Passive Components

Passive components are the parts of a circuit that do not amplify a signal or add energy of their own; they store, dissipate or redirect electrical energy. The three fundamental passive components are the resistor, the capacitor and the inductor, and most other passive parts are built from or related to these. Together they set currents and voltages, filter signals, store charge and energy, and shape the behaviour of a circuit, but they always need an active device or an external source to drive them.

A resistor opposes the flow of current, converting electrical energy into heat, and is used to set currents, divide voltages and limit power. A capacitor stores energy in an electric field between two conductive plates separated by an insulator; it blocks steady direct current but allows changing signals to pass, which makes it useful for filtering, smoothing and coupling. An inductor, a coil of wire, stores energy in a magnetic field and opposes changes in current, complementing the capacitor in tuning and filtering circuits.

Several other components are conventionally grouped with the passives. A transformer uses two or more coupled coils to transfer energy between circuits and to change voltage levels, relying on the same magnetic principles as the inductor. A relay is an electrically operated switch in which a small current through a coil moves contacts to control a larger circuit. Although a relay contains a coil and contacts, it serves to switch power rather than to amplify a signal.

Frequently asked questions

What makes a component passive?

A passive component cannot amplify a signal or supply energy of its own. It only stores, dissipates or redirects electrical energy, such as a resistor turning current into heat.

What is the difference between a capacitor and an inductor?

A capacitor stores energy in an electric field and opposes changes in voltage, while an inductor stores energy in a magnetic field and opposes changes in current. They behave in complementary ways.

Is a relay a passive component?

A relay is usually grouped with passive parts because it does not amplify a signal. It is an electrically operated switch whose coil moves contacts to control a separate circuit.