*Question E5D03:
What is the phase relationship between current and voltage for reactive power?*  

*Correct Answer is  C: They are 90 degrees out of phase*  

### AI Response: (may contain errors)  

In AC circuits, reactive power is associated with the energy stored in inductors and capacitors. This energy is not used for work but is temporarily stored and then returned to the source.

1. **Correct Answer: They are 90 degrees out of phase**

   - In a purely reactive component (like an ideal inductor or capacitor), the current and voltage are 90 degrees out of phase. This means that when the voltage reaches its peak, the current is at zero, and vice versa. This phase difference is a key characteristic of reactive power.

2. **Why the other choices are incorrect:**

   - **A: They are out of phase**  
     This is too vague. While it is true that they are out of phase, the specific phase difference for reactive power is 90 degrees.

   - **B: They are in phase**  
     If current and voltage were in phase, it would indicate purely resistive power, not reactive power. In resistive circuits, all the power is used for work, and there is no reactive component.

   - **D: They are 45 degrees out of phase**  
     A 45-degree phase difference would indicate a combination of resistive and reactive components, not purely reactive power.

3. **Useful Formula:**

   - The relationship between real power ($P$), reactive power ($Q$), and apparent power ($S$) in an AC circuit is given by:
     $$ S = \sqrt{P^2 + Q^2} $$
   - The phase angle ($\phi$) between the current and voltage can be found using:
     $$ \tan(\phi) = \frac{Q}{P} $$
   - For purely reactive power, $P = 0$, so $\phi = 90^\circ$.

Understanding these concepts is crucial for analyzing AC circuits and their power components.