A capacitor is considered a passive device in electronics. Passive devices are those that do not require an external power source to function but rather respond to applied voltages or currents. In the case of a capacitor, it stores electrical energy in an electric field between its plates when a voltage is applied across it. This storage of energy happens without the need for any active control or amplification from an external power source.
Contrastingly, active devices such as transistors or operational amplifiers require an external power source to operate and can actively amplify or switch electrical signals. They rely on an external power supply to function and actively control the flow of current through them.
Inductors, like capacitors, are also passive devices. They store energy in a magnetic field when current flows through them and oppose changes in current. Both capacitors and inductors are fundamental passive components used extensively in electronic circuits for filtering, tuning, energy storage, and other applications where their unique electrical properties are beneficial.