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A second-order circuit contains two irreducible energy storage elements, such as capacitors and inductors. Examples include RLC circuits and RC or RL circuits with dual capacitors or inductors. These circuits are characterized by a second-order differential equation that correlates input and output signals, where inputs typically originate from voltage or current sources.

Apply Kirchhoff's voltage law to the RLC circuit, then take the time derivative of the resulting equation. This yields a second-order differential equation whose coefficients are expressed in terms of resistance, capacitance, and inductance. These coefficients manifest as the damping coefficient and resonant frequency, which characterize the circuit's behavior.

The damping coefficient indicates the damping caused by the resistor and determines the rate of energy dissipation in the system. It directly influences how fast energy is lost due to resistance. A higher damping coefficient means faster energy loss, while a lower coefficient allows energy to persist longer in the circuit.

Resonant frequency represents the natural oscillation frequency of the circuit as energy is exchanged between the inductor and capacitor. It measures how quickly energy shifts between these two storage elements, illustrating the circuit's inherent tendency to oscillate at a particular frequency independent of external driving forces.

The damping coefficient controls the rate of energy loss due to resistance, while the resonant frequency highlights the circuit's natural oscillation speed. Together, these two factors determine the overall response of the circuit. The damping coefficient dictates how quickly oscillations decay, whereas resonant frequency determines the frequency at which oscillations occur.

Input signals generally originate from voltage or current sources. Output signals are usually the voltage across the capacitor and/or the current through the inductor. In circuits with initial energy stored in these elements, the stored energy drives the circuit response without external input.

Common examples include RLC circuits, which contain a resistor, inductor, and capacitor. Other second-order circuits include RC circuits with dual capacitors and RL circuits with dual inductors. Understanding types of responses of series RLC circuits helps predict how these configurations behave under different conditions.