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# Equivalent Circuit Diagram for a Capacitor

Thus far, we have considered capacitors and coils as "ideal" components possessing a pure reactance *X*_{C} (capacitor) or *X*_{L} (coil) in an AC circuit. In reality, however, both components also possess an active resistance which must be accounted for in detailed investigations. In such cases, instead of ideal components, an *equivalent circuit diagram* is substituted that includes the active resistance.

A capacitor charged by a DC voltage and subsequently disconnected from the voltage source loses its charge over a variable time period. Because the insulator between the capacitor plates is still conductive, albeit slightly, a correspondingly weak *insulation current* flows through it, by which the charges seek to equalise. This undesired leakage current in conjunction with the capacitor voltage results in a power loss termed *insulation loss*.

When a capacitor is connected to an AC voltage, the insulator's molecules are re-polarised continuously at the same frequency as the alternating voltage, causing a *displacement current* to flow in alternate directions. This results in a power loss dependent on the frequency and amplitude of the AC voltage as well as the capacitance of the component. This is termed *dielectric loss*.

Both losses can be accounted for by the following equivalent circuit diagram for a real capacitor. It consists of an ideal (loss-free) capacitance *C* connected in parallel with an ohmic resistance *R*_{V }(representing the losses). This size of this resistance is such that the associated power loss is equal to the sum of the insulation and dielectric losses.