Helmholtz energy function

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Hermann Ludwig Ferdinand von Helmholtz

Definition of A (for arbeit):

\left.A\right.=U-TS

where U is the internal energy, T is the temperature and S is the entropy. (TS) is a conjugate pair. The differential of this function is

\left.dA\right.=dU-TdS-SdT

From the second law of thermodynamics one obtains

\left.dA\right.=TdS -pdV -TdS-SdT

thus one arrives at

\left.dA\right.=-pdV-SdT.

For A(T,V) one has the following total differential

dA=\left(\frac{\partial A}{\partial T}\right)_V dT + \left(\frac{\partial A}{\partial V}\right)_T dV

The following equation provides a link between classical thermodynamics and statistical mechanics:

\left.A\right.=-k_B T \ln Q_{NVT}

where kB is the Boltzmann constant, T is the temperature, and QNVT is the canonical ensemble partition function.

[edit] See also

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