Law encompasses several principles: Conservation of energy, which says that energy can be neither created nor destroyed, but can only change form. A particular consequence of this is that the total energy of an isolated system does not change. The concept of internal energy and its relationship to temperature. If a system has a definite temperature, then its total energy has three distinguishable components, termed kinetic energy (energy due to the motion of the system as a whole), potential energy (energy resulting from an externally imposed force field), and internal energy. The establishment of the concept of internal energy distinguishes the first law of thermodynamics from the more general law of conservation of energy. Work is a process of transferring energy to or from a system in ways that can be described by macroscopic mechanical forces acting between the system and its surroundings. The work done by the system can come from its overall kinetic energy, from its overall potential energy, or from its internal energy. For example, when a machine (not a part of the system) lifts a system upwards, some energy is transferred from the machine to the system. The system's energy increases as work is done on the system and in this particular case, the energy increase of the system is manifested as an increase in the system's gravitational potential energy. Work added to the system increases the potential energy of the system: When matter is transferred into a system, the internal energy and potential energy associated with it are transferred into the new combined system. where denotes the internal energy per unit mass of the transferred matter, as measured while in the surroundings; and denotes the amount of transferred mass. The flow of heat is a form of energy transfer. Heat transfer is the natural process of moving energy to or from a system other than by work or the transfer of matter. In a diathermal system, the internal energy can only be changed by the transfer of energy as heat: Combining these principles leads to one traditional statement of the first law of thermodynamics: it is not possible to construct a machine which will perpetually output work without an equal amount of energy input to that machine. Or more briefly, a perpetual motion machine of the first kind is impossible. Second law The second law of thermodynamics indicates the irreversibility of natural processes, and in