What is a capacitor?

• A. A capacitor is a device that stores charge
• B. A resistor
• C. An inductor
• D. A transformer

Answer: (A)

A capacitor’s defining feature is that it stores electric charge on two facing conductors separated by an insulating dielectric. When a voltage is applied, charges accumulate on the plates with equal magnitude and opposite sign, creating an electric field in the dielectric and storing energy there. The amount of charge is proportional to the voltage across the plates, Q = C V, where C is the capacitance determined by plate area, separation, and the dielectric constant (C = εA/d). In DC steady state, once charged, no net current flows, so it behaves like an open circuit. In AC and transient situations, it acts as a reactive element with impedance Z = 1/(j ω C), meaning it passes changing signals while blocking DC. The energy stored in a capacitor is (1/2) C V^2. This distinguishes it from a resistor, which dissipates energy as heat; an inductor, which stores energy in a magnetic field; and a transformer, which transfers energy via mutual inductance.