Glossary
capacitor
A capacitor is a component of two plates separated by an insulator that stores charge and gives it back, so it blocks steady current and passes a changing signal.
A capacitor stores charge in an electric field. Two conductive plates, separated by an insulator — air, ceramic, plastic film or an electrolytic paste — sit very close together; put a voltage across them and charge collects on the plates, and the part holds it the way a small reservoir holds water. Capacity is measured in farads, and a practical part is usually a microfarad (a millionth) or less.
Two behaviors answer most questions. It passes a changing voltage and blocks a steady one, so a capacitor charges up and then stops DC, and it can be used to smooth a rectified supply. And it opposes any change in the voltage across it, which is why it is paired with a resistor in a timing circuit.
Its symbol is two parallel bars with a gap between them, and the gap is the thing to notice: the plates never touch, because charge is stored rather than conducted. Keep it apart from the resistor, which deliberately limits current in both directions, and from the diode, which conducts one way only.
Taught in
Related terms
- diode — A diode is a semiconductor one-way valve: current passes in one direction only, and its symbol is a triangle pointing into a bar.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)