Which statement best defines a capacitor?

• A. A resistor that dissipates energy
• B. An inductive coil used to store charge
• C. A diode used to rectify current
• D. An electronic device for storing charge

Answer: (D)

A capacitor is an electronic device that stores electrical energy as charge on two conductors separated by an insulating material called a dielectric. When voltage is applied, charge builds up on the plates, with the amount of charge Q proportional to the voltage (Q = C·V, where C is the capacitance). The energy stored is (1/2)·C·V². This stored energy can be released back into a circuit when needed, such as smoothing power supplies or providing short bursts of current in timing circuits.

This is different from a resistor, which dissipates energy as heat; from an inductive coil, which stores energy in a magnetic field; and from a diode, which controls the direction of current. Because of its defining role of storing charge, the best description is simply that it is an electronic device for storing charge.