What constitutes a capacitor?

• A. Two conductors separated by an insulator.
• B. A single conductor connected to a voltage source.
• C. A resistor in series with a source.
• D. A coil wrapped around a ferromagnetic core.

Answer: (A)

A capacitor is two conductors separated by an insulating material, called a dielectric. When a voltage is applied across the plates, charge collects on each plate and an electric field stores energy between them, while the dielectric prevents direct current from flowing between the plates. This stored energy in the electric field is what defines a capacitor. The other descriptions don’t fit: a single conductor with a voltage source has no second plate to form a capacitor; a resistor in series with a source dissipates energy as heat; a coil around a ferromagnetic core is an inductor, storing energy in a magnetic field.