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Hi, I have some questions about the derivation of the Boltzmann distribution, for instance as in Mandl's "statistical physics".
Put a system (1) in a heatbath (2) with temperature T. In thermal equilibrium system 1 will also then have temperature T. The energy of system (1) is not fixed due to heat exchange, so we say that the energy of (1) lies in the interval [itex][E,E+\delta E][/itex]. My first question is: how can we fix the temperature with the heatbath but introduce a small deviation in the energy? Shouldn't we also say something that the temperature of (1) lies in the interval [itex] [T, T+ \delta T][/itex]? Or are the effects of different order?
Now we label the microstates of (1) as {1,2,...,r,...} with corresponding energies [itex]{E_1,E_2,...,E_r,...}[/itex] and assume that [itex]E_1 \leq E_2 \leq ... \leq E_r \leq ... [/itex]. The interval [itex]\delta E [/itex] is smaller than the minimum spacing between these [itex]E_r [/itex]'s. Now comes the thing that confuses me:
"The probability [itex]p_r [/itex] that system (1) will be in a definite (micro)state r with energy [itex]E_r [/itex] will be proportional to the number of states of the heat bath compatible with this, given that the total energy has a constant value [itex]E_0 [/itex]. These heatbathstates must have an energy lying in the interval [itex][E_0-E_r,E_0-E_r + \delta E] [/itex] (due to the fact that [itex]\delta E [/itex] is smaller than these minimum spacings). There are [itex]\Omega_2(E_0 - E_r) [/itex] such states, SO THAT
[tex]
p_r = const.\Omega_2(E_0 - E_r)
[/tex]
My question is: why not
[tex]
p_r = const. \Omega_1(E_r)\Omega_2(E_0 - E_r)
[/tex]
?
It has to do something with the fact that [itex]E_r [/itex] is the variable here and that we have to choose one of the energies (of system 1, OR system 2) as our independent variable (so fixing one energy fixes the other), but I can't reason why my formula would be wrong. And the resulting distribution depends on it (you get also the entropy of system 1 exponentiated in the distribution). If we fix [itex]E_0 - E_r [/itex] we also fix [itex]E_r [/itex] so that makes [itex]\Omega_1(E_r) [/itex] constant, but that shouldn't allow us to drop it from the derivation, right?
So why is my proposal wrong, and how can we fix T but allow for an uncertainy in E? :)
Put a system (1) in a heatbath (2) with temperature T. In thermal equilibrium system 1 will also then have temperature T. The energy of system (1) is not fixed due to heat exchange, so we say that the energy of (1) lies in the interval [itex][E,E+\delta E][/itex]. My first question is: how can we fix the temperature with the heatbath but introduce a small deviation in the energy? Shouldn't we also say something that the temperature of (1) lies in the interval [itex] [T, T+ \delta T][/itex]? Or are the effects of different order?
Now we label the microstates of (1) as {1,2,...,r,...} with corresponding energies [itex]{E_1,E_2,...,E_r,...}[/itex] and assume that [itex]E_1 \leq E_2 \leq ... \leq E_r \leq ... [/itex]. The interval [itex]\delta E [/itex] is smaller than the minimum spacing between these [itex]E_r [/itex]'s. Now comes the thing that confuses me:
"The probability [itex]p_r [/itex] that system (1) will be in a definite (micro)state r with energy [itex]E_r [/itex] will be proportional to the number of states of the heat bath compatible with this, given that the total energy has a constant value [itex]E_0 [/itex]. These heatbathstates must have an energy lying in the interval [itex][E_0-E_r,E_0-E_r + \delta E] [/itex] (due to the fact that [itex]\delta E [/itex] is smaller than these minimum spacings). There are [itex]\Omega_2(E_0 - E_r) [/itex] such states, SO THAT
[tex]
p_r = const.\Omega_2(E_0 - E_r)
[/tex]
My question is: why not
[tex]
p_r = const. \Omega_1(E_r)\Omega_2(E_0 - E_r)
[/tex]
?
It has to do something with the fact that [itex]E_r [/itex] is the variable here and that we have to choose one of the energies (of system 1, OR system 2) as our independent variable (so fixing one energy fixes the other), but I can't reason why my formula would be wrong. And the resulting distribution depends on it (you get also the entropy of system 1 exponentiated in the distribution). If we fix [itex]E_0 - E_r [/itex] we also fix [itex]E_r [/itex] so that makes [itex]\Omega_1(E_r) [/itex] constant, but that shouldn't allow us to drop it from the derivation, right?
So why is my proposal wrong, and how can we fix T but allow for an uncertainy in E? :)
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