Capacitors slow the change of current in a circuit because they take time to charge.

• A. They store magnetic energy
• B. They require constant current
• C. They are indestructible
• D. They slow the change of current by charging over time

Answer: (D)

Capacitors resist rapid changes in current by charging gradually. They store energy in the electric field between their plates, and the current through a capacitor is i = C dv/dt, so the flow of current is tied to how fast the voltage across the capacitor changes. When a capacitor first connects to a circuit, the voltage across it begins at zero, so dv/dt is large and current is high. As it charges, dv/dt slows, causing the current to drop. The rate is set by the time constant RC, so after a few time constants the capacitor is near full, and the current becomes very small. That time-delayed charging is why capacitors slow the change of current.

The other statements aren’t correct: a capacitor stores electric energy, not magnetic energy (that’s an inductor); current through a capacitor isn’t required to be constant; and capacitors aren’t inherently indestructible.