*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, the phase relationship between current and voltage is crucial for understanding reactive power. 

1. **Correct Answer Explanation**:  
   The correct answer is **C: They are 90 degrees out of phase**.  
   - In a purely reactive circuit (either inductive or capacitive), the current and voltage are indeed 90 degrees out of phase. This means that when the voltage reaches its maximum, the current is at zero, and vice versa. This phase difference is a characteristic of reactive power, which is associated with the energy stored in the electric and magnetic fields of inductors and capacitors.

2. **Incorrect Choices Explanation**:  
   - **A: They are out of phase**: This is too vague. While it is true that they are out of phase, it does not specify the exact phase difference, which is critical in understanding reactive power.
   - **B: They are in phase**: This is incorrect because in a purely resistive circuit, current and voltage are in phase (0 degrees). However, for reactive power, this is not the case.
   - **D: They are 45 degrees out of phase**: This is incorrect as well. A 45-degree phase difference would imply a combination of resistive and reactive components, which does not represent the condition for purely reactive power.

3. **Useful Formulas and Rules of Thumb**:  
   - The relationship between voltage ($V$) and current ($I$) in an AC circuit can be expressed using the power factor ($PF$):
     $$PF = \cos(\phi)$$  
     where $\phi$ is the phase angle between the voltage and current.
   - For purely reactive power, the power factor is zero, indicating that all the power is reactive and the phase angle is 90 degrees.

Understanding these concepts will help you grasp the behavior of AC circuits and prepare effectively for the Extra Class exam.