Which leads: V or I?
"Which leads: V or I?" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "AC through an inductor" in Alternating Current. Rotating V and I phasors for a pure R, L or C beside the sine curves their vertical projections trace, with the quarter-cycle shift marked. NEET has asked about this idea 1 time in 10 years of papers. JEE Main asked 37 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson AC through an inductor in Alternating Current. Change the values and watch what happens.
What it shows
NCERT §7.3–7.5
Rotating V and I phasors for a pure R, L or C beside the sine curves their vertical projections trace, with the quarter-cycle shift marked.
Controls
4 to try
- Segmentedelement R / L / C
- Sliderω 50–500 rad/s
- Buttonplay / pause
- Dragphasor angle to scrub time
Try this
Pick the element whose current peaks a quarter period AFTER the voltage, then freeze the clock where v is maximum and i is zero.
Common mistake
Saying the current in an inductor leads the voltage.
In NEET
10 years of papers
Asked 1 time in 10 years of NEET.
See the past questions: LCR Circuits, Alternating Current (chapter)
In JEE Main
42 shifts analysed
37 questions from Alternating Current across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §7.4
- For a pure inductor (negligible resistance), the loop rule gives v − L di/dt = 0, so di/dt = (vm/L) sin ωt.
- Integrating, i = −(vm/ωL) cos ωt = im sin(ωt − π/2), with im = vm/ωL. There is no constant term, because the source swings evenly about zero.
- The quantity ωL plays the part of resistance. It is the inductive reactance XL = ωL, measured in ohm, and im = vm/XL.
- XL grows in proportion to both the inductance and the frequency.
- The current lags the voltage by π/2, a quarter of a cycle: it reaches each maximum a quarter period after the voltage does.
- The average power into a pure inductor over a cycle is zero. Energy stored in its field during one quarter cycle is handed back to the source in the next.
- Example 7.2: a pure 25.0 mH inductor on 220 V, 50 Hz has XL = 2π × 50 × 25 × 10⁻³ = 7.85 Ω and an rms current of 220/7.85 = 28 A.
Questions
5 answers
- What does the "Which leads: V or I?" simulation show?
- Rotating V and I phasors for a pure R, L or C beside the sine curves their vertical projections trace, with the quarter-cycle shift marked.
- What mistake does the "Which leads: V or I?" simulation fix?
- A common mistake it clears up: saying the current in an inductor leads the voltage.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 1 time in 10 years of papers.
- Does JEE Main ask questions from Alternating Current?
- Yes. JEE Main asked 37 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "AC through an inductor" in Alternating Current (Class 12 Physics).
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