A theoretical gas that differs from a real gas that consists of elastic molecules in which the effective volume occupied by the molecules is zero and the gas obeys the ideal gas law: where p is the pressure, V is the volume occupied by the gas, n is the number of molecules, R is the universal gas constant, and T is the absolute temperature. It applies to a gas with a low density in which the intermolecular forces between the molecules are neglected. Perfect and ideal gases are often used interchangeably although perfect gases assume a constant specific heat at constant volume.