Alternating Current Class 12: CBSE weightage, key formulas and quick prep guide
Author: Shivam Yadav | Updated on Apr 14, 2026
Alternating Current Class 12 usually contributes around 6–8 marks in the Physics paper, while Unit IV (Electromagnetic Induction + AC) carries 8 marks in CBSE. For JEE, AC questions appear as 2–3 items in Main and Advanced papers.
Alternating Current Class 12 — CBSE & JEE weightage
The chapter is short but high-yield. Expect a mix of one-mark theory, 2–3 mark numericals and occasional case-based problems. Below is a quick weightage summary students report in recent exams.
| Exam / Unit | Typical weightage | Notes |
|---|---|---|
| CBSE Unit IV (EMI + AC) | 8 marks total | Unit IV = 8/70 (~11–12%) |
| AC chapter alone (CBSE) | 2–3 marks | Combined with EMI can reach 5–6 marks |
| JEE Main | 6–8 marks (~2–3 Qs) | Numericals + concept MCQs |
| JEE Advanced | 8–12 marks (~2–3 Qs) | Multi-step numericals common |
Alternating Current Class 12 — high-yield topics and formulas
Focus on Series LCR, resonance, impedance, power factor, phasor diagrams and transformers. These topics make up the bulk of both board and competitive questions.
Key formulas (memorise):
- RMS relations: Irms = I0/√2, Vrms = V0/√2
- Resonance: ωr = 1/√(LC), fr = 1/(2π√(LC))
- Impedance: Z = √(R^2 + (XL - XC)^2)
- Power: Pavg = Vrms Irms cosφ
- Reactances: XL = ωL, XC = 1/(ωC)
Short derivations and phasor sketches often fetch easy marks. Transformers and simple efficiency/turns-ratio numericals are low-effort, high-return topics.
How to prepare: time, pattern and priorities
Recommended revision time depends on your target. For board-focused revision aim for 4–6 hours total (theory + diagrams + 5–6 numericals). For JEE-level practice plan 8–12 hours of problem-solving across LCR, resonance and phasor problems.
| Target | Prep focus | Time (recommended) |
|---|---|---|
| CBSE boards | Theory, one-step numericals, transformers | 4–6 hrs |
| JEE Main / Advanced | Multi-step numericals, graph analysis, case problems | 8–12 hrs |
Work on NCERT questions first, then solve previous year JEE numericals and 10 carefully chosen LCR/resonance problems. Use phasor diagrams to avoid sign/phase mistakes.