Which principle is obeyed by linear systems when multiple inputs are applied?

• A. Superposition Principle
• B. Equivalence Principle
• C. Orthogonality Principle
• D. Reciprocity Principle

Answer: (A)

The key idea is linearity: in a linear system, outputs add up when inputs are combined. If you apply more than one input at once, you can treat each input separately and then add the results to get the total output. Mathematically, if an input x1 produces output y1, and x2 produces y2, feeding both at once (x1 + x2) yields y1 + y2. The output scales proportionally with the input as well, so a scaled input leads to a scaled output. This additive, scale‑consistent behavior is the superposition principle, and it’s what lets you analyze complex inputs by breaking them into simpler parts.

Other principles don’t describe this overall additive behavior. Equivalence is about replacing a circuit with an equivalent one for easier analysis, not about how multiple inputs combine. Orthogonality helps with separating components or minimizing error via projections, not the rule for summing responses. Reciprocity concerns how a network responds when you swap source and measurement points, which is a different property from how multiple inputs combine.