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arXiv 1302.4903cond-mat.stat-mech
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长程相互作用系统的局域热力学和广义Gibbs-Duhem方程

Local thermodynamics and the generalized Gibbs-Duhem equation in systems with long-range interactions

Ivan Latella, Agustín Pérez-Madrid

摘要:

利用平均场近似研究了具有长程相互作用的d维系统的定域热力学。长程相互作用是通过对相互作用势引入的,对相互作用势在粒子间距离中按幂律衰减。我们分别计算了微正则、正则和巨正则系综中每个粒子的局部熵、亥姆霍兹自由能和巨势。由粒子的局域熵,利用局域热力学平衡条件,得到系统的局域状态方程。该局部状态方程具有理想气体状态方程的形式,但密度取决于表征长程相互作用的势。通过对局部热力学势之间关系的体积积分,得到了全局热力学势所满足的相应方程。结果表明,势能进入作为一个热力学变量,修改全球热力学势。因此,我们找到了一个广义吉布斯-迪昂方程,该方程将势能与温度、压力和化学势联系起来。对于衰变相互作用势的功率等于空间维数的边缘情形,恢复了通常的Gibbs-Duhem方程。作为应用这个方程的例子,我们考虑空间均匀相互作用势和自引力气体。我们还指出了与小系统热力学的密切关系。

英文摘要:

The local thermodynamics of a system with long-range interactions in d dimensions is studied using the mean-field approximation. Long-range interactions are introduced through pair interaction potentials that decay as a power law in the interparticle distance. We compute the local entropy, Helmholtz free energy, and grand potential per particle in the microcanonical, canonical, and grand canonical ensembles, respectively. From the local entropy per particle we obtain the local equation of state of the system by using the condition of local thermodynamic equilibrium. This local equation of state has the form of the ideal gas equation of state, but with the density depending on the potential characterizing long-range interactions. By volume integration of the relation between the different thermodynamic potentials at the local level, we find the corresponding equation satisfied by the potentials at the global level. It is shown that the potential energy enters as a thermodynamic variable that modifies the global thermodynamic potentials. As a result, we find a generalized Gibbs-Duhem equation that relates the potential energy to the temperature, pressure, and chemical potential. For the marginal case where the power of the decaying interaction potential is equal to the dimension of the space, the usual Gibbs-Duhem equation is recovered. As examples of the application of this equation, we consider spatially uniform interaction potentials and the self-gravitating gas. We also point out a close relationship with the thermodynamics of small systems.

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