| Energy circuit: A pathway whose flow is proportional to the quantity in the storage or source upstream.
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| Source: Outside source of energy delivering forces according to a program controlled from outside; a forcing function. |
| Tank: A compartment of energy storage within the system storing a quantity as the balance of inflows and outflows; a state variable.
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| Heat sink: Dispersion of potential energy into heat that accompanies all real transformation processes and storages; loss of potential energy from further use by the system. |
| Interaction: Interactive intersection of two pathways coupled to produce an outflow in proportion to a function of both;control action of one flow on another; limiting factor action; work gate. |
| Consumer: Unit that transforms energy quality, stores it, and feeds it back autocatalytically to improve inflow.
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| Switching action: A symbol that indicates one or more switching actions.
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| Producer: Unit that collects and transforms low-quality energy under control interactions of high-quality flows.
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| Self-limiting energy receiver: A unit that has a self-limiting output when input drives are high because there is a limiting constant quality of material reacting on a circular pathway within.
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| Box: Miscellaneous symbol to use for whatever unit or function is labeled.
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| Constant-gain amplifier: A unit that delivers an output in proportion to the input I but is changed by a constant factor as long as the energy source S is sufficient. |
| Transaction: A unit that indicates a sale of goods or services (solid line) in exchange for payment of money (dashed line). Price is shown as an external source.
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