Physics Alternating Current NUMS 2019
PMDC Verified Question 90 of 127
In an AC circuit containing only a pure inductor, what is the phase relationship between voltage and current?
A
Current leads voltage by \(90^\circ\)
B
Voltage leads current by \(90^\circ\)
C
Voltage and current are in phase
D
Voltage lags current by \(180^\circ\)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: Voltage leads current by \(90^\circ\)
Concept:

In an inductor, back-EMF opposes rapid current changes. The voltage across the inductor peaks a quarter-cycle before the current.

Formula / Reaction:

$$v_L(t) = V_0 \sin(\omega t), \quad i(t) = I_0 \sin\left(\omega t - \frac{\pi}{2}\right)$$

Solution:

  • Self-induction causes current to lag voltage by \(90^\circ\).


  • Equivalently, the voltage across the inductor leads current by \(90^\circ\) (\(\pi/2\text{ rad}\)).


Why other options are incorrect:

  • Opt_A: Current leads voltage by \(90^\circ\) in a pure capacitor.


  • Opt_C: Voltage and current are in phase in a pure resistor.


  • Opt_D: A \(180^\circ\) lag represents complete phase opposition.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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