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

Choices:  
A: They are out of phase  
B: They are in phase  
C: They are 90 degrees out of phase  
D: They are 45 degrees out of phase    

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

Correct answer: C — They are 90 degrees out of phase.

Why this is correct:
- Reactive power is associated with ideal inductors and capacitors, where voltage and current are in quadrature (±90°).
- For a purely reactive load, the phase angle is $\phi = \pm 90^\circ$, so no real power is consumed, only energy storage and return.

Useful formulas and rules:
- Real (average) power: $P = VI \cos \phi$
- Reactive power: $Q = VI \sin \phi$
- Apparent power: $S = VI$, with $S^2 = P^2 + Q^2$
- For purely reactive loads: $\phi = \pm 90^\circ \Rightarrow \cos \phi = 0 \Rightarrow P = 0$, and $Q = \pm VI$
- Mnemonic: “ELI the ICE man”
  - Inductor (L): Voltage (E) leads Current (I) by 90° — current lags
  - Capacitor (C): Current (I) leads Voltage (E) by 90°

Why the other choices are incorrect:
- A: “They are out of phase” (commonly taken as 180°) — that would imply a negative power factor, not purely reactive. Reactive specifically means 90°, not 180°.
- B: “They are in phase” (0°) — that is a purely resistive case: $P = VI$, $Q = 0$.
- D: “They are 45 degrees out of phase” — that indicates a mix of real and reactive power (both $P$ and $Q$ nonzero), not purely reactive power.