Why are the equations for mean energy in the canonical ensemble equal?

[Abigale]

Hi,
I regard an equation for an canonical ensemble.
I do not understand why both equations should be equal.

\bar{E}= -\frac{ \partial \ln{(Z)}}{ \partial \beta} \overset{\text{?}} = k T ^{2 } \frac{\partial \ln{(Z)}}{\partial T}

Z is a canonical partition function.
\beta = \frac{1}{kT}.
Thx
Abby

 

[Jorriss]

Try the chain rule.

 

[vanhees71]

Do you under stand the first equation? That simply comes from the definition of the partition sum
Z=\mathrm{Tr} \exp(-\beta \hat{H}),
where \hat{H} is the Hamiltnian of the system. The Canonical Statistical Operator is
\hat{R}=\frac{1}{Z} \exp(-\beta \hat{H}).
The mean energy is
\overline{E}=\mathrm{Tr}(\hat{R} \hat{H})=-\frac{1}{Z} \frac{\partial Z}{\partial \beta}=-\frac{\partial \ln Z}{\partial \beta}.
As it turns out by comparing with the first Law of thermodynamics, you have
\beta=\frac{1}{k T},
where T is the temperature (contrary what you've written in your posting!).

Then of course you have for any function of the temperature
\frac{\partial f}{\partial \beta}=\frac{\partial f}{\partial T} \frac{\partial T}{\partial \beta}.
From the above formula you find T=1/(k \beta) and thus
\frac{\partial T}{\partial \beta}=-\frac{1}{k \beta^2}=-k T^2.
This gives
\overline{E}=k T^2\frac{\partial \ln Z}{\partial T}.