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In thermodynamics, an open system is one whose border is permeable to both energy and mass. A closed system, by contrast, is permeable to energy but not to matter. The definition of a "system" is arbitrary; a system may be defined as the region of space under study being characterized by a collection of components or related in some way. Open systems have a number of consequences. A closed system contains limited energy. The definition of an open system assumes that there are supplies of energy that cannot be depleted; in practice, this energy is supplied from some source in the surrounding environment, which can be treated as infinite for the purposes of study. One type of open system is the so-called radiant energy system, which receives its energy from solar radiation – an energy source that can be regarded as inexhaustible for all practical purposes. Some hypothetical open systems receive energy from space in the form of vacuum energy (sometimes referred to as zero-point energy). Examining processes with knowledge of open system mechanics can lead to interesting insights. An example of an open system is the space tether, which uses the ground and the ionosphere as parts of an electrical circuit. This circuit acts as an electrical generator, converting the tether's kinetic energy to electrical energy by electromagnetic induction as it travels through the Earth's magnetic field. Open systems can also occur in suspended circuits, in which the conductor has no ground and uses the environment to complete a circuit.
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