Electron gas, classical fluid and liquid helium

Three fundamental subjects of many-body theory of condensed matter: electron gas, classical fluid and liquid helium are discussed. The general treatment of gas-liquid condensation in classical statistical mechanics is presented. It is concluded that the van der Waals equation has been obtained in the long-range limit of the attractive potential for arbitrary dimensions. With the help of the Maxwell construction it is possible to obtain a stable phase separation curve of the distribution function. The case of two-dimensional electron correlations is considered. It is shown that the collective interactions in fluids depend not only on density but also on temperature