- #1
Piamedes
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I'm not sure if this is the proper location for this thread, but its for a math course and I think my issues concern the math portion of the problems. If it should be moved please do so.
Note: I know the post is quite long, so I'll just pull out my few main issues from all the junk.
1) What exactly are 'absolute probability' and 'total probability' from what I assume is a statistical mechanics standpoint.
2) How do you find the mean value a variables assumes over all allowed values weighted by their probablities?
Those I believe are my main two problems, but I'm not even entirely sure of that.
The Actual Question
There are actually two questions, one for a specific instance and the other for a more general occurrence. I've typed out both questions verbatim so anything that looks or sounds weird is just the wording of the question.
The Specific One: A magnetic moment [tex] \mu [/tex] in a magnetic field [tex] h [/tex] has energy [tex] E_{}\pm=\mp \mu h [/tex] when it is parallel (antiparallel) to the field. Its lowest energy state is when it is aligned with [tex] h [/tex]. However at any finite temperature, it has a nonzero probabilities for being parallel or antiparallel given by P(par)/ P(antipar) = exp[-E+/T] / exp[-E-/T] where T is the absolute temperature. Using the fact
that the total probability must add up to 1, evaluate the absolute probabilities for the two orientations. Using this show that the average magnetic moment along the field [tex] h [/tex] is [tex] m=\mu\tanh{\frac{\mu h}{T}} . [/tex]
Sketch this as a function of temperature at fixed [tex] h [/tex]. Notice that if [tex] h=0 [/tex], [tex] m [/tex] vanishes since the moment point up and down with equal probability. Thus [tex] h [/tex] is the cause of a nonzero [tex] m [/tex]. Calculate the susceptibility, [tex] \frac{dm}{dh} \right|_{h=0}[/tex] as a function of [tex] T [/tex].The General Case: Consider the previous problem in a more general light. According to the laws of statistical mechanics if a system can be in one of n states labeled by an index i, energies [tex] E_{i} [/tex], then at temperate T the system will be in state i with a relative probability [tex] p(i)=e^{-\beta E_{i}} [/tex] where Beta is one over T. Introduce the partition function [tex] Z= \sum_{i} e^{-\beta E_{i}} [/tex]. First write and expression for [tex] P(i) [/tex], the absolute probability (which must add up to one). next write a formula for <V>, the mean value of a variable V that takes the value [tex] V_{i} [/tex] in the state i. i.e., <V> is the average over all allowed values, duly weighted by probabilities. Show that <E> [tex] = -\frac{d ln{Z}}{d \beta}} [/tex]. Give an explicit Formula for Z for the specific case described above. Show that [tex] -\frac{d ln{Z}}{d \beta}} [/tex] gives the mean moment along h. Use the formula for Z, evaluate this derivative and verify that it agrees with the result you got in the last problem. My Attempts to solve them
Yes I realize that is a very long set of questions, but I typed both of these problems because I've only made a little progress by working them together. I don't know if that is the best approach, but here's what I so far.
By reading the second problem I realized that the weird proportion in the first one relating the probabilities of being parallel and antiparallel wasn't actually a proportion; it was just a very confusing way of stating:
[tex] P_{par} = e^{\mu h} [/tex] and
[tex] P_{anti} = e^{-\mu h} [/tex] .
Now is where I first encountered problems. I am completely uncertain what the question means by 'absolute probabilities', and google unfortunately has been no help. I at first assume that the sum of those two equations summed to one, but that didn't really accomplish anything.
I then tried to work backwards, starting with the hyperbolic tangent function. I did the last part of the first problem, the simple derivative:
[tex] m=\mu\tanh{\frac{\mu h}{T}} [/tex]
[tex] \frac{dm}{dh}=\frac{\mu^2}{T} \text{sech}^2 {\frac{\mu h}{T}} [/tex]
At h=0 the hyperbolic secant term equals one, so the final equation is just:
[tex] \frac{dm}{dh}=\frac{\mu^2}{T} [/tex].
I then tried manipulating any of the exponential equations into some sort of hyperbolic tangent form and couldn't do so.I then moved onto the second problem and did a little research into statistical mechanics. The wikipedia page on the Partition Function looked quite appealing until I realized it still didn't answer my main problems.
So if anyone could please answer my original two questions at the top of the post I would be most thankful and then I could move onto doing the math portion of this homework instead of fumbling around blindly.
Note: I know the post is quite long, so I'll just pull out my few main issues from all the junk.
1) What exactly are 'absolute probability' and 'total probability' from what I assume is a statistical mechanics standpoint.
2) How do you find the mean value a variables assumes over all allowed values weighted by their probablities?
Those I believe are my main two problems, but I'm not even entirely sure of that.
The Actual Question
There are actually two questions, one for a specific instance and the other for a more general occurrence. I've typed out both questions verbatim so anything that looks or sounds weird is just the wording of the question.
The Specific One: A magnetic moment [tex] \mu [/tex] in a magnetic field [tex] h [/tex] has energy [tex] E_{}\pm=\mp \mu h [/tex] when it is parallel (antiparallel) to the field. Its lowest energy state is when it is aligned with [tex] h [/tex]. However at any finite temperature, it has a nonzero probabilities for being parallel or antiparallel given by P(par)/ P(antipar) = exp[-E+/T] / exp[-E-/T] where T is the absolute temperature. Using the fact
that the total probability must add up to 1, evaluate the absolute probabilities for the two orientations. Using this show that the average magnetic moment along the field [tex] h [/tex] is [tex] m=\mu\tanh{\frac{\mu h}{T}} . [/tex]
Sketch this as a function of temperature at fixed [tex] h [/tex]. Notice that if [tex] h=0 [/tex], [tex] m [/tex] vanishes since the moment point up and down with equal probability. Thus [tex] h [/tex] is the cause of a nonzero [tex] m [/tex]. Calculate the susceptibility, [tex] \frac{dm}{dh} \right|_{h=0}[/tex] as a function of [tex] T [/tex].The General Case: Consider the previous problem in a more general light. According to the laws of statistical mechanics if a system can be in one of n states labeled by an index i, energies [tex] E_{i} [/tex], then at temperate T the system will be in state i with a relative probability [tex] p(i)=e^{-\beta E_{i}} [/tex] where Beta is one over T. Introduce the partition function [tex] Z= \sum_{i} e^{-\beta E_{i}} [/tex]. First write and expression for [tex] P(i) [/tex], the absolute probability (which must add up to one). next write a formula for <V>, the mean value of a variable V that takes the value [tex] V_{i} [/tex] in the state i. i.e., <V> is the average over all allowed values, duly weighted by probabilities. Show that <E> [tex] = -\frac{d ln{Z}}{d \beta}} [/tex]. Give an explicit Formula for Z for the specific case described above. Show that [tex] -\frac{d ln{Z}}{d \beta}} [/tex] gives the mean moment along h. Use the formula for Z, evaluate this derivative and verify that it agrees with the result you got in the last problem. My Attempts to solve them
Yes I realize that is a very long set of questions, but I typed both of these problems because I've only made a little progress by working them together. I don't know if that is the best approach, but here's what I so far.
By reading the second problem I realized that the weird proportion in the first one relating the probabilities of being parallel and antiparallel wasn't actually a proportion; it was just a very confusing way of stating:
[tex] P_{par} = e^{\mu h} [/tex] and
[tex] P_{anti} = e^{-\mu h} [/tex] .
Now is where I first encountered problems. I am completely uncertain what the question means by 'absolute probabilities', and google unfortunately has been no help. I at first assume that the sum of those two equations summed to one, but that didn't really accomplish anything.
I then tried to work backwards, starting with the hyperbolic tangent function. I did the last part of the first problem, the simple derivative:
[tex] m=\mu\tanh{\frac{\mu h}{T}} [/tex]
[tex] \frac{dm}{dh}=\frac{\mu^2}{T} \text{sech}^2 {\frac{\mu h}{T}} [/tex]
At h=0 the hyperbolic secant term equals one, so the final equation is just:
[tex] \frac{dm}{dh}=\frac{\mu^2}{T} [/tex].
I then tried manipulating any of the exponential equations into some sort of hyperbolic tangent form and couldn't do so.I then moved onto the second problem and did a little research into statistical mechanics. The wikipedia page on the Partition Function looked quite appealing until I realized it still didn't answer my main problems.
So if anyone could please answer my original two questions at the top of the post I would be most thankful and then I could move onto doing the math portion of this homework instead of fumbling around blindly.