Sit a full JEE Main paper, free
25 questions, 45 minutes, out of 100, marked the way the exam marks it. No payment, no card, and no account needed to start.
1,000 XP coins free the first time you open your portal — enough to open a whole chapter level there and then.
Alternating Current is a chapter of definite, repeatable questions. Work out an impedance, find a power factor, locate the resonant frequency, use a transformer ratio. JEE Main asks these almost verbatim each year, and they take under a minute each once the formulas are in place. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 2 questions every year, which is 8 marks.
- Series LCR impedance and power factor are asked every year.
- Resonance, transformers and rms values fill the rest.
What the syllabus says
In the syllabus (Unit 14, Electromagnetic Induction and Alternating Currents): Alternating currents, peak and RMS values of alternating current and voltage, reactance and impedance, the LCR series circuit and resonance, power in AC circuits, wattless current, the AC generator and the transformer.
Taken out in 2024: quality factor.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| rms values | Irms = I0/√2, Vrms = V0/√2 |
| Reactances | XL = ωL, XC = 1/ωC |
| Impedance | Z = √(R2 + (XL − XC)2) |
| Power factor | cosφ = R/Z |
| Average power | P = VrmsIrms cosφ |
| Resonance | XL = XC, f = 1/(2π√(LC)) |
| Phase in a capacitor | the current leads the voltage by π/2 |
| Phase in an inductor | the current lags the voltage by π/2 |
| Transformer | Vs/Vp = Ns/Np = Ip/Is |
At resonance the two reactances cancel, so Z = R and the power factor becomes 1. Everything about a series resonant circuit follows from that one line: smallest impedance, largest current, and all the power actually used.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. In a series LCR circuit R = 30 Ω, XL = 50 Ω and XC = 10 Ω. The impedance is
- (a) 30 Ω
- (b) 40 Ω
- (c) 50 Ω
- (d) 90 Ω
Q2. For that same circuit, the power factor is
- (a) 0.5
- (b) 0.6
- (c) 0.8
- (d) 1.0
Q3. In a circuit containing only a capacitor, the current
- (a) leads the voltage by π/2
- (b) lags the voltage by π/2
- (c) is in phase with the voltage
- (d) leads the voltage by π
Q4. The resonant frequency of a circuit with L = 2 H and C = 2 µF is about
- (a) 79.6 Hz
- (b) 50 Hz
- (c) 500 Hz
- (d) 8 Hz
Q5. A transformer has 100 turns in the primary and 500 in the secondary. With 220 V across the primary, the secondary voltage is
- (a) 44 V
- (b) 220 V
- (c) 1100 V
- (d) 550 V
Q6. The rms value of a sinusoidal supply whose peak voltage is 311 V is about
- (a) 311 V
- (b) 220 V
- (c) 440 V
- (d) 155 V
Q7. A choke coil is used to control current in an alternating current circuit because it
- (a) has a very high resistance
- (b) cuts down the current while wasting almost no power
- (c) raises the supply voltage
- (d) stores charge
Q8. At resonance, the power factor of a series LCR circuit is
- (a) 0
- (b) 0.5
- (c) 1
- (d) infinite
Q9. In a series LCR circuit at resonance, the power factor is
- (a) zero
- (b) 0.5
- (c) 1
- (d) always less than 1
Q10. An alternating current ammeter reads the
- (a) peak value
- (b) rms value
- (c) average value over a cycle
- (d) instantaneous value
Answer key, with a hint for each
| Q1 | (c) | Z = √(302 + 402) = √2500. |
| Q2 | (b) | cosφ = R/Z = 30/50. |
| Q3 | (a) | A capacitor must be charged before a voltage appears across it, so the current gets there first. |
| Q4 | (a) | f = 1/(2π√(2 × 2 × 10−6)) = 1/(2π × 2 × 10−3). |
| Q5 | (c) | 220 × 500/100. This is a step-up transformer. |
| Q6 | (b) | 311/√2. This is why the Indian mains is quoted as 220 V. |
| Q7 | (b) | A pure inductor has a phase angle of 90°, so cosφ = 0 and it consumes no power. A resistor would waste it as heat. |
| Q8 | (c) | The reactances cancel, so Z = R and the current is exactly in phase with the voltage. |
| Q9 | (c) | At resonance Xₗ = Xᴄ, so the impedance is purely resistive and the current is in step with the voltage. cosφ = R/Z = 1 — which is exactly why the circuit draws its largest power there. |
| Q10 | (b) | It is calibrated in rms, because that is the value which gives the same heating as a steady current. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
The Foundation paper for Alternating Current is waiting for you on the portal: 25 questions, 45 minutes, marked out of 100, with the answer key and a hint for every question. Sit it tonight and you will know exactly where this chapter stands.
Free the moment you register
1,000 XP coins, free
A Foundation paper opens for 230 coins, so your first four are already paid for. And every paper you sit sincerely pays you more.
Every chapter of JEE Main Physics has three papers on the portal: Foundation, Intermediate and Expert. You move up only when you clear the one below. 30 chapters, 90 papers, open 24×7.
Want to know how many marks this chapter usually carries? See the JEE Main Physics chapter-wise weightage.
All the formulas for every chapter in one place: JEE Main Physics formula sheet.
One JEE Main Physics question every morning
It comes at 7 AM on Telegram. Tap the option you think is right and you will know at once whether it was. The worked hint follows the next morning.