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It is a principle that applies to mainly linear systems. Superposition implies that the output of the system to a signal composed of two different inputs (i.e x1+x2) can calculated just by calculating the response to the system to every of the inputs (y1, y2) and then adding them (y1+y2). \[y = f(x), y_1 = f(x_1), y_2 = f(x_2) \rightarrow y = f(x_1+x_2) = y_1+y_2\]
it is a theorem used for simplyfing the networks consisting of linear elements.....
Yes, more or less. In simple terms, it's used to determine how each independent source contributes to the current or voltage of a particular variable within the circuit. You analyze one source at a time, turning off all other sources (voltage sources become short circuit, current sources become open circuit).
Here's a verbatim quote from some old lecture notes of mine:
"The principle of superposition is based on the linearity property of the circuit and the various elements that the circuit consists of.
The superposition principle states that the voltage across or the current through a given linear element of a given linear circuit is the algebraic sum of the voltages across or the
currents through the element in question due to each independent source acting alone when all other independent sources are zero or become inactive.
The independent sources are considered acting one at a time while all other independent sources are omitted:
–Voltage source becomes equal to zero, which implies that the voltage source is replaced by a short-circuit.
–Current source becomes equal to zero, which implies that the current source is replaced by an open-circuit.
Dependent sources are left intact as they are controlled by circuit variables."