Alternating Current
Article updated: Apr 16, 2026.
Alternating Current appears as a high-yield topic for Class 12 Physics and competitive exams. The official NCERT/CBSE syllabus places Unit IV (Electromagnetic Induction and Alternating Currents) at 8 marks , but analysis of CBSE 2026 papers shows AC-related questions totaling 12–15 marks (about 18–22% of the unit in that year).
Alternating Current: CBSE and JEE weightage
Here are the standard weight trends from official papers and recent 2026 analysis.
| Exam / Source | Typical weight / questions |
|---|---|
| CBSE (official Unit IV) | 8 marks (unit total) |
| CBSE 2026 papers (actual) | 12–15 marks (6–8 questions) |
| JEE Main | 6–8 marks (2–3 questions) |
| JEE Advanced | 8–12 marks (2–3 questions) |
Around 50–60% of 2026 AC questions focused on series LCR and resonance; the paper mix was roughly 60% numerical and 40% conceptual .
Alternating Current: Key formulas and facts
Memorise these for quick reference. They appear frequently in board and JEE questions.
| Quantity | Formula / Value |
|---|---|
| RMS current/voltage | I_rms = I0/√2 ; V_rms = V0/√2 (≈ 0.707×peak) |
| Average current (half-cycle) | I_avg = 2I0/π ≈ 0.637 I0 |
| Inductive reactance | X_L = ωL |
| Capacitive reactance | X_C = 1/(ωC) |
| Impedance (series LCR) | Z = √(R^2 + (X_L − X_C)^2) |
| Phase angle | tan φ = (X_L − X_C)/R |
| Resonant frequency | f_r = 1/(2π√(LC)) |
| Average power | P_avg = V_rms I_rms cos φ |
| Transformer turns ratio | E_s/E_p = N_s/N_p |
At resonance X_L = X_C, Z = R and φ = 0 (power factor = 1). These are frequent test points.
How much time to revise
Recommended revision from official exam trends: spend 4–6 hours to revise theory, diagrams and basic numericals for CBSE Boards. For JEE-focused numerical practice, allow 8–12 hours on series LCR, resonance and transformer problems.
Coverage notes from official papers
CBSE 2026 included more numerical and case-based questions on resonance and LCR than usual. JEE papers continue to test multi-step numericals, phasor reasoning and power-factor questions.