How it works
Superposition principle: in a linear circuit with multiple independent sources, the response (current or voltage) at any element is the algebraic sum of responses due to each independent source acting alone, with all other independent sources set to zero (voltage sources → short circuit; current sources → open circuit). The principle holds only for linear elements — resistors, linear capacitors, inductors, and linear dependent sources. Dependent sources are never deactivated; they remain active in every partial circuit. Each partial circuit is solved using basic techniques: voltage divider, current divider, or mesh/nodal analysis.
Key points to remember
Superposition applies to voltages and currents, not directly to power — since P = V²/R = I²R, power is a quadratic function of current or voltage, so P_total ≠ P₁ + P₂ from individual sources. This is the most common mistake in exam answers. The theorem is valid only for circuits with linear bilateral elements. Superposition is most useful when independent sources are of different types (AC and DC, or two different frequencies) — it is the only clean way to handle mixed-frequency circuits where impedances differ. For n independent sources, n partial circuits must be solved, making superposition increasingly laborious compared to mesh/nodal analysis for large circuits.
Exam tip
The examiner always asks you to use superposition to find a current or voltage, followed by a power calculation — compute the total voltage or current first by superposition, then calculate power from the total, never by summing partial powers.