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In thermodynamics, the fundamental thermodynamic relation is generally expressed as an infinitesimal change in internal energy in terms of infinitesimal changes in entropy, and volume for a closed system in thermal equilibrium in the following way. : Here, ''U'' is internal energy, ''T'' is absolute temperature, ''S'' is entropy, ''P'' is pressure, and ''V'' is volume. This is only one expression of the fundamental thermodynamic relation. It may be expressed in other ways, using different variables (e.g. using thermodynamic potentials). For example, the fundamental relation may be expressed in terms of the Helmholtz free energy (''F'') as: : and in terms of the enthalpy as : == Derivation from the first and second laws of thermodynamics== The first law of thermodynamics states that: : where and are infinitesimal amounts of heat supplied to the system by its surroundings and work done by the system on its surroundings, respectively. According to the second law of thermodynamics we have for a reversible process: : Hence: : By substituting this into the first law, we have: : Letting dW be reversible pressure-volume work, we have: : This equation has been derived in the case of reversible changes. However, since , , and are thermodynamic functions of state, the above relation holds also for non-reversible changes. If the system has more external parameters than just the volume that can change and if the numbers of particles in the system can also change, the fundamental thermodynamic relation generalizes to: : Here the are the generalized forces corresponding to the external parameters . The are the chemical potentials corresponding to particles of type . 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「fundamental thermodynamic relation」の詳細全文を読む スポンサード リンク
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