Thévenin's theorem

Thévenin's theorem states that any linear electrical network viewed from two terminals can be replaced by an equivalent voltage source in series with an impedance.

Open-circuit voltage: The voltage across a pair of terminals when no load is connected. It becomes the Thévenin source voltage.
Linear circuit: An electrical circuit whose responses obey superposition and scale proportionally with its inputs. Linearity is the condition that makes a fixed two-parameter equivalent possible.
Load line: A graph of the voltage-current combinations permitted by a load or source circuit. The Thévenin model gives a simple straight-line source relation.
Millman's theorem: A circuit theorem for finding the voltage at a node connected to multiple branches with voltage sources and impedances. It solves a common circuit structure directly rather than replacing an arbitrary network at its terminals.
Short-circuit current: The current that flows between two terminals when they are joined by a zero-resistance connection. Together with open-circuit voltage, it can determine the equivalent resistance.
Electrical network: An arrangement of electrical components connected by conductive paths. The theorem applies to the terminal behavior of a network.
Norton theorem: A theorem representing a linear two-terminal network as a current source in parallel with an impedance. Its parallel-source form predicts the same terminal behavior.
Superposition theorem: A method for finding a linear circuit's response by summing the contributions from individual independent sources. It analyzes source contributions, while Thévenin's theorem compresses terminal behavior.
Equivalent impedance: An impedance that reproduces a circuit's terminal voltage-current behavior under specified conditions. It is the series element that completes the Thévenin model.
Impedance: The frequency-dependent opposition that a circuit presents to alternating current, combining resistance and reactance. The theorem uses impedance for general linear AC networks.

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