Reactance of a coil and a capacitor
"Reactance of a coil and a capacitor" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the lesson "AC through a capacitor" in Alternating Current. Switch between the two and slide the frequency. Watch the reactance, the current and the angle between the V and I phasors. XL = ωL rises with frequency and XC = 1/ωC falls with it. In a coil the current lags the voltage by π/2; in a capacitor it leads by π/2. Either way the average power is zero. JEE Main asked 37 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson AC through a capacitor in Alternating Current. Change the values and watch what happens.
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 §7.5
- On dc a capacitor passes current only while it charges; once full, the current stops. On ac it is charged, discharged and charged the other way every cycle, so current keeps flowing in the leads.
- With v = q/C = vm sin ωt, the current i = dq/dt = ωCvm cos ωt = im sin(ωt + π/2), where im = ωCvm.
- So 1/ωC acts like a resistance. It is the capacitive reactance XC = 1/ωC, in ohm, and im = vm/XC.
- XC falls as the frequency or the capacitance rises: a capacitor blocks dc (zero frequency) and passes high frequencies easily.
- The current leads the voltage by π/2: it reaches each maximum a quarter period before the voltage.
- As with the inductor, the average power into a pure capacitor over a cycle is zero.
- Example 7.3: a lamp in series with a capacitor stays dark on dc but glows on ac. Making C smaller raises XC and dims the lamp on ac.
- Example 7.4: 15.0 μF on 220 V, 50 Hz has XC = 212 Ω, rms current 1.04 A and peak current 1.47 A, leading the voltage by π/2. At double the frequency XC halves and the current doubles.
- Example 7.5: a bulb in series with an inductor dims when an iron rod is pushed into the coil. The rod raises L and so XL, and more of the supply voltage appears across the coil.
Questions
3 answers
- What does the "Reactance of a coil and a capacitor" simulation show?
- Switch between the two and slide the frequency. Watch the reactance, the current and the angle between the V and I phasors.
- 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 a capacitor" in Alternating Current (Class 12 Physics).
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