this task too difficult. Anyone having any comments to this? Did I start into wrong direction with this problem, or can the [itex]\rho(r)[/itex] be solved from what I started?
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