What is the name of the power that is alternately stored in a capacitor’s electric field and then returned to the source?

Introduction:
AC circuits feature different kinds of power: true (real), reactive, and apparent. This item asks you to identify the kind associated with energy shuttling in and out of reactive fields without net dissipation over a cycle.

Given Data / Assumptions:

• Ideal capacitor with negligible losses.
• Sinusoidal AC excitation.
• Periodic steady-state conditions.

Concept / Approach:
Reactive power Q describes energy exchange between the source and the reactive element's field. For capacitors, energy is stored as W = 0.5 * C * V^2 and then returned to the source later in the cycle. Average real power P over a full cycle is zero for the ideal component, but Q is nonzero and measured in VAR (volt-ampere reactive).

Step-by-Step Solution:

Verification / Alternative check:
Phasor analysis: For a capacitor, current leads voltage by 90°, so cos(phi) = 0 and sin(phi) = −1. Real power P = V * I * cos(phi) = 0; reactive power magnitude |Q| = V * I is nonzero.

Why Other Options Are Wrong:

• Apparent power: S = V * I (VA) includes both P and Q; not specifically the stored-and-returned component.
• Impedance power: Not a standard term in AC power theory.
• True power: Represents average dissipation; zero in an ideal capacitor.

Common Pitfalls:
Equating apparent with reactive power, or thinking reactive power is ‘‘waste’’; actually it represents necessary energy exchange to sustain fields in reactive elements.

Final Answer:
Reactive power.