Class 12 Physics Chapter 7 Alternating Current
Alternating Current is a high-scoring chapter in Class 12 Physics; CBSE weightage is 8–12 marks and typical board questions from this topic total around 6–8 marks . The article was updated on Apr 17, 2026 and lists the formulas and facts you must remember.
Class 12 Physics Chapter 7 Alternating Current: Weightage & trends
Quick facts for exam planning. CBSE and JEE share much of the same content — about 80% overlap — so focus on NCERT plus a few JEE-level numericals.
| Exam | Typical weightage |
|---|---|
| CBSE boards | 8–12 marks |
| JEE Main | 6–8 marks (2–3 questions) |
| JEE Advanced | 8–12 marks (2–3 questions) |
Transformer questions have been repeated in recent papers in 2024, 2023, 2020, 2017 . Series LCR and resonance appear in roughly 50–60% of questions; together with transformers they cover 85–90% of the chapter’s exam content.
Class 12 Physics Chapter 7 Alternating Current: Key formulas you must memorise
Write these on your one-page formula sheet. Use RMS values for power problems and draw phasor diagrams for LCR circuits.
| Topic | Formula |
|---|---|
| RMS value | Vrms = Vm / √2 ≈ 0.707 Vm |
| Average (half-cycle) | Iavg = 2Im/π ≈ 0.637 Im |
| Inductive reactance | XL = ωL = 2πfL |
| Capacitive reactance | XC = 1/ωC = 1/(2πfC) |
| Impedance (series LCR) | Z = √(R² + (XL − XC)²) |
| Resonance condition | XL = XC ⇒ Z = R, power factor = 1 |
| Average power | Pavg = Vrms·Irms·cosφ = Irms²·R |
| LC angular frequency | ω = 1/√(LC) |
| Transformer turns ratio | Vs/Vp = Ns/Np = Ip/Is |
Practical transformer efficiency ranges from 90–98% . Major losses are copper (I²R), eddy current, hysteresis, and flux leakage.
How this helps your revision
Target audience: Class 12 Physics students preparing for CBSE and JEE/NEET. These notes match the latest marking scheme and NCERT emphasis. Practice mock tests and graded numericals to convert formulas into quick answers under exam time pressure.
Quick exam tips
- Always use RMS values for power calculations and show cosφ = R/Z. Draw neat phasor diagrams even if not asked explicitly.
- For transformers, mention turns ratio and list losses; box the efficiency result.
- For resonance problems, write XL = XC and substitute XL = 2πfL or XC = 1/(2πfC).
Article updated: Apr 17, 2026
FAQs
How do you derive the RMS value of alternating current and voltage?
A1: Use i = Im sin ωt and average of sin² over one cycle = 1/2. Irms = Im/√2 =
0.707 Im
.
Q2: What is the phase difference in a pure inductive circuit?
A2: Voltage leads current by
90°
; XL = ωL =
2πfL
.
Q3: Give the impedance and current expression for a series LCR circuit.
A3: Z = √(R² + (XL − XC)²); i = Im sin(ωt + φ) with tanφ = (XL − XC)/R and cosφ = R/Z.
Q4: How is average power calculated and what is wattless current?
A4: Pavg = Vrms·Irms·cosφ. For pure L or C, cosφ = 0 so average power = 0 (wattless current).
Q5: Difference between peak, RMS and average values?
A5: Peak = Im; RMS = 0.707 Im; Average (half-cycle) = 0.637 Im. Household
220 V
is RMS; peak ≈
311 V
.
Q6: What is the resonance angular frequency for an LC circuit?
A6: ω = 1/√(LC).