What is a capacitor?

• A. Resistor
• B. Inductor
• C. Diode
• D. An electronic device for storing charge. Passive two-terminal electrical component used to store energy in an electrical field.

Answer: (D)

Capacitors are passive two-terminal devices that store energy in the electric field between two conductors separated by a dielectric. When voltage is applied, charge gathers on the plates, and the stored energy is ½ C V². This ability to hold energy makes capacitors useful for smoothing power supplies, coupling signals, and timing in circuits.

They’re different from other components in fundamental ways: a resistor dissipates energy as heat rather than storing it; an inductor stores energy in a magnetic field, not an electric field; and a diode is a nonlinear device that controls current direction, not primarily a storage element.

In alarm systems, capacitors help stabilize power and support timing-related functions, ensuring reliable operation during transient power changes.