- #1
Mikhail_MR
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Homework Statement
I must calculate chemical potential using the Boltzmann equation in relaxation time approximation $$f=f^0-\tau v_z^2 \partial f^0/\partial z,$$ where ##f^0## is given as
$$f^0 = 2(\frac{m}{2\pi\hbar})^3 \frac{1}{\exp{\beta(z)(\frac{mv^2}{2}-\mu(z))}+1}$$
I have to consider only heat current without electrical current. In this case, I can make use of ##<v_z>=0##.
Homework Equations
##0=<v_z>=\int d^3 v (v_z f^0 - \tau v_z^2 \partial f^0/\partial z)##. I can also use the limit ##\mu \beta \gg 1## and ##I_n=\int_{-\infty}^{+\infty} dx x^n \frac{e^x}{(e^x+1)^2}## with ##I_0=1, ~I_1=0, ~I_2=\pi^2 / 3##
The Attempt at a Solution
Because I have velocity only in ##z## direction, I can calculate the first integral
$$c_0 \int dv_z v_z \frac{1}{\exp{\beta (mv_z^2 / 2 - \mu)}+1} = c_0 \frac{\beta m v_z^2/2 - \ln(\exp{\beta m v_z^2 /2}+\exp{\beta \mu})}{\beta m}$$ Now I can make use of ##\beta \mu \gg 1## and I get $$c_0 (v_z^2/2-\mu/m)$$ with ##c_0 = 2(\frac{m}{2\pi\hbar})^3##
To calculate the second integral I need to calculate the derivative first.
$$\partial f^0/ \partial z = - e^x /(e^x+1)^2 dx(v, z)/dz$$ with ##x=\beta (m v^2 / 2 - \mu)##
$$\Rightarrow \partial f^0/ \partial z = - e^x /(e^x+1)^2 [\mu d\beta / dz - m v^2/2 \cdot d\beta / dz - \beta d\mu /dz]$$
Now I must somehow get the form of the integrals ##I_n##. But I do not see a way I can do it. Have I missed some step or is my attempt at a solution completely wrong?.
Any help would be appreciated