In real gases at lower pressures, when the average intermolecular distances are still relatively large, intermolecular attractive forces are exerted between considered pairs of molecules. The repulsive forces between molecules were investigated by the British mathematician, John Edward Lennard-Jones. The positive part corresponds to repulsive forces and the negative part represents attractive forces. The behaviors of real gases are described by different analytical state equations in order to correlate them with the experimental reality. One of the earliest and best-known state equations of real gases is the semiempirical van der Waals equation, named after Dutch physicist Johannes Diderik van der Waals. The internal energy of an ideal gas is the sum of individual kinetic energies of molecules. At adiabatic expansion, when intermolecular distances increase, the real gas system performs work to the detriment of its kinetic energy, which leads to a reduction of molecular kinetic energy.