Several Energy in Thermodynamics

Energy appears in all areas of physics, and thermodynamics is essential for those who seek to understand its fundamental nature.

In thermodynamics, there are three categories of state functions containing the word “energy”: Internal energy, free energy, and non-exploitable energy, though the latter is perhaps less commonly used.

Internal energy is the only one that corresponds to the quantity consistently recognized as energy across other domains of physics. Internal energy is defined as a quantitative work performed by adiabatic operation, i.e. measurable mechanical operation in thermally isolated condition. The absolute magnitude of internal energy is indeterminable because it cannot be measured directly. Only changes in internal energy can be determined experimentally through the measurable mechanical work extracted from or exerted to the system, and hence considered physically significant in thermodynamics. Despite this, internal energy is believed to obey the conservation law: if no energy enters or leaves the system, the internal energy remains constant.

Free energy, including the Helmholtz and Gibbs free energies, reflects the work that a system can perform under specified constraints. For instance, the Helmholtz free energy indicates the maximum exploitable work the given system can exert at constant temperature. Since free energy depends on the conditions of its surrounding conditions, such as temperature or pressure, it is not conserved. When people refer to energy usage, consumption, or efficiency, they always mean free energy. Once some work is performed, corresponding free energy vanishes. Free energy, therefore, is not an inviolable conserved quantity that we refer to as energy.

Non-exploitable energy captures the fraction of internal energy that cannot readily perform work under the given thermodynamic conditions. The second law of thermodynamics introduces entropy. Increasing entropy reduces the free energy. Like free energy, non-exploitable energy depends on the surrounding and available technology. A change in the surrounding can turn previously non-exploitable energy into free energy. Consequently, it is not a form of energy; rather, it is a context-dependent quantity that can only be defined in relation to technological factors.